Theory of Evolution
Theory of Evolution
Author: Santiago Collado González
Published in: Philosophica: Enciclopedia filosófica on line
During the 18th century a group group of researchers, who were called naturalists, managed to gather a large amount of information on the fauna and flora in many different areas of our planet. B One problem posed by the accumulation of such a large volume of information was its organisation. The classification of living beings was initially carried out by means of extensive descriptions of the morphology and origin of the different individuals found. This subject of descriptions did not constitute a real financial aid to achieve classifications that were sufficiently univocal[Velázquez 2007: 131-142].
The system devised and developed by Linnaeus (1707-1778) was an important improvement in the organisation of information available. It consisted of proposing a series of rules to assign to all known living beings a label of genus and species. This classification, whose first edition was published in 1735, was called Sistema Naturae. Logically, at that time, it was the morphological properties of the different living beings that made it possible to assign genus and species to a particular individual. Although not Exempt of arbitrariness, the work done by Linnaeus greatly simplified the task of classifying animals and plants. In general terms, the tree structure he developed is still valid today, despite the changes that biology has undergone since then.
For Linnaeus, the species identified were groups of well-differentiated beings without any relationship of origin. The criterion of kinship, as we have indicated, was merely morphological. This so-called fixist perspective considered that each species was created as it was, and its individuals did not undergo changes over time.
However, the accumulation of data provided by naturalists, and the advances made in their organisation, led to the adoption of approaches quite different from the fixist one. Soon the idea that some species came from others and that, therefore, a classification had to be achieved that reflected the affinities between the different living beings from other perspectives: what was called a natural classification had to be achieved.
Buffon (1707-1788) had already questioned Linnaean fixism, but the first to propose a hypothesis as to how some species could be derived from others was the Frenchman Jean Baptiste de Monet, Chevalier de Lamarck, known simply as Lamarck (1744-1829). In his Zoological Philosophy, written in 1809, he gave a systematic description of the evolution of living things.
For Lamarck, species are derived from one another, from the simplest to the most complex. The organs of each species would develop as a consequence of reaction and adaptation to the environment. The changes would therefore be gradual and would take place over long periods of time. Lamarck thought that fixism was absurd because animals could not have survived, without evolving, the changing climatic conditions, which in some periods of time were very aggressive.
The originality of Lamarck's proposal consists in defending that changes occur through adaptation to the environment. Certain organs are strengthened by the animal's use of them, conditioned by the environment, while other organs atrophy and are eventually eliminated through disuse. Lamarck considered that these modifications in the various organs are transmitted by inheritance to descendants. The latter is what has been called "inheritance of acquired characters". In reality the idea that Lamarck was advocating was a version of "function creates the organ". An important consequence of the Lamarckian proposal was that the transformation of organisms must be necessary, gradual, upward and continuous. That is, from worms, for example, we would eventually end up with men again.
It can therefore be said that it was Lamarck who was the first to formulate an evolutionary hypothesis in the strict sense, although at the time the word evolution was reserved for the development of the embryo, and his proposal was called transformationalist. Unlike Darwin's proposal , the subject of Lamarckian evolution is the individual: it is the individual that undergoes transformation through adaptive use or disuse, and it is this transformation that is then transmitted to its offspring.
Lamarck's proposal , although it gained much support and seemed to explain in a natural way the increased complexity and diversity observed in nature, was also opposed by scientists of the stature of Cuvier (1792-1832), professor of comparative anatomy, who, using what Brentano later called the teleological principle[Brentano 1979: 244], gave guidelines for deducing animal forms from other forms of the same animal. These guidelines have since been developed by modern palaeontology.
Certainly, in living beings, particularly in higher animals, slight modifications of certain organs can be observed as a consequence of their use and, above all, it is easier to observe the atrophy of those organs that are not used. This does not mean that the function creates the organ, but rather that the functionality of the organ can be reinforced by its use. What science has so far strongly rejected is the inheritance of acquired traits. Neither experimental evidence nor any mechanism has been found by which individuals can pass on the supposed improvements acquired in the course of their lives. The principles governing the transformation of individual characters, which are now commonly accepted by science, were first established by Darwin and Wallace. On the other hand, the principles governing the transmission or inheritance of such characters were first established by Mendel.
As is well known, Charles R. Darwin (1809-1882) took part as a naturalist in the Beagle expedition to South America and the Pacific in 1831. The voyage, which began when he was only 22 years old, ended five years later. During this period Darwin had time to make many observations, compile information and reflect on the data he was collecting and on some texts such as Charles Lyell's Principles of Geology, where he found good syntheses of evolutionary arguments such as those defended by Lamarck. All this led him to embrace a transformist perspective of nature. In the years following his voyage, Darwin developed his own ideas and collected new data with which to produce a work in which he wanted to set out, in an orderly fashion, his vision of nature. Perhaps one of the texts that most influenced the development of his theses was the book by Thomas R. Malthus (1766-1834), first published in 1798: An Essay on the Principle of Population. In this book Malthus defended the thesis that the struggle for survival was necessary as a consequence of the fact that population tends to grow in a geometric progression while food grows in an arithmetic progression.
In 1858 Darwin received a parcel in the post from a remote island in the Malay Archipelago, present-day Indonesia. The package contained a text summarising the results of research carried out by Alfred Russel Wallace (1823-1913). It contained an extraordinary exposition of "the theory of evolution by natural selection". Its clarity of exposition means that the text still retains great pedagogical value today. Darwin had been working for two decades on a theory equivalent to the one in the paper and was on the point of abandoning his project when he read the work. It was Charles Lyell and the botanist Joseph Dalton Hooker who intervened in favour of their friend Darwin's interests. Wallace's paper was published in the Proceedings of the prestigious Linnean Society, preceded by another contribution by Darwin, containing some fragments of an unpublished essay from 1844 and a letter written to the botanist Asa Gray. The writings were published in August 1858, thus saving Darwin's right to claim originality for the work he had been preparing for so long and which had not yet seen the light of day. It was in the following year, 1859, that Darwin published the results of the work he had done during the preceding years in a book graduate "On the Origin of Species by Means of Natural Selection". The success of this book makes it possible to affirm that it was at this time that the "theory of evolution by means of natural selection" was born.
1) Offspring inherit the characters of their parents from generation to generation. Darwin, however, was not aware of the laws of heredity that were being worked on precisely in the years in which he made his theory known. The now scientifically accepted laws of heredity discovered by Mendel were not known until the beginning of the 20th century. Darwin's proposed explanations for the inheritance of traits proved to be erroneous and were soon rejected. These explanations, however, were not part of the content of the "Origin of Species".
2) In the process of inheritance, spontaneous variations occur that are either random or blind. We speak of random or blind variations in a double sense. On the one hand, their causes cannot be determined. On the other hand, these variations are not directed towards a better adaptation of the organism to the environment, i.e. there is no a priori orientation in them. In the first edition of the "Origin of Species", Darwin explicitly rejected the inheritance of acquired characteristics advocated by Lamarck. Later, however, he qualified this rejection.
3) There is differentiated reproduction in the individuals of a population. The reason is twofold: either some individuals are more fertile than others, or they are better adapted to the environment. Better adaptation to the environment will result in better survival and, consequently, in more offspring.
