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

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 significantly different from that of all other species of the same genus, whether allopatric or sympatric. In 6 of 7 pairs, the Pacific member had eggs that were smaller than those of the Atlantic one, a pattern attributed to adaptation to the higher levels of primary productivity in the eastern Pacific. Therefore, shifts in mean egg size are possible in three million years of independent evolution. However, there was no egg size characteristic for each ocean, and (except for the sand dollar Clypeaster, in which one species has evolved a facultatively planktotrophic larva) the differences between congeneric species were small compared with the differences between members of different genera. Thus, though adaptation to different environments can push egg size in a particular direction, differences in the basic biology of genera, accumulated over the course of their phylogenetic history, either constrain the possible sizes to be attained by each genus or cause selective regimes within the same environment to be dissimilar.