what is referred to as the species problem?
It is difficult to define a species in a way that applies to all organisms. The debate about species delimitation is called the species problem.
Species
It is difficult to define a species in a way that applies to all organisms. The debate about species delimitation is called the species problem. 
when are organisms considered to belong to the same species When any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. Some other ways of defining them include their karyotype, DNA sequence, morphology etc. how are they defined as? A species is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. was there any scientists who contributed to this classification? Some of the early ones include Linnaeus, Ernst Mayr, R. L. Mayden and John Wilkins. who is the second mentioned person? He was an evolutionary biologist,  taxonomist, tropical explorer, ornithologist, philosopher of biology, and historian of science. how did he summarized the darwin's theory? UNANSWERABLE what are morphological species? A group of organisms in which individuals conform to certain fixed properties (a type). The clusters of variations or phenotypes within specimens (such as longer or shorter tails) would differentiate the species. who proposed this aforementioned concepts? It was used by Linnaeus in early evolutionary theory. how are the organisms named in common and scientific names? The commonly used names for kinds of organisms are often ambiguous and can vary from place to place. In contrast, the scientific names of species are chosen to be unique and universal; they are in two parts used together. were any identification codes devised? With the rise of online databases, codes have been devised to provide identifiers for species that are already defined. name few places were these online databases are used? UNANSWERABLE
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Species [SEP] The species problem
It is difficult to define a species in a way that applies to all organisms. The debate about species delimitation is called the species problem. The problem was recognized even in 1859, when Darwin wrote in "On the Origin of Species":