The impact of Darwin/Wallace's ideas was enormous. Very soon after the publication of "The Origin of Species", already in the 1960s, evolution based on natural selection as advocated by Darwin was, in practice, universally accepted. However, very soon the first objections to it began to be raised proposal. The objections from the 1960s onwards were not directed against the fact that there was evolution, i.e. that the various species descended from common, earlier species, but were directly directed against what made his proposal original, i.e. that the engine of evolution was random variation and natural selection.
In relation to the development of Darwin's proposal in the following years, and the criticism it has received up to the present day, it must be said that Darwin paid great attention to the possibility of explaining the development of Structures complex on the basis of chance variations and natural selection as the main cause of this development. Indeed, although for Darwin such a theory explained many aspects of the evolution of living things, including the origin of species, this did not imply that the evolution of organisms could be explained by natural selection alone. Darwin accepted the existence of other mechanisms of evolutionary change. Darwin's reasons at the time for maintaining his pluralistic view of the causes of evolution were, however, very poor or flawed when viewed from today's perspective.
Darwin personally confronted many of the objections to his theory of evolution that have been raised to this day. His views were set out in successive editions of the Origin of Species[Darwin 2002: 183]. He not only focused on the problem of the origin and increasing complexity of living things, but also, for example, addressed problems such as the paucity of the fossil record available of the supposed living things that must have existed as a consequence of gradual evolution as advocated in his proposal [Darwin 2002: 349].
The weight of the objections to his theory, together with his ignorance of the laws of genetics, led Darwin, after 1959, to play down the importance of the mechanism of natural selection and even to accept the existence of mechanisms of subject lamarkian as an explanation of the transformations in living beings.
One of the main objections to Darwin's theory in these years was raised by William Thomson (Lord Kelvin). Kelvin shared with Darwin a way of understanding the transmission of hereditary characters that led him to conceive of the process of evolution by natural selection as extraordinarily slow. Not only were the changes that served as subject for natural selection minute and gradual, but in order to transmit the traits to offspring without any loss of variation it was necessary for the novelty to appear in two individuals and for them to mate with each other. The probability of things happening in this way was so small that in order to explain the evolution and variety of life on Earth as it is presented to our experience it was necessary for the process to have taken billions of years.
The problem was that the estimated time for the earth was much shorter. At that time it was thought that the energy we received from the Sun came exclusively from gravity. The approximate mass of the Sun and the energy it emitted could be calculated. With these assumptions Kelvin's calculations predicted a lifetime for the Sun of no more than a few hundred million years. Logically, life on earth could not exceed that time, which was much shorter than the estimated time needed for life as we know it development . Radioactivity, the real source of the energy we receive from the Sun, was discovered in the decade beginning in 1890. These considerations must have had a significant influence on Darwin's approach, who, as we have noted, in successive editions of the Origin of Species, downplayed the importance of blind variation and gave it to other mechanisms such as the inheritance of acquired environmentally induced characters.
Despite admitting a plurality of mechanisms as the driving force of evolution, for Darwin there was evolutionary continuity among all species, including humans. However, Darwin did not defend that the higher human Schools were the result of natural selection. Wallace was arguably stricter than Darwin in defending the mechanism of natural selection. His panselectionism led him to regard random variation and natural selection as the sole force of biological evolution. However, Wallace admitted the influence of another force, of a "spiritual" nature, when it came to explaining the origin of life, the emergence of consciousness in animals and, above all, the higher human Schools such as, for example, their ability to do mathematics or their artistic skills. For Wallace, the world of subject was clearly subordinated to that other world of the spirit in which natural selection did not fit as an explanation. Wallace was stricter in his defence of natural selection in organic evolution than Darwin was, and also more net in his defence of a "spiritual" realm for which natural selection was not an explanation[Sarkar 2007: 31-32].
Another very important biologist in the 19th century was the German August Weismann (1834-1914). Weismann, also a panselectionist, completely rejected the possibility admitted by Darwin of the existence of Lamarkian subject mechanisms. The distinction he drew between germ cells, isolated from environmental influences, and somatic cells, points to what would later become the general framework of modern evolutionary theory.
Despite the initial success of Darwin's theory, and the efforts of biologists such as Weismann to defend natural selection and undermine the credibility of Lamarkism, the 1990s saw the beginning of a period in which the "blind variation plus selection" mechanism lost popularity in favour of other mechanisms of subject or those that could also be framed within the so-called orthogenesis (evolution with a specific direction). One of the advocates of neo-Lamarckism in these years, Herbert Spencer, was the one who coined the expression "survival of the fittest", which has often been translated as "survival of the fittest", and which has helped so little in the correct understanding of the theory proposal by Darwin/Wallace.
The reasons for the regression of the Darwinian proposal are varied. We have already mentioned the serious difficulties arising from Kelvin's considerations. The probabilistic arguments did not seem to support the theory initially proposal by Darwin. Scepticism was growing about the possibility that natural selection alone could explain the emergence of species diversity. This scepticism was fuelled by the lack of knowledge of the mechanisms of genetics and also by the lack of quantitative experimental data to support the thesis of the "Origin of Species".
On the other hand, even in classical philosophy, arguments had been made on the basis of finality to defend the existence of a higher being on which the world depends. Talk of a mechanism that seemed to steal finality from nature provoked and continues to provoke the liveliest debates.
The misgivings about the new theory of evolution became more acute when the continuity between animals and man was emphasised. Darwin explicitly defended this continuity in a book published in 1871 entitled "The Descent of Man". The gradualness of the higher human Schools (intelligence and linguistic capacity, for example) compared to animals did clash openly, for example, with the doctrine held by all Christian denominations about the peculiar way of being of human beings. Darwin proposed a selective explanation for certain moral qualities found in man and also, in his own way, in animals: group cooperation, common defence, transmission of knowledge from parents to children, for example. But the difficulties in supporting the evolution from animals of Schools such as intelligence or human linguistic capacity forced Darwin to resort to the use of inheritance inheritance typical of Lamarkism and other hypotheses that today are completely untenable. What Darwin never renounced was the continuity between animals and man, which meant reducing human cultural dimensions to pure biology.
Both neolamarkism and orthogenesis served in the last decade of the 19th century as an alternative, or at least as a complement, to Darwin's and Wallace's theory in the way of explaining what, already in those years, was admitted by scientists as a certain and incontrovertible fact: the fact of evolution or descent of all living beings from common ancestors, including human organic characteristics. What was questioned in those years, or even flatly denied, was the ability of natural selection alone to generate the diversity of species and the Degree complexity achieved by living beings.
The discussion would be placed on a new framework with the development experienced by genetics at the beginning of the 20th century.
In our review of the ideas that make up the modern theory of evolution, we have examined one of the pillars supporting this theory: the ideas set out in the "Origin of Species" about small variations and natural selection. The other important pillar is the ideas published in 1866 by the Augustinian monk Gregor Johann Mendel (1822-1884), born in Heinzendorf (then in Austria and now part of the Czech Republic). Although his work laid down the fundamental principles of modern genetics, the importance of its content was not recognised until the beginning of the 20th century.
Mendel obtained the principles of inheritance by experimenting with certain pea plants which showed a number of well-defined characters: flower size and colour, seed shape and colour, etc. He made crosses between plants with different characters and quantified and interpreted the results obtained by crossing several generations of plants. He reached a series of conclusions that were later known as Mendel's laws and are still valid today.
