ESTIMATION OF CONSERVATISM OF CHARACTERS BY CONSTANCY WITHIN BIOLOGICAL POPULATIONS

The concept of conservatism is basic to much of taxonomy. A conservative character is usually defined as one that has changed more slowly than a non-conservative one during phylogeny. The reliability of a character for placing organisms properly in evolutionary schemes is also included in conservatism. It is argued quite validly that a character which changes slowly is the best kind of character for discerning the evolutionary relationships of organisms. This is true in practice because of the very incompleteness of the historical record of phylogeny. When not all the forms of a line are known, there will be gaps in the record of rapidly-changing characters. But slowly-changing ones may still have a fairly complete series represented, even if only a few forms are known. The patterns of evolutionary relationship are easier to trace in characters that evolve slowly. It is usually assumed that not all characters are equally reliable, and many taxonomic problems can be reduced to the process of recognizing the most reliable ones. It has long been the practice to select conservative characters by the criteria of constancy within known groups and estimated biological importance. This approach has been criticized by numerical taxonomists (Sokal and Sneath, 1963), who point out that biological importance is seldom known, and that calling a character conservative if it is constant within a known group constitutes circular reasoning, if the group was constructed on the assumption that the character was conservative. It is not the concept of conservatism itself that is invalid, but only the means that have been suggested for finding conservative characters. There is little doubt that if a character is constant throughout a group that is natural in an evolutionary sense, then that character is a more reliable indicator of evolutionary relationship than is a character that varies widely in the same group. I shall present an argument to show how it may be inferred which characters are most likely to be conservative. Suppose that a given character is conservative in that it varies little in a given natural group. The variability of that character between the populations of that hypothetical group can only be small if the variability within single biological populations is also small. Otherwise random variation alone would be expected to produce wider variation within the group. Now it has been argued that the fitness of a population is maximized when the population is carrying as much heterogeneity as it can (Mayr, 1963). But then we expect a character to vary little within populations only if the centripetal selective forces acting on it are relatively intense. Thus we would expect that a conservative character would be characterized by a low variability within populations, in turn due to heavy selection on that character. On the other hand, it seems fair to say that a character with high centripetal selection acting on it is of great adaptive importance to the organism, and conversely. Then we expect a character with low variability within populations to be conservative, at least in the sense that it is of great biological importance: it is important to the survival of the organism. It is likely that it would be relatively difficult for such a character to change during the course of evolution. Because of the intense centripetal selection acting on it, a population un-