Papers in the Biological Sciences
Date of this Version
3-1959
Abstract
A "conservative" taxonomic character is one which preserves evidence of evolutionary relationships. As Mayr (1942: 296) points out such characters are rare because structures are usually lost quickly when no longer needed and selection seldom neglects a functional structure for very long but continually modifies it as adaptive fitness is increased. However, because different characters evolve at different rates it is possible to find some characters which have changed more slowly than others and to use these as phylogenetic indicators. In birds, for example, plumage color and structure apparently change more rapidly than the skeleton or the muscular system. This is demonstrated by studies such as that of Hudson and Lanzillotti (1955) on the wing muscles of the Corvidae. In 19 species representing 14 genera of crows, nutcrackers, magpies and jays the wing musculature showed "remarkable uniformity" and the small differences noted did "not suggest any taxonomic groupings." The wing musculature in this instance was a conservative family level character which had not been basically modified during the evolutionary changes involved in the proliferation of the species which were studied.
Under domestication it is possible to accelerate hereditary changes by artificial selection (Lerner, 1958). In the Domestic Fowl (Gallus gallus) a remarkable array of characters has been established as the traits of different "breeds," many of them within the past century or two. The various breeds range in size from tiny bantams weighing less than two pounds to large exhibition varieties weighing ten or 12 pounds. Body shapes range from the slim "game" breeds to the heavy-bodied, meat-producing types. Plumage variations in color and form are remarkable and include tbe continually growing central rectrices of the Phoenix Fowl. Other breeds are characterized by crests, feathered feet, "silky" plumage and other variations. The Houdan has a double hind toe and a "split" 'comb. The wild ancestral Jungle Fowl has a well-defined breeding period and clutch size, but some domesticated breeds lay almost continuously and have nearly lost their brooding tendencies. Breeds vary in their resistance to disease, parasites and vitamin deficiencies; in egg size, egg shape, egg color, hatchability and freedom from blood spots in the eggs (Lerner, 1958). Variations are also found in dominance tendencies and in the frequency with which the various male displays are given (Williams and McGibbon, 1955).
It seems likely that no important feature of the Domestic Fowl has remained entirely exempt from modification. If the various breeds were judged taxonomically on a purely morphological basis they would certainly constitute many species and several genera. In this situation the search for "conservative" characters is especially difficult and only those features important for survival or which have not been singled out by artificial selection are likely to show stability.
It might be expected that the composition of such substances as the egg white would not be affected by selection for size, color, etc., but the possibility of pleiotropic effects cannot be disregarded. In an electrophoretic study of egg-white proteins of 37 species of birds McCabe and Deutsch (1952) found no evidence of differences among several breeds of chickens, but no systematic study had been made. Therefore, an investigation of protein composition, as determined by paper electrophoresis, was initiated.
Comments
Published in The American Naturalist 93:869 (March-April 1959), pp. 107-115. Copyright 1959 University of Chicago Press. Used by permission.