LEVELS OF SELECTION AND MACROEVOLUTIONARY PATTERNS IN THE TURRITELLIDGASTROPODS

This analysis examines the evolution of the greater diversity of speci es with non-planktonic larval types relative to species with planktoni c larval types in the turritellid gastropods. This sort of trend has b een documented in both the fossil and recent biota of several gastropo d families. Two mechanisms for generating diversity gradients in larva l types have been proposed in the literature. The first, species selec tion, focuses on the population biology of larval types. The second pr oposes that factors in development that are mediated by organismal ada ptation are responsible. Turritellids have been cited as a classic exa mple of species selection. In order to examine the relevance of these two proposed mechanisms, a phylogenetic analysis of the turritellids u sing molecular sequence data was performed to determine the evolution of larval types in this clade. The resultant phylogeny suggests that s pecies selection is not the only process driving the trend toward incr easing numbers of non-planktonic species through time. Developmental p rocesses, apart from those involving organismal adaptation (except in the trivial sense), are implicated as playing a role in this trend. In particular, these processes may involve changes in the timing of germ -line sequestration in organisms. Germ-line sequestration governs how accessible organisms are to heritable variation during ontogeny. Embry ological evidence from gastropods suggests that non-planktonic species have early germ-line sequestration relative to planktonic species, ma king them more resistant to developmental change. Thus, non-planktonic lineages will only rarely revert to a planktonic larval mode.