Final answer:
The classical criticism of the theory of evolution by natural selection regarding transitional species is the paucity of transitional species. According to the classical theory, if evolution occurs gradually and species gradually transition into new and distinct species, there should be numerous fossils showing evidence of these transitional forms. However, while fossils are rare, fossils showing evidence of transitional forms are even rarer.
Step-by-step explanation:
The classical criticism of the theory of evolution by natural selection regarding transitional species is the paucity of transitional species. According to the classical theory, if evolution occurs gradually and species gradually transition into new and distinct species, there should be numerous fossils showing evidence of these transitional forms. However, while fossils are rare, fossils showing evidence of transitional forms are even rarer. This has led some biological anthropologists to question if evolution is truly gradual.