BUILDING A REGIONAL SPECIES POOL: DIVERSIFICATION OF THEENALLAGMADAMSELFLIES IN EASTERN NORTH AMERICA

Abstract

We use a phylogeny of the North American Enallagma damselflies, derived from molecular and morphological data, to examine how the patterns of local and regional assemblage structure developed in this taxon across North America. The two primary clades in the genus have nearly identical numbers of extant species, but the centers of diversity and the diversification rates for the two clades are quite different. One clade has its center of diversity in and radiated very recently from three species to give the current 18. Although most of this radiation involved the creation of new species in the ancestral fish-lake habitat, at least two independent lineages invaded and adapted to a new habitat: ponds and lakes lacking fish but supporting large numbers of large predatory dragonflies. The other clade, with greatest diversity in the southeastern United States, contains species that inhabit only water bodies that support fish populations. This south- eastern clade diversified at a much slower and more steady pace within the fish-lake habitat than the New England clade, but four speciation events in this clade appear to have occurred at the same time as the northern radiation. Combined with our current understanding of local community structure in fish and fishless lakes, these results indicate that most of the species in this regional assemblage were created by speciation mechanisms other than filling empty niches, which have resulted in many locally coexisting species that are very similar in their ecological characteristics. Damselflies in North American ponds and lakes appear to exemplify features of both a regulated component of the littoral food web (i.e., a functional group) and an assemblage whose local community composition is influenced by nonadaptive macroevolutionary processes that have operated on a much larger regional scale.