Convergence and Parallelism

Convergence and parallelism are two very similar evolutionary phenomena. Convergence occurs when species from two unrelated lineages develop very similar adapta-tions—sometimes to the point at which their identities can be confused. Parallelism is the evolution of similar species from closely related lineages—with the sense that it is more likely that similar organisms will evolve in lineages that are quite closely related. In practice, however, it is often difficult to distinguish cases of convergence from cases of parallelism—and the term convergence will be used as a (near) synonym throughout the rest of this entry.

Convergence is one of the most persuasive lines of evidence for the importance of adaptation in the evolutionary process. Because there are often optimal solutions to particular mechanical or design problems faced by animals and plants in their daily lives, natural selection has fashioned similar solutions to such problems time after time in evolutionary history.

Consider the classic example of evolutionary convergence: the body shapes of sharks, dolphins (porpoises), and the extinct ichthyosaurs. The body is fusiform in many species within each of these groups—that is, streamlined for the rapid swimming needed for a predatory mode of existence. Although early observers were confused by the superficial similarity between sharks and dolphins, biological research soon showed that sharks are a type of primitive cartilaginous fish (see Chon-drichthyes), while dolphins, with their mammary glands, three middle ear bones, and pla-cental development of the young are just as obviously true mammals. When ichthyosaurs were discovered in the nineteenth century as fossils of the Mesozoic Era, it was soon realized from the details of the bones in their heads that, whatever their dolphinlike shape might suggest, they were actually a unique group of reptiles.

Similarly, the wings of birds, bats, and the extinct flying reptiles known as pterosaurs were also developed in separate evolutionary histories. That bats are mammals is obvious from the presence of hair plus the other mammalian features mentioned above for dolphins.

That birds are actually a kind of surviving feathered dinosaur—which were reptiles—is perhaps not so obvious, but it is nonetheless the conclusion of paleontologists and anatomists. Birds share many features with dinosaurs like Tyrannosaurus rex. On the other hand, the pterosaurs, though Mesozoic reptiles like dinosaurs, were nonetheless not as closely related to the dinosaurs as are modern birds. Moreover, the internal bony structure of the wings of birds, bats, and pterosaurs is different—each group having a different configuration of the webbing or feathers attachment to one or more of the fingers of the "hand" of the forelimb. This in itself is clear evidence that the similar-looking wing of each of these groups has been evolved independently from separate four-legged ancestors.

But convergence can take on a larger dimension—as when even larger scale groups appear very similar and often occupy the same general niche. As an example of what perhaps might be best considered as "parallelism," the Rugosa, one of the major groups of corals that lived almost the entirety of Paleozoic time, became extinct in the great mass extinction in the Late Permian Period (the so-called Permo-Triassic mass extinction of 245 million years ago, the greatest of all mass extinctions to have struck life so far). For several millions of years thereafter, there were no longer any corals left on earth. But then modern corals (Scleractinia) suddenly appeared. Anatomical and molecular biological (genetic) evidence indicates that the closest relatives, living or extinct, of modern corals are sea anemones, which live just like corals but lack the calcified external shell of true corals. Paleontologists now believe that modern corals evolved from the naked sea anemones after the Rugose corals of the Paleozoic had become extinct—an example of parallel evolution from another branch of close relatives.

—Niles Eldredge

See also: Adaptation; Chondrichthyes (Sharks, Rays, Chimaeras); Cnidarians (Sea Anemones, Corals, and Jellyfish); Evolution; Mammalia; Mass Extinction; Natural Selection

Bibliography

Eldredge, Niles. 1991. Fossils. New York: Harry N. Abrams; Eldredge, Niles. 1999. The Pattern of Evolution. New York: W. H. Freeman; Futuyma, Douglas J. 1997. Evolutionary Biology. Sunderland, MA: Sin-auer; Maynard Smith, John. 1993. The Theory of Evolution. Cambridge: Cambridge University Press.

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