Adaptation and Phylogeny as Determinants of Egg Size in Echinoderms from the Two Sides of the Isthmus of Panama

Abstract

Theoretical models and empirical observations have attempted to determine selective pressures that influence the egg size of marine invertebrates. Both approaches generally assume that egg size is free to evolve toward an evolutionarily stable point without any constraints imposed by phylogeny. The size of eggs in 22 species of echinoids and 2 species of asteroids from the two sides of the Isthmus of Panama was determined in an effort to assess the rate at which differences between species can accumulate. Of these 24 species, 14 belong to pairs of geminate species, presumed to have been divided by the rise of the isthmus 3.1-3.5 million years ago. Comparisons between egg size in related species evolving in separate environments for the same period of time, and in related and unrelated species living in the same environment, were used to evaluate the relative importance of adaptation and phylogeny in determining the egg size of a species. In 12 of 13 comparisons, the mean egg size of each species was signi...