Convergence

The term "convergence" is used to describe the presence of a similar feature in two or more taxa that are not closely related. Convergent features evolve independently often as a result of natural selection operating on unrelated taxa that occupy similar environments. The recognition of convergence requires an accurate phylogeny. With a phylogenetic hypothesis, homologous characters can be distinguished from analogous traits. Convergent structures are usually derived from different morphological features or by different developmental pathways (although this is not the case in one special class of convergence described below).

Convergence can be further broken down into specific phenomena. Analogy describes the convergent modifications of a nonhomologous trait. For example, analogous organs may share a common function but develop from different tissue types in unrelated organisms. The wings of insects and the wings of birds have the same functional role (flight) but they are derived from nonhomologous structures and are structurally very different. Therefore, they are considered analogous structures. The term "parallelism" refers to apomorphic, or derived, traits. Apomorphies are identical traits that are found in different taxa but that do not share a common evolutionary origin. Apomorphies may arise independently (even in closely related taxa) as consequences of similarities in development among species (often due to developmental constraints imposed by similarities in genes that regulate developmental processes). Parallelism differs from other types of convergent evolution in that parallel traits are the product of the same genes and developmental processes operating in different taxa. In this case, convergence is found not only in physical traits but also in the developmental processes that produce them.

A striking example of convergent evolution in animals is the evolution of flight in three different vertebrate taxa: pterosaurs (extinct flying reptiles), birds, and bats. Structural similarities in the wings of each of these groups are indications of common constraints imposed by phylogeny and biomechanics. All vertebrate forelimbs have similar developmental patterns, regardless of whether they will become limbs or wings in the adult. In order to achieve flight, the ratio of the surface area of the wings relative to body mass must be great enough to provide sufficient lift to overcome gravity. Differences among them indicate that in each lineage, unique solutions have evolved under particular historical and functional constraints, resulting in different structural patterns with similar functions. Pterosaurs, like birds, had hollow bones and keeled sterna (breastbones), a short and stout humerus (upper arm bone), and wing fibers that were analogous to bird feathers. The pterosaur wing was supported primarily by an elongated fourth digit. In birds, digits of the forelimb are reduced and the wing is supported primarily by the radius and ulna (bones of the lower arm) and bones of the wrist. Feathers provide rigidity and increased surface area to the wing. In pterosaurs and bats the digits are elongated and provide support for patagia (thin membranes of skin). In birds, feathers provide a unique structural solution to the challenge of flight while elongate digits and patagia have evolved convergently in other groups that lack feathers.

see also Biological Evolution.

Andrew G. Gluesenkamp

Bibliography

Cockburn, Andrew. An Introduction to Evolutionary Ecology. Oxford, U.K.: Blacwell Scientific Publications, 1991.

Futuyma, D. J. Evolutionary Biology. Sunderland, MA: Sinauer Associates, 1986.

McKinney, M. L. Evolution of Life. Englewood Cliffs, NJ: Prentice Hall, 1993.