Mendel distinguished between character and factor. Traits were the visible properties manifested by the plants: colour, shape, etc. The manifestation of the various "characters" depended on a set of independent and discrete "factors" that were present in the plants[Curtis-Barnes 1996: 207 ff.]
Mendel's first law is called the "principle of segregation" and states the hypothesis that each individual carries pairs of factors for each trait, and that the factors of each pair segregate or separate from each other when the gametes (the germ or reproductive cells) are formed. Thus, in the offspring, when the paternal and maternal gametes unite, one factor of the new pair is inherited from the father plant and the other from the mother plant. Later, these factors were called genes, the units of inheritance, and the varieties that had these factors or genes were called alleles.
The result of the experiments carried out by Mendel led him to conclude that one of the two factors of the pair is always "dominant" with respect to the other, which is then called recessive. In other words, when both the dominant and recessive factors were present in the plant, the character presented by the plant was always that of the variant of the dominant factor or allele.
Mendel's second law is called the "principle of independent transmission". It states that when gametes are formed, the alleles of one gene segregate independently of the alleles of another gene. Therefore, the possible combinations of the different characters when crossing different plants should also be independent. In other words, the colour trait, for example, was not linked to the size trait, but size and colour could be combined independently in breeding.
These laws were the interpretation of the distribution of characters that Mendel obtained by experimentally crossing different pea plants. This interpretation was able to perfectly quantify the results of the proportions of characters obtained in the experiments.
For some, the outline proposed by Mendel constituted an achievement for biology of even greater importance than Darwin's proposal . It could be said that outline introduced biology into the realm of quantification, which is the ideal to which every science that seeks to rely on experimentation aspires.
As we have indicated, Mendel's work went unnoticed until it was rediscovered simultaneously by three botanists in 1900. All three recognised Mendel's proposal as a predecessor of their own work. At the turn of the century, the zoologist William Bateson (1861-1926) emerged as the greatest defender of Mendel's laws. Bateson was at the centre of a new controversy that pitted him against Darwinian evolutionists of the time such as Karl Pearson and, in particular, the zoologist Walter Frank Raphael Weldon (1860-1906).
Bateson thought it was more in line with Mendel's finding that the variations that gave rise to evolution were discontinuous and not small variations as hypothesised by Darwinian theory. In fact he did not believe that evolution took place along the lines of the outline presented by Darwin. On the other hand, Pearson and Weldon thought that Mendel's laws only worked in very exceptional cases. They also rejected the distinction between character and Mendelian factor and formulated a set of laws that omitted this distinction and relied only on the external characters presented by individuals. Weldon attempted the construction of a statistical theory of evolution that conformed to Darwin's ideas. The confrontation between Bateson and Weldon ended with Weldon's death in 1906, but it did not end the dispute between the Mendelians and the so-called "biometricians". The former emphasised, contrary to Darwin's theory, the importance of discontinuity in the changes transmitted by inheritance. The latter were faithful to the Darwinian evolution of subject , which emphasised gradual changes in traits. Mendel thus contributed to a further weakening of confidence in the Darwinian thesis in the early years of the 20th century.
The wall separating the positions of Mendelians and biometricians began to crumble in 1918. In this year R. A. Fisher (1890-1962) was able to show that the laws formulated by the latter could be explained within the framework established by Mendel's laws. This contribution, together with the work of other authors such as John Burdon Sanderson Haldane (1892-1964), made it possible to construct a theory of natural selection based on the Mendelian model of inheritance. The modern theory of evolution had its beginnings in the work of these years, which reached maturity in the early 1930s.
The distinction made by Wilhelm Johannsen (1857-1927) in 1909 between the notion of genotype and phenotype was very important in the construction of the new theoretical framework . The latter is constituted by the set of detectable characteristics in an organism (structural, physiological or behavioural) that are determined by the expression in the living being of the genotype and its interaction with the environment. The latter is constituted by the set of detectable characteristics of an organism (structural, physiological or behavioural) that are determined by the expression of the genotype in the living being and by its interaction with the environment. This distinction updated the one originally made by Mendel, proposal , between character and factor. The notion of gene, also coined by Johannsen, was then postulated in order to achieve a theory consistent with experience, but although much was already known about how genes were involved in the inheritance of traits, it was not really known at that time what genetic material was or what it consisted of.
One point that is core topic at the junction of the two competing perspectives in these early years of the century was the assumption that the development of every living thing, from embryo to adulthood, is like a black box, i.e. any consideration of how genes interact with the organism and its environment was omitted. This is undoubtedly a major simplification, but it made such a synthesis approachable. The new outline assumed that natural selection could be modelled on the basis of changes occurring only in the genome. In other words, only modifications in the genome are responsible for evolutionary change, and these modifications are not conditioned in their production either by the phenotype or by the environment, but are random modifications, from agreement with the ideas of Darwin and Wallace. The non-influence of phenotype on genotype is related and can be considered equivalent to what was later called the central dogma of biology.
In the 1920s, Haldane, Fisher and Wright had a great influence on the development of the Theory of Evolution. Haldane published several articles in which he dealt with natural selection from a genetic perspective: he analysed a wide variety of genetic models and also the different forms in which natural selection could occur: weak or intense, constant, cyclical, etc. One of the conclusions he reached was that the process of natural selection acting on blind variation was faster than previously thought. The fear that there was not enough time for natural selection to lead to major evolutionary modifications did not seem to be justified in the light of this work. The competing theories of natural selection in the early years of the century - those advocating orthogenesis and those of subject neo-Lamarckianism - were dealt a severe blow by this work. These three authors are considered today as the fathers of population genetics, which is still the foundation for the current theory of evolution.
Haldane focused on the study of the evolutionary consequences of various genetic models. Fisher and Wright tried to offer general theories to explain the history of life on Earth. The two had some differences regarding the role of natural selection in evolution. Fisher believed that the best explanation of evolution is provided by natural selection acting on small variations that occur in large populations in which individuals mate randomly. Wright, on the other hand, thought that small isolated populations in which large fluctuations could occur, precisely because of the small number of individuals in them, were more important in explaining change. This hypothesis later became known as "genetic drift". The discussion between these two positions has been relevant in the development of modern evolutionary theory.
The integration of the above work with the rest of biology was the task of Theodosius Grygorovych Dobzhansky (1900-1975), who managed to unify the empirical results of natural populations with the theoretical models of Haldane, Fisher and Wright. His most important book was published in 1937 and was entitled "Genetics and the Origin of Species". One of the topics that focused his interest was that of speciation: the appearance of new species from existing ones. This is the problem that appeared in the title of Darwin's famous book and which, in reality, he did not clarify. Dobzhansky highlighted the importance of geographical isolation as one of the most important causes for the emergence of a new species. This speciation subject was called allopatric speciation. Its study, and the study of speciation in general, has been further developed by, among others, Ernst Mayr (1904-2005). Today, this speciation subject is considered no more important than sympatric speciation, where species formation does not require geographic isolation. Julian Huxley (1887-1975) popularised the new theoretical framework achieved by the above authors in 1942 in a widely circulated book whose title called the new theory of evolution the "modern synthesis"[Huxley 1946]. Since then this theory has been known as the "synthetic theory of evolution".
