North American Enallagma Phylogeny

Concensus phylogeny based on 815 bp of COI and COII and 51 morphological characters for 34 of the 38 Nearctic Enallagma (McPeek and Brown 2000, Brown et al. 2000), and the 4 Palearctic Enallagma.  The tree was derived via maximum parsimony analysis.  Ischnura ramburii, I. posita, and representatives of three other genera were used as outgroups.  The four species found as larvae in Dragonfly Lakes are identified in blue. All other Enallagma are found in Fish Lakes.  The branch lengths are calculated assuming a molecular clock and the GTR+G+I model of nucleotide substitutions. Branch lengths are based only on the DNA sequence data.

The basal split in the genus dates to ca. 10-15 million years ago, using the standard molecular clock estimates for insect mitochondrial DNA and depending on the nucleotide substitution model that is assumed. This basal split also divides the genus into a "Northern" and a "Southern" Clade based on the biogeography of the extant members. The center of diversity for the Southern Clade is in the southeastern United States, and the center of diversity for the Northern Clade is in New England. Both clades show a rapid increase speciation rates within the last 250,000 years, with a much more dramatic increase in the Northern Clade. In fact, maximum likelihood estimates derived from this phylogeny suggest that speciation rates increased by 24X in the Northern Clade. The lineage accumulation curves for each clade quantitatively illustrate this increase in speciaton rates.

Given their biogeography, we believe that this increase in speciation was triggered by climate change during the Pleistocene. Within the Northern Clade, two Nearctic lineages radiated to give 17 extant species. Another lineage invaded the Palearctic ca. 1.0-1.3 million years ago and radiated at the same time as the Nearctic clades to produce the four extant Palearctic species. Most of the species derived from these radiations have distributions that would have been largely covered with ice 15,000 years ago.