TESTING EVOLUTIONARY CONSTRAINT HYPOTHESES WITH EARLY PALEOZOIC GASTROPODS

The evolution of higher taxa among early Paleozoic gastropods is simil ar to that among early metazoans as a whole, as higher taxa diversifie d rapidly and early. There are two issues pertinent to this pattern. F irst, were greater morphologic changes concentrated in the early phase s of evolution? Second, does the pattern better Bt models of increasin g phylogenetic constraints or increasing ecologic restrictions? This p aper presents a phylogeny-based method designed to test whether amount s of morphologic evolution decreased over time. It also explores wheth er the data better fits models of increasing phylogenetic (i.e., devel opmental or genetic) constraint or increasing ecologic restriction. Tw o metrics of morphologic separation (i.e., the morphologic difference between sister-species) are used: (1) Euclidean distance in morphospac e and (2) transition magnitude. The latter metric is calculated by a m ultivariate analysis of sister-species contrasts, which determines bot h types and magnitudes of morphologic transitions. The advantage of us ing transition magnitudes is that it balances the effects of transitio ns that either affect more morphometric characters or occur more frequ ently. Both metrics indicate that larger morphologic separations betwe en sister-species were concentrated early in gastropod evolution. Amon g gastropods, gross shell morphology of ten reflects basic trophic str ategy and function whereas basic internal anatomy does not. Transition magnitudes can be broken down into transitions associated with differ ences in basic trophic strategies and shell functional biology (''exte rnal''), and those associated with differences in basic internal anato my (''internal''). internal transition magnitudes show a highly signif icant decrease over time (p < 10(-04)) whereas external transition mag nitudes show a much less significant decrease over time (p < 0.10) and no significant decrease after the earliest Ordovician (p congruent to 0.50). The results therefore suggest that increasing phylogenetic con straints played a greater role in the early evolution of gastropods th an did increasing ecologic ones.