Another important milestone in the shaping of the theory of evolution took place with Watson and Crick's design in 1953 of the double helix model of the DNA molecule. It had been known since the 1940s that genetic information was contained in DNA (deoxyribonucleic acid) molecules. In 1953, the structure of this information was determined. It was discovered that DNA molecules encode genetic information along linear sequences of 4 nitrogenous instructions sequences or nucleotides called Adenine, Cytosine, Guanine and Thymine. These instructions constitute the four letters of an alphabet with which the information that is expressed in the genome is written on the development of the living being.
The distinction between genotype and phenotype was thus firmly established. The most basic level of phenotype would be the proteins: macromolecules made up of amino acids that constitute the fundamental structural part of the various living organisms. The correspondence between the different sequences of instructions of the DNA with each of the 20 different types of amino acids is known. Specifically, each amino acid is encoded by three of the basic letters of the genetic code. Each three-letter group that codes for an amino acid is called a "codon". Not all DNA is coding. In addition, there are amino acids that are associated with different codons. This is why the genetic code is said to be degenerate. In turn, the 20 amino acids give rise by composition to a great variety of proteins that perform a multitude of functions in the organism at very different levels and form part of a great diversity of organ systems.
We will now briefly outline some of the most important notions that were established by the genetics that developed from the 1950s onwards and that are decisive in the way in which evolution is understood today[Ayala 2006a: 223 ff].
DNA is, as we have indicated, the molecule in which the genetic information is encoded. It is a long molecule in the form of a helix and can be represented as two long molecular filaments coiled and joined by the instructions or nucleotides. There are four types of instructions and each strand is linked to the other by the complementary instructions of the other.
DNA molecules are packaged together with proteins in dense bodies called chromosomes. Each species has a certain number of chromosomes. The human species has 46 chromosomes. In this case, which is a case of sexed reproduction, 23 chromosomes correspond to the father and the other 23 to the mother. We have 23 pairs of homologous chromosomes.
The gene is the discrete unit of inheritance first identified by Mendel. In the current paradigm, each gene corresponds to a morphological characteristic of the organism, for example, the colour of a body part such as hair or eyes. The gene is a segment of the chromosome at a specific location called a locus. Each chromosome can have many thousands of gene loci. The loci are on both homologous chromosomes. Each gene at a particular locus can have variant forms that are called alleles. This means that allele genes vary in one or more parts of their nucleotide sequence. Genes therefore occur in pairs, one on a maternal chromosome and the other on the corresponding paternal or homologous chromosome. The two homologous genes occupy one locus on each of the homologous chromosomes. The existence of alleles is the prerequisite for evolution to take place. It has been proven that there is a great genetic diversity, i.e. a great diversity of alleles within the different populations. Artificial selection is a sample that there is a large genetic variability in natural populations.
A notion core topic in evolutionary theory is that of species. In the "modern synthesis" the notion of biological species was characterised by Dobzhansky and Mayr, for sexually reproducing organisms, as "groups of inter-fertile natural populations that are reproductively isolated from other groups"[Ayala 2006b: 258]. This notion is the most widely accepted today, despite its obvious limitations, such as the fact that it is valid only for groups that reproduce sexually, or that its application is not possible for species that are already extinct. The notion is important, among other reasons, because defined in this way, each species constitutes a discrete and independent evolutionary unit (there is no exchange of genes between different species). Much has been written about the mechanisms that lead to the formation of a species. In all that has been written, the importance of reproductive isolation mechanisms, of which several types have been identified, is emphasised.
The discoveries of the 1950s in genetics and biochemistry have led to countless studies and research from the new theoretical framework and have already yielded concrete practical results. These studies have resulted, for example, in the completion of the project Human Genome in 2003. During the 13 years of project , the approximately 20,000-25,000 genes in our DNA were identified and the sequence of the three billion instructions that make up DNA was determined. In addition, the theory of evolution has been refined considerably. It is now possible, for example, to taxonomise living organisms on the basis of the genetic heritage of each species and not on the basis of external morphological aspects, which are more arbitrary. We now know, among other things, what was not known when the synthetic theory was first formulated: what genetic material consists of. The very meaning of genetic information, which has to do with its expression in the living organism, is gradually being understood, a task that has only just begun. All this knowledge has opened up many expectations, for example in medicine and also in theoretical biology in general. On the other hand, however, the extraordinary complexity of living organisms has also become apparent. As far as the process of evolution is concerned, the above-mentioned advances have resolved old questions, but have also opened up new ones that are even more difficult challenges for science than the old ones.
It can be said at final that there is a common framework accepted by most scientists, which includes, among others, the ingredients described above. The essential nucleus on which there is common agreement among the entire scientific community could be summarised by saying that "evolution occurs through the action of mechanisms such as natural selection on, primarily, small blind variations that occur at the genetic level"[Sarkar 2007: 69]. But within this framework there are issues that remain the subject of lively debate. Questions already raised at the beginning of the formulation of evolutionary theories are also maintained, but now seen from the new perspective and, therefore, from a better understanding of their complexity.
We have also seen that, from its inception, the theory of evolution that grew out of Darwin's ideas has not been accepted peacefully. We have already mentioned some of the controversies that arose in the final years of the 19th century and their roots. Outside the scientific sphere, the controversies have not been minor. One of the movements that has offered most resistance to Darwin's ideas has been Creationism. The confrontations with the theory of evolution, which continue to this day, provide the possibility of tracing a history of which even a simple outline is beyond the scope of this voice [see the voice design intelligent].
Some of the most important scientific issues that have been the subject of controversy in recent years in relation to the theory of evolution are briefly outlined below. They are listed here because understanding them also allows for a better understanding of the theory of evolution and its scope. On the other hand, the philosophical discussion , to which the last part of this section is devoted, is not unrelated to the scientific discussion .
In the 1960s, a new technique, gel electrophoresis, began to be used, with which the Degree genetic variation of populations could be checked quite accurately. Studies carried out with this technique, and others that are also easy to perform at laboratory, led to the determination that the proportion of heterozygous loci, i.e. those with different alleles on homologous chromosomes, ranges between 5 and 20 percent. Taking into account that the technique used detects variations in proteins and that the coding of proteins by DNA is degenerate, it was reasonable to think that the Degree of genetic variation was even higher than these percentages[Ayala 2006c: 280].
The Degree of variation resulting from such experiments was much higher than expected. An explanation for this phenomenon was proposal by the Japanese geneticist Motoo Kimura in 1968. For this scientist, and those like him who defend the so-called "neutralist theory", most genetic differences neither favour nor hinder the survival of organisms, leading them to conclude that whether they survive or are eliminated from a population is simply a matter of chance. Neutralists say that if most genetic differences were subject to natural selection, variation would be much smaller and therefore natural selection would not have the impact on evolution that synthetic theory suggests.
Another phenomenon used by neutralists to defend their proposal is the constancy of the rate of genetic change over generations. Different studies have been carried out linking the common evolutionary history of various species and the number of differences in the respective DNA sequences. The results suggest that genes can be considered as molecular clocks since the rate of change experienced is relatively constant over long periods of time and, moreover, these values are similar in different species. If natural selection were to act as proposed by the synthetic theory, say the neutralists, the rates of change would be more variable as a consequence of different selective pressures over time and across species.
