Document: THO-3-46-8

Evolutionary consequences of skewed distributions of body size.

KIRCHNER, T.B.*

New Mexico State University, Carlsbad, NM 88220 ¹

Abstract:
The mechanism giving rise to Cope's Rule, the tendency for body size to increase within lineages across evolutionary time, has remained a puzzle since Cope first published his observations over a century ago. An individual-based model of intraspecific competition is used to show that the positive skewness in distributions of body sizes observed within many species can bias selection toward larger body sizes while maintaining variability in body size. The model also demonstrates that size-based competition can easily explain the relatively rapid changes in body size observed in insular faunas and, coupled with bottlenecking events, may also help explain Bergmann's rule. The key feature of this model is that there is no a priori assumption of an optimum size. The model is implemented as a stochastic, individual-based population model of a sexual, diploid species that is constrained by density-dependent mortality. Body size is determined by a function of the values of alleles at 10 or fewer independent locii. Natural selection is implemented as small increment to random mortality with the size of the increment determined by phenotypic (body size) differences. Competitive disadvantage (= higher mortality) is proportional to the similarity in size of an individual compared to the other individuals in a population. Thus individuals near the mode of the distribution will generally be at a greater competitive disadvantage than individuals within the tails. This assumption is at odds with the adaptive landscape metaphor, in which it is typically assumed that the mode of a distribution of an attribute will be associated with the adaptive optimum. The determination of fitness of an individual due to differences among phenotypes as compared to the fitness a genotype in relation to a hypothetical optimum enables the model population to undergo changes in mean size without the need to postulate a general adaptive advantage for large size.

Keywords: Cope's rule, body size, evolution

This abstract is being presented at: 10:30 AM in session:
Poster Session #9: Fish, Lakes, Streams and Wetlands.