An Explanation for Cope's Rule

Whether body size will increase or decrease in an evolving population depends on whether mean body size is larger or smaller than the optimum for the population. Cope's Rule, the generalization that most animal groups have evolved toward larger body size, cannot be explained by intrinsic advantages of large size. Rather, it is the tendency of groups to arise at small body size relative to their optima that produces the widely observed pattern of net size increase. The specialized nature of large species of a given body plan, required by problems of similitude, renders these forms unlikely potential ancestors for major new descendent taxa. The adaptive discontinuity that must be crossed for invasion of a new adaptive zone at large body size exists because of the need for descendent taxa to be specialized along new lines. These factors tend to restrict large-scale adaptive breakthroughs to small body sizes. Size changes probably tend to occur sporadically, during speciation events. Size increase is not inherently favored in speciation, but prevails during diversification because origin of a higher taxon at small body size concentrates many early species in the small size range. Nearly all diverse animal orders and classes, and many families and super-families, are composed of species whose body sizes are distributed as positively skewed histograms. The typical pattern of size change during diversification of such a group can be determined from time-series plots for fossil species of diversifying higher taxa. A major taxon normally arises at small body size relative to its potential size range, and a slightly skewed histogram is rapidly formed. The histogram may expand or contract slightly in the small size range as diversification proceeds, but spreads continually farther in a positive direction, to develop a strongly attenuated tail in the large size range. Skewing occurs very rapidly because possible increments of size change with speciation are not constant throughout a taxon's size range, but are a direct function of body size, so that early spreading of the range proceeds more rapidly in a positive direction than in a negative direction. Nearly always an increase in mean size results. Just as taxonomic and morphologic diversification approach limits as a group's potential adaptive zone is filled, the size-frequency plot approaches a limiting distribution. The attenuated right flank of a high-diversity distribution reflects not only well known ecological factors, but also the fact that structural specialization at large relative body size for a given higher taxon gradually limits the range of potential morphologies (and hence diversity). The left flank is often steeper even than that of a Gaussian distribution partly because the onset of factors determining minimum size limits tends to be abrupt. In the Aves and Mammalia, for example, the curve for metabolic rate versus size turns sharply upward in the 4-5 gram range. In both groups many species are only slightly larger than this size, but hardly any are smaller. For most highly diversified poikilothermous metazoan groups, the minimum space required for fundamental organ systems is abruptly limiting. The probabilistic explanation offered for Cope's Rule implies that the rule is more fruitfully viewed as describing evolution from small size rather than toward large size. Although his interpretation was erroneous, Cope himself adopted this viewpoint. Strangely, it has been abandoned by most modern workers who have analyzed Cope's Rule.