This proposal has perhaps been the most hotly debated in the world of biology from the time synthetic theory was formulated to the present day. Indeed, in 1969 the neutralists King and Jukes announced, somewhat provocatively, the birth of an evolutionary model in which allele-neutral drift had replaced selection as the evolutionary force.
In fact, the debates on neutralism-selectionism have not yet ceased. Authors such as Ayala or Sarkar think that, at present, discussion is not about determining the success of one of the proposals and the exclusion of the other. It is rather a question of determining to what extent selection acts and to what extent neutralism is valid and financial aid to understand evolution. In fact, Kimura himself accepted that natural selection is the determining evolutionary force at the morphological level and that neutralism at the molecular level presents problems when trying to explain the adaptive differences that become apparent at higher levels of organisation. Ayala further argues that synthetic theory does not force the rate of evolution to be as irregular as the neutralists assume. The enormous time intervals over which molecular evolution takes place mean that the fluctuations compensate for each other, giving the impression that they occur at a constant rate. Mathematical models have even been formulated in which the molecular clock is made compatible with evolution governed by natural selection.
A so-called "quasi-neutral" theory has also been proposed, which tries to maintain the consistency of the neutralist thesis with a lot of data indicating that natural selection is at work. Most biologists still believe in the importance of natural selection as one of the drivers of evolution, but this discussion is still alive and many hope that the data being obtained from the different genome sequencing projects will help determine the weight that the contributions of both proposals have in the overall framework of evolutionary theory[Sarkar 2007: 65-67].
One of the classic distinctions underlying some of the debates about evolution has been that of microevolution versus macroevolution. Current genetics has introduced an equivocality into these terms that is worth bearing in mind. Microevolution can be understood as evolution that occurs as a consequence of small observable variations within the same species. Macroevolution, on the other hand, would be that which brings about large changes such as the diversification of species over long periods of time. The new theoretical framework leads to a different, though related, understanding of these notions. Microevolution is now what we can observe at the biochemical level: a modification in a pair of instructions of a gene, a mutation, would be the most elementary fact of microevolution. This subject of modifications at the genetic level, with their repercussions at the phenotypic level, are not in doubt: they can be provoked, observed and experimented with directly.
As a consequence of the influence of the gradualist thesis defended by Darwinism, it is often argued that the difference between microevolution and macroevolution is only a matter of time: macroevolution would be nothing more than the accumulation of microevolutionary changes. This assumption has been the subject of nuance and discussion even among Darwinists themselves. It is not disputed that microevolutionary changes are at the basis of macroevolutionary changes. What is disputed is the reducibility of one to the other, that is, that the explanation of the laws and mechanisms of microevolution leads to a complete explanation of those of macroevolution. In other words, it is doubted or denied that the laws of macroevolution that we can establish can be derived from those established for microevolution. Ayala, for example, states the following: "macroevolution is an autonomous field of evolutionary study and, in this important epistemological sense, macroevolution is decoupled from microevolution"[Ayala 2006b: 268].
Closely related to the difference between microevolution and macroevolution is one of the most important debates that has arisen within the scientific community and which, initially, seemed to break with the very foundations of the theoretical framework of the modern synthesis. This is the confrontation between the gradualism of the synthetic theory and what has been called punctuationism or saltationism. This name derives from that given to the theory proposal by Niels Elredge (1943-) and Stephen Jay Gould (1941-2002) in 1972, which the latter called "Punctuated Equilibrium".
The problem behind the origin of this proposal is the contrast between the gradualism that seems to derive from the synthetic theory and the existing jumps in the fossil record, which are far from being a gradual continuity. Until punctuated equilibrium saw the light of day, the most common, though not the only, way to justify the existence of these gaps in the fossil record was the easiest and most straightforward: not all living things that have ever existed have been fossilised, or else we have not yet discovered many of the fossils that will allow us to fill in the existing gaps. Mayr, on the other hand, went somewhat ahead of saltationism in 1954 by arguing that the existence of gaps in the fossil record was a consequence of the fact that "the founder populations in the process of speciation are very restricted in space and time and, therefore, are very unlikely ever to appear in the fossil record"[Mayr 2005: 212]. The fact is that the accumulation of new fossils did not seem to support the easy solution, and there was reason to doubt the compatibility of gradualist orthodoxy with the data provided by palaeontology.
Gould and Eldredge argued in their work that the fossil record showed positively that in evolution there were short periods in which evolutionary changes occurred very rapidly, and that these were followed by long periods of stasis in which the various forms remained stable. In other words, evolution seemed to leap from one species to another with no species in between. It is not that we did not have the fossils of the intermediate links - missing links - but that these links simply did not exist.
Initially punctuated equilibrium seemed to some to be an alternative theory to the modern synthesis, received much criticism and provoked a lively discussion . It soon became clear that the outline punctuationist or saltationist was not a real difficulty in continuing to uphold the principles of the modern Darwinian synthesis. The authors of the theory themselves explained that the problem lies in the fact that we are playing with two different time scales. On the one hand we have to consider the time in which evolution takes place following a model of small and gradual changes and, on the other hand, the time that is relevant to the fossil record called geological time, whose scale is much larger than that of the former. In Gould's words: "What punctuated equilibrium theory attempts to explain is the macroevolutionary role of species and speciation as expressed in geological time. Its statements about rapidity and stability describe the history of individual species, and its statements about rates and styles of change deal with the tracing of these individual histories in the unfamiliar domain of geological time, where the duration of a human life is below any possible appreciation, and the entire history of human civilisation is to the duration of primate phylogeny as a blink of an eye to a human life"[Gould 2004: 797].
Many biologists have pointed out that macroevolutionary patterns of stasis and jumps could be produced by models based on microevolution. It also seems to have been shown in recent decades that rapid morphological changes can occur in natural populations. It thus seems to be confirmed that even if jumping was the predominant pattern of macroevolutionary change, the processes involved remain within the framework of the modern synthesis.
Gould himself has defended the compatibility of his proposal with the theses of current synthetic theory: "Nor does punctuated equilibrium attempt to redefine or criticise conventional microevolutionary mechanisms at all (because it arises as the anticipated expression, after change of scale, of microevolutionary theories of speciation in the radically different domain of geological time)"[Gould 2004: 812]. Nevertheless, Gould argues that his proposal is original, and this originality lies in the change of perspective with which evolution is observed. For Gould, on the geological time scale, the subject of evolutionary selection would no longer be the individual in a population, but the species itself. As Gould puts it: "But the crux of the potential novelty of punctuated equilibrium for biological theory is that these classical microevolutionary mechanisms are not exclusive to evolutionary explanation, and that their domain of action must be restricted (or at least shared) at the level of macroevolutionary guideline at the geological scale, because punctuated equilibrium ratifies an effective macroevolutionary mechanics based on the recognition of species as Darwinian individuals. In other words, the main contribution of punctuated equilibrium to macroevolutionary theory is not the revision of microevolutionary mechanics, but the individuation of species (which establishes the basis for an independent macroevolutionary theoretical domain)[Gould 2004: 812].
Therefore, according to this author, saltationism could be seen as an extension of synthetic theory in which its principles are reaffirmed but in a different theoretical domain. These statements indicate, as we have also seen Ayala does, a certain level of independence between macroevolution and microevolution, but always within the common framework of synthetic theory.
