19. evolutionary convergence

“Evolutionary Convergence“

Tracing genealogies fascinates many people, and reconstructing the genealogies (which they call “phylogenies”) of groups of organisms is a favorite sport of biologists. A persisting mystery has been the evolutionary relationships of various groups of birds. Which birds are similar because they are descended from relatively recent common ancestors (true evolutionary relationship), and which are similar because, although coming from different recent ancestors, they have evolved similar structures in response to similar ways of life (evolutionary convergence). This mystery is exemplified by a long debate over who are the relatives of the Wrentit. Confusion is indicated by its name: does it share recent common ancestors with wrens, or titmice, or members of some entirely different group? At one time or another, the Wrentit has been declared a near relative of wrens, bushtits, titmice, mockingbirds, Old World warblers (which include Dusky and Arctic Warblers which stray into North America), and babblers (Eastern Hemisphere insect eaters).

Normally evolutionary family trees are constructed by carefully comparing details of structural features, because taxonomists known that overall similarity in form can be misleading. In spite of their fishlike shapes, whales have long been recognized as phylogenetically much more closely related to people than to fishes, because the presence of mammary glands and hair (scanty as it is) and the structure of their brains, hearts, and many other features show them to be mammals. The superficial similarity of fishes and whales is an example of convergence. The whale-fish convergence indicates that streamlining is the evolutionary solution to minimizing drag on large creatures moving rapidly through water.

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Published Stanford Essay found at
http://www.stanford.edu/group/stanfordbirds/text/essays/Birds,_DNA.html

Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye