Wednesday, January 14, 2009

Not everyone accepts the theory of evolution. This month, as part of our Darwin forum, Paul Craze invites you to discuss the question "just how good is the evidence for evolution?"

Scientific theories stand or fall on their ability to account for the available evidence and to accurately predict the outcome of experiments designed to test them. That is all. No matter who came up with the theory, no matter how clever they are or were, no matter how elegant their ideas, no matter how many programs have been made about them by the BBC or years dedicated to honour their birthdays; if there is enough good, reliable evidence that the theory is wrong then it is wrong.

And so the question “Just how good is the evidence for evolution?” is of vital importance. In fact, we should probably recognise two questions here; firstly, “Just how good is the evidence for natural selection?” and then “Just how good is the evidence for the larger scale processes of evolution?”.

There is an enormous body of reliable evidence that natural selection is a real biological process capable of altering the characteristics of populations over time. Such classic examples as industrial melanism in the peppered moth are the tip of a very large iceberg of studies that confirm the reality of natural selection.

As a rule, the larger scale processes of evolution (those that have caused the diversification of a single, ancient, ancestral lineage into the full array of organisms we see today) cannot be seen directly and so are more difficult to test. Except in a few unusual cases they take far too long for us to have any hope of seeing them actually taking place.

The fact that they have occurred and continue to occur must be inferred from indirect observations, in much the same way as the guilt or innocence of someone tried for a murder is inferred from the evidence by the judge and jury, none of whom actually witnessed the crime.

We have essentially two lines of evidence: the snapshots we have of the process of evolution occurring in the past (the fossil record) and the organisms we see alive today (which have evolved and continue to evolve).

Increasingly the evidence from current organisms includes their DNA, which is being shown to contain many indications of past evolution. The critical role of this evidence means it has to be considered objectively and honestly with any assumptions and uncertainties clearly stated. It also means multiple lines of evidence have to be considered together in order to come up with a conclusion based on “the balance of probabilities”.

Again, the analogy of a criminal trial is useful here. Convictions are usually made based on conclusions that are “beyond reasonable doubt” rather than definite. The judge and jury must weigh up multiple lines of evidence some of which may be uncertain or even appear contradictory.

It works most of the time and in those cases where mistakes are made it is always because new evidence comes to light. The similarities to the process of science are very clear. And so in this theme there is the opportunity to present and examine the available evidence openly and honestly and subject the theory of evolution to yet another re-trial. Let us see if it emerges “beyond reasonable doubt” as the most likely explanation.