This discussion serves to allude to a discussion that also has resonances in philosophical studies on evolution: the determination of what is the unit of selection. The gene, the individual and other population groups such as the species have been proposed as selective units. Geneticists, and more specifically neutralists, are more likely to consider the gene as the unit or target of selection. Mayr considers that "since no gene is directly exposed to selection, but only in the context of the whole genotype, and since a gene can have different selective values in different genotypes, it seems inappropriate to consider it as the target of selection"[Mayr 2005: 218]. This author thinks that it is the individual that is the main target of selection, although he also admits the possibility of the so-called selection of group as, for example, the selection of species.
It has already been mentioned that one notion core topic, as is evident from all that has been seen so far, is the notion of species. In any work on evolutionary theory, such as this one for example, it is one of the most frequently used words. The notion has been the subject of important debates since the beginning of evolutionary theory. The discussion on this notion also has special philosophical connotations, which is why it is also important to deal with it here, albeit very briefly.
The problem under discussion could be expressed simply as an alternative: do species have a real existence or are they instead a product of our mind that simply facilitates the organisation of our knowledge of nature? Darwinian gradualism blurs its contours and opposes a notion of species conceived as something perfectly determined morphologically and temporally. If one species is derived from another by gradual evolution, where do we draw the line between the two species? Or, what differences must there be between two individuals to be considered as belonging to different species? Darwin, for example, stated: "I regard the term species as given arbitrarily, for the sake of convenience, to a group of very similar individuals, and as not differing essentially from the term variety, which is given to less precise and more fluctuating forms"[Darwin 2002: 104]. For Haldane the species concept was a concession to our linguistic habits and neurological mechanisms[Sarkar 2007: 70].
The difficulties in providing a definition of species that does not present some problem or limitation seem to support these views and detract from the reality, or rather realism, of the species notion. However, despite its limitations, much importance has been attached to the previously defined notion of biological species. In fact, this notion is useful within the conceptual outline that serves to explain evolution itself and, as we have seen, in some authors such as Gould, it even becomes the target of Darwinian selection, that is, a Darwinian subject. This notion of biological species is clear and clearly defines what a species is, but it does not completely avoid the general problem of the lack of delimitation between species when gradualism in evolution is accepted.
Currently this discussion is still open. The definition of biological species is seen by some as insufficient. For example, the field of microbiology offers a greater diversity than the one we are used to contemplating ordinarily and which has been the object of the most usual taxonomic proposals. In this field, the definition of species biology is useless, since the main reproductive subject is not sexual. Despite the difficulties, taxonomic criteria are still being sought that are useful for making classifications and reduce as much as possible the inadequacies of the existing ones. What seems clear to all biologists is the need for a criterion to differentiate species, although they do not always agree on agreement to establish the most appropriate one. Beyond the disagreements, what does seem to be agreed upon is that the reality of the species is related to the existence of population units grouped in ecological niches in which natural selection avoids confusion between them. It is also acknowledged that there may not be a single, optimal characterisation for the species, but that one or the other should be used depending on the level or branch in the tree of nature being studied[Zimmer 2008: 72-73].
In reality, the greatest difficulties on this point arise, above all, for those who defend the proposals of subject creationist or subject fixist. In reality, such fixity belongs only to our way of thinking about natural beings, that is, it belongs to our objectification of them. It is not easy to think objectively about the very movement of life. A movement that, as in the case of evolution, involves periods of time that completely escape the magnitudes that our ordinary knowledge captures.
The Degree intervention of natural selection in the evolutionary process has also been a constant subject of discussion since the formulation of Darwin's theory. The controversy is still open in the purely scientific sphere. It should be noted that this mechanism has always been given a central role within the orthodoxy of synthetic theory. Much of the originality of Darwin's proposal rests on it.
However, reasons are now being advanced for attenuating its importance in evolution, such as those put forward by the aforementioned neutralists. Important reasons continue to be given for their importance. One argument employee often used in their defence is the observation, especially in the field of macroevolution, of the existence of "convergent evolution": living organisms are phyletically very distant, or have evolved in isolation, but have developed similar organisms and have reached remarkably similar functional solutions.
The problem of the importance of natural selection is parallel to the problem of the Degree contingency of evolution. The difficulty that arises is the following: if evolutionary history were to start all over again, would we have a picture in nature similar to the one we find today? If one accepts that it is mutations that generate variety, and that mutations are blind, the answer to the question has to do with how strong or weak the role of natural selection in evolution is. The answer to the question has pitted a number of scientists against each other. Gould, for example, has made contingency one of the central points of his thesis. Simon Conway Morris, on the other hand, has emphasised convergence in a special way. In general it seems that among biologists there is agreement that there is both one and the other, and they accommodate contingency and convergence within modern synthetic theory. Some scientists have even formulated the action of natural selection in the form of a theorem, specifying which are the necessary premises that must be fulfilled for natural selection to act or not[Meléndez-Hevia 2001: 18]. These formulations try to explain the reason for the contrasts pointed out within the framework of the synthetic theory and to offer a perspective as close as possible to the importance of the action of natural selection in evolution.
These are by no means the only issues debated. In any case, none of the important 20th century authors who have contributed to the establishment of synthetic theory think that this large number of controversies jeopardises the validity of modern synthetic theory for the time being. Mayr says in this respect: "for many evolutionary problems there are many possible solutions. But all of them are compatible with the Darwinian paradigm. The lesson of this pluralism is that, in evolutionary biology, generalisations are almost never correct. Even when something happens "as a rule", this does not mean that it must always happen"[Mayr 2005: 223].
The reality is that there are many issues at discussion. The pluralism to which Mayr refers may seem excessive to some who are considering the need to achieve greater unity and simplicity, perhaps with the formulation of a new synthesis. This new "postmodern" synthesis, as it is described with some apprehension in an article in Nature[Whitfield 2008], should be able to explain what is happening in areas of biology that have not yet been satisfactorily integrated with synthetic theory. One such area is, for example, the biology of development, on which genetics is currently providing a great deal of information. The emerging discipline evo-devo (evolution and development) tries to bring these two areas of biology together, but it is still far from maturity. There are many mysteries to be unravelled in this area of biology, and the data that are accumulating lead to the question, for example, of the need to assume a richer relationship between genotype and phenotype than the one accepted by synthetic theory. This relationship should not be, for example, as unidirectional as the aforementioned central dogma of biology establishes. Or, at least, it should admit an influence of the environment that is not exclusively reduced to a selective function.
All that has been said so far may lead one to think that the conceptual framework of modern synthetic theory explains a lot, but that it is still insufficient to give true unity to all the phenomena we witness in the biological world. In any case, what these debates clearly show is that life, in its apparent simplicity and straightforwardness, presents great complexity when analysed from a scientific point of view. Biology is not physics and does not seem to be caught in the nets of a perfectly unified, defined and finished method. In fact, physics, although more amenable to mathematics, does not seem to allow it to do so either. In any case, the large amount of scientific knowledge that we have on biology, and in particular on evolution, makes it possible and at the same time strongly invites us to make a philosophical reflection.
Philosophy is a discipline that seeks to achieve a global perspective on reality. There is nothing that can escape the gaze of philosophy in its attempt to find the synthesis or connection with the globality of the real, that is, how each portion of the real fits into the broad landscape of reality[Polo 1995: 21].
