Do Life History Tactics Exist at the Intraspecific Level? Data from Freshwater Snails

Abstract

To determine if life history tactics were discernible at the intraspecific level, I used discriminant analysis to study tactics at three taxonomic levels in freshwater snails. Data on seven life history traits were compiled from field rearing experiments and studies in the literature. Clustering of tactics in discriminant space was more obvious at the family than at lower taxonomic levels or across habitat types. Phenotypic correlations among traits were also somewhat greater at the family than at the population level. Prosobranchs have longer reproductive intervals and lower reproductive rates than pulmonates. Since pulmonates are more common in temporary ponds, one explanation for the divergence among families may be that pulmonates as a group are better adapted to uncertain habitats. Other explanations, however, may be that variation at higher taxonomic levels is unrelated to selection regimes, because of either phylogenetic constraints or other confounding factors. I suggest therefore that the adaptive basis of life history variation is still best studied at the intraspecific level.