• Harry Nelson and Robert Jurmain : “Evolution of similar adaptive traits in unrelated forms” is called converge evolution
• Stein and Rowe (1990): “The development of similar forms in different evolutionary lines ’ is known as convergent evolution
• John Buettner-Janusch : A process that refers to the evolutionary development of “similar traits and adaptations in two groups of organisms that are not closely related phylogenetically is called convergent evolution
Convergence is defined as development of similarities in adaptive relationships or structures in two animal species or major groups that are not closely related. A widespread example of convergent evolution is the evolution of wings and powered flight in birds, bats and (now extinct) pterosaurs, each of which belong to a different class of organism and therefore have very distant common ancestors.
Fossil evidence has determined that flight evolved in pterosaurs (flying reptiles of the late Triassic period) around 225mya and in birds around 150mya, while mammalian bats evolved wings around 50-60mya. The evolution of powered flight has only happened once in each of these lineages, although there are certain organisms, for example, ostrich birds, which have subsequently reverted back to being flightless while retaining their wing structures.
The different wing structures of birds, bats and pterosaurs are each supported by a modified five-fingered limb. Each limb consists of a humerus a radius and ulna, a thumb and finger bones, and is a homologous structure, containing the same bones that make up the limbs of many animals including humans, whales and crocodiles; however, the shape of each bone differs greatly between each form.
An elongated fourth finger shapes the pterosaur wing, with the other digits used as claws. In birds, an elongated radius and ulna, as well as finger bones fused together for strength, support the wing. Finally, the wings of a bat differ in that they are formed of membrane that is stretched over four elongated fingers. The reason each of these different bone formations results in the same eventual wing shape is due to the basic physics of flight: wings that were shaped much differently would not allow an animal to fly.
Although birds and pterosaurs share a very distant common ancestor, and birds also share a common ancestor with bats, none of these ancestors had wings or were able to fly. In each of these lineages, the wing is therefore an analogous structure because the bones have been arranged differently in order to independently achieve a functionally similar structure.
Whales , fish looks similar because of same aquatic environment but different species.
Differences Between Convergent and Divergent Evolution
Three main differences between convergent and divergent evolution are:
•1. Convergent evolution shows how species have evolved separately but have similar (analogous) structures. Divergent evolution demonstrates how species can have common (homologous) anatomical structures which have evolved for different purposes.
•2. Convergent evolution happens in organisms that are not closely related while divergent evolution is observed in organisms that are closely related.
•3. The relationship between the analogous structures in different species that evolved through convergent evolution can be less distinct compared to the homologous structures seen in divergent evolution which have the same basic structural plan.