For this reason, philosophy always transcends the field on which it focuses its attention. Its vocation is to confront the most radical questions. Philosophy is a discipline that searches for the first principles or causes of reality. This is the most demanding way of adopting a global perspective. To say that philosophy tries to reach the principles of the reality it is concerned with is equivalent to saying that what is expected of philosophy is that it offers ultimate answers to the problems it raises, which is not the same as saying that definitive answers are expected of it.
This does not mean that philosophy is a kind of discipline independent or apart from what the ordinary or scientific knowledge offer to our understanding. There is no such thing as a pure philosophy uncontaminated with questions that are considered of lesser or superficial importance. All genuine philosophy must be well rooted in what is known, whatever the method or the way in which that knowledge has been made present to us.
Therefore, the peculiarity and also the difficulty of the philosophical knowledge consists in its aspiration to achieve a global perspective. This aspiration means that philosophy cannot always be easily discerned from doctrines that we could call pseudo-philosophies. One could also speak of the existence of pseudo-sciences. These pseudo-doctrines have as their real point of support and are largely nourished by ideologies for which they serve as a mouthpiece. It is normal that pseudo-philosophy and pseudo-science take advantage of the limitations of the scientific knowledge to try to fill their gaps with considerations that often contain an ideological component. Biology, with its complexity, its topics and its current Degree of development, is a fertile ground for this subject of pseudo-doctrines.
On the other hand, there is no need to justify the need for a philosophy of biology. It is sufficient to note that such a philosophy is unavoidable, as is shown by the numerous publications and works carried out in this discipline.
The fundamental problem of the philosophy of biology, which is life, will not be dealt with here. Only some of the philosophical questions raised by the theory of evolution will be briefly addressed. Some of them appear implicitly, or sometimes explicitly, underpinning the debates referred to above.
It is important to distinguish between evolutionary theory, which we have presented here as a strictly scientific theory, and evolutionism.
All science is associated with a method that can be more or less explicit or defined. The method does not simply consist of a set of operational rules but includes elements of very different subject and reaches a great complexity in real science. In all cases, the use of a method always entails a reduction in the scope of the studied reality. This reduction is especially necessary if we want to achieve one of the objectives pursued by empirical science, which is to control, in some way, reality: empirical science is "that human activity in which we seek a knowledge of nature that allows us to obtain a controlled control of it"[Artigas 1999: 15].
The methodical reduction that determines the way in which we look at reality, what we observe and what we leave out of our consideration, is absolutely necessary to achieve the goals of scientific activity. Problems arise when one forgets that employing a method implies reduction, or simply asserts in a positive way that only that which is made present through a particular method, however complex it may be, is real. This assertion, in reality, gives a global character, which is proper to philosophy, to a particular science. The problem lies in the fact that this way of proceeding leaves out of reality, in an arbitrary way, aspects that are real but cannot be captured by such a method. As these omitted or denied aspects belong to reality, they will sooner or later claim their presence in our knowledge and, then, inadequate explanations will be offered for them because they do not fit the method by which they are explained. It will also create a situation conducive to ideological responses to the problems that arise as a consequence of the aforementioned mismatch. The discipline that tries to embrace the totality from its particular method slides down the slope of reductionism, and so the suffix "ism" can be aptly added to the name of this discipline .
Evolutionism would mean, in this context, a worldview in which the natural world is viewed and explained in its entirety through the method developed by the theory of evolution. This claim, which can be seen in some current authors, is not at all legitimate[Artigas-Giberson 2007]. The situation is parallel, albeit with its own characteristics, to that which resulted from the birth of mechanics. The physics of the 17th century constituted a true novelty in the way of understanding natural reality and brought with it a multitude of benefits for mankind. But along with the scientific discipline, a globalising and therefore philosophical way of thinking also developed, which was called mechanicism or mechanical philosophy. The birth of a new science, in which satisfactory results and answers to previously unsolved problems are offered, and in which prospects for important new knowledge are opened up, is always an opportunity to engage in reductionism. The more powerful the method and the more spectacular the results achieved by the new science, the more tempting the opportunity.
Mechanism exerted a great influence on thought for three long centuries. It went into crisis as a consequence of the progress of physical science itself. Evolutionism, as reductionism, is also very influential in many fields today and is present in the writings of some science popularisers who have gained a wide audience today.
It would be evolutionary reductionism, therefore, to attempt to explain all of reality from the methodical elements employed by the theory of evolution. To try to explain with the theory of evolution all the phenomena of our experience, including such human realities as love, for example, the reality of God, morality, etc., would be to turn this theory into a kind of philosophy in which elements alien to it would necessarily have to be introduced. The experience of mechanics is very illustrative of what is involved in the pretension of covering the whole of reality with a scientific method. In the case of mechanics, not only did it prove to be insufficient to assume a role that is proper to philosophy, but it did not even serve to explain the whole reality of its own topic: that of physical motion.
The confusion of the theory of evolution with evolutionism is frequent and has given rise to controversies such as the one that has pitted Darwinism against creationism or, more recently, against "design Intelligent". The struggles of this subject never reach any conclusion because, ordinarily, the discussion is centred on philosophical aspects. This is precisely the area that the disputants cannot reach if they want to stay within science. The resource to ideologies, at least implicitly, makes the agreement impossible.
The above distinction is related to the accusation levelled by some against evolutionary theory that it is not properly science but philosophy. This accusation is not equivalent to what authors like Artigas point out when they say that all science has a series of philosophical presuppositions. What they are really saying is that the assertions that fall within the topic of such a science are philosophical in scope and are not supported by a properly scientific method. At the basis of this accusation is the failure to take sufficiently into account the distinction we are discussing and to understand evolutionary theory as one of the forms of evolutionism.
The problems that philosophy had to face, especially during the first half of the 20th century, in relation to the so-called "problem of demarcation", i.e. the problem of determining whether something is science or not, have led to the adoption of rather broad and nuanced criteria in the delimitation of what constitutes a discipline as science. If it were required, for example, that a theory, in order to be scientific, had to possess predictive capacity, as is the case with physics, then the theory of evolution would indeed have to be bracketed or denied as scientific. Dobzhansky himself states: "Those who claim that predictability is essential to a scientific theory may deride the theory of evolution as unscientific"[Dobzhansky 1983: 405-406]. Today, the emphasis is rather on systematicity as a peculiarity of science[Hoyningen-Huene 2008], and there is no attempt to establish such a precise demarcation of its limits that disciplines that are scientific, even if their method does not respond to a paradigm as clear and well-established as that of mathematics or physics, for example, are denied scientific consideration.
The discussion on naturalism also arises in this context. Naturalism, in its most common and strongest sense, holds that all reality is resolved and explained by natural laws: ontological naturalism. Some critics of evolutionary theory have accused it of being naturalistic. Again, it seems fairer to accuse evolutionism of naturalism, in this strong sense. On the other hand, it seems justified to argue that one can only have recourse to natural laws when one wants to explain phenomena that do not fall outside the realm of material nature. To defend the latter would be to defend what could be called methodological naturalism. Science is legitimately naturalistic in the latter sense, i.e. when it does not set itself up as a global knowledge , which is specific to philosophy.
The theory of evolution has been a powerful incentive for philosophical reflection since its first formulations. Today, numerous studies are published under the name label that are philosophical and that focus on the field of biology. Many of them, directly or indirectly, deal with issues related to evolution. On the one hand, there are the epistemological problems related to the theory, which have already been mentioned in the previous section and which have to do with the consideration of its scientific status. Also in the epistemological sphere is the problem of the reducibility of the theory of evolution, and of biology in general, to other disciplines such as physics. This topic also has ontological implications. Adaptation, the role of natural selection and its legitimacy as a non-tautological notion, chance, the notion of function, what are the units of selection, the emergence of properties, the concept of progress in biology and the evolutionary continuity of man with respect to the rest of the animals, are some of the many other questions related to evolution that are currently the subject of philosophical reflection. It is not possible for us to deal with them in this encyclopaedic paper. It is worth considering here, albeit briefly, the fact that the background to most of the questions raised revolves around reflection on the causes of evolution.
Reflection on causes, especially when it focuses on the most radical or first causes of any reality, is genuinely philosophical and obliges us to adopt a approach that is global, that is to say, philosophical. A danger that, directly or indirectly, is present in the consideration of the causes of evolution is to try to offer solutions that must be given from a global perspective, that is, philosophical, with elements proper to the scientific method and that, therefore, do not have that scope. For example, the affirmation of chance as the driving principle of evolution, as proposed by Monod[Monod 1987], for example, incurs in a reduction of this subject. In reality, chance is part of a mechanism that, on its own, is not capable of explaining evolution from a global approach .
This subject of reductionism is often contested by science itself. Dobzhansky himself states: "I do not think that the modern biological theory of evolution is based on "chance" to the extent Degree feared by Auden or claimed by Monod. The known and the unknown of this question deserve detailed consideration"[Dobzhansky 1983: 394-395]. In the same paper he states the following: "Adaptability through culture and extragenically transmitted symbolic language has developed in a single species - man. To call this 'chance' is a meaningless solution. To attribute it to predestination is incompatible with all that is known about the causes that produce evolution. The analogy with artistic creativity is, at least descriptively, more adequate, since no obvious differences are opposed to the opposite"[Dobzhansky 1983: 422-423]. The search for deeper causes drives the reflection that leads Dobzhansky to see the analogy with artistic creativity as the best way to express his ideas about the causes of evolution. But the very notion of creativity he employs has great limitations and presents problems when attributed to natural selection. The sincerity that drives his reflection is evident in the following words: "We did not appear by chance, nor were we predestined to appear. In evolution, chance and fate are not alternatives. We have here an occasion in scientific theory, in which we must invoke some subject of Hegelian or Marxist dialectics. We need a synthesis of the "thesis" of chance and the "antithesis" of predestination. My philosophical skill is insufficient for this task. I implore the financial aid of fellow philosophers"[Dobzhansky 1983: 419].
The discussion about causes in nature is as old as philosophy. Reflections on motion are at the centre of the reflections of the early Greek philosophers. The most mature fruits of this reflection are to be found in the Aristotelian doctrine of the four causes: material, formal, efficient and final. The evolution of thought after Aristotle affects in one way or another the way in which these causes are understood. Experimental science, since its inception, has had an important impact on this understanding. Of particular importance in the field of biology is the way in which the final cause has been understood. Finality, its peculiar way of causing or its non-existence as a cause, is a constant in philosophical reflection. The birth of mechanics, for example, substantially modified the way of understanding the four causes and, in a particular way, the final cause. The effect of this modification is important to take into account in order to understand the direction taken by many of the philosophical debates surrounding the theory of evolution.
The most important change introduced by mechanics with respect to the final cause is that it began to be regarded as a cause external to nature. For Aristotle, finality is in the nature of things, which for him was especially evident in living beings. This perspective is maintained in the great medieval masters who see in finality a way to access the existence of God: the argument from finality. The change of perspective introduced by mechanics also led to a barely perceptible reformulation of the argument from finality. St. Thomas' fifth way argument, the argument from finality, is no longer the teleological argument employee by Paley (1743-1805) to prove the existence of God. Both understand nature and its causes differently. Paley's argument leads to the affirmation of the existence of a God who is the explanation of the complexity of living beings, but who causes from outside. The example he uses of complexity is that of a watch: the order of its parts cannot be explained by "natural causes". The causes of Paley's argument are no longer Aristotelian causes. In particular, the final cause is different. The finality of the clock is external or extrinsic to the clock itself: a different conception from that understood by Aristotle and the Thomistic tradition for the complexity of any living thing. In it, the notion of nature is very important, in which there is a unity, which we could call intrinsic, between the formal cause and the final cause.
Before Darwin, Paley's argument seemed to be convincing as an argument for access to God. Mechanical philosophy thus fulfilled an apologetic role. Problems arise with Darwin because his proposal seems to render the argument from finality unfounded. What should be noted is that the argument that is directly affected by Darwin's proposal is the one made from the mechanical philosophy. The theory of evolution seems to offer a way of explaining complexity without the need for external agents to design or order the various organisms. This is immediately interpreted by many as an elimination of purpose as a cause of nature. Mechanics seemed to erase finality from the inanimate world and Darwin, for many, succeeded in doing the same in the living world. But to eliminate finality is to leave one of the most important arguments for access to God without a basis. Science, it is claimed from these positions, has been taking away the causal role of supernatural agents in favour of science. Ayala, for example, states: "The scientific advances of the 16th and 17th centuries had brought the phenomena of the inanimate subject -the movements of the planets in the sky and of physical objects on Earth- into the realm of science: explanation by means of natural laws. In the same way, natural selection provided a scientific explanation of design and the diversity of organisms, something that had been omitted by the Copernican revolution. With Darwin, all natural phenomena, inanimate or living, became topic of scientific investigation"[Ayala 2007: 24-25].
Ayala's words do not explicitly presuppose the expulsion of God from rationality, but they may lead one to think that God is confined to the world of the subjective and that, therefore, in the best of cases, there is no incompatibility between God and science because they belong to fields that have no points in common: the thesis of the double magisterium defended by Ayala and also by Gould, for example.
These dangers derive from a finalist vision filtered through the filter of mechanicism. It is enough to read a text by Thomas Aquinas to see that in his proposal finality does not explain complexity in an external way but from nature itself and, therefore, through natural laws: "Nature is, precisely, the plan of a certain art (specifically, the divine art), impressed on things, by which things themselves move towards a determined end: as if the craftsman who makes a ship could grant the logs to move by themselves to form the structure of the ship" [S. Thomas Aquinas, Commentary on the Physics of Aristotle, book II, lectio 14, n.8]. Thomas Aquinas, Commentary on Aristotle's Physics, book II, lectio 14, n.8]. Thomas Aquinas is not opposed to a methodological naturalism.
The causal pluralism of the realist tradition is richer than the one derived from the mechanical philosophy and on which many of the debates concerning finality and causes in general are still based. Causal pluralism confronts the monisms of various kinds that have been proposed as causal explanations of evolution, the most important of which are subject materialist. The proposal of the realist tradition does not confront a methodological naturalism such as the one that is evident in Ayala's words quoted above. The philosophy of the realist tradition assumes all that science can say in its domain, but frames finality, as cause, in a wider context than corresponds to the scientific method. This implies that the topic of God does not cease to be a fully rational topic and that the scientific sphere necessarily contributes to philosophical reflection: science, to which the theory of evolution belongs, through philosophy, has to do with God.
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