Anole Annals

As impressive as their current diversity may be, anoles have a rather pathetic fossil record.  Aside from a smattering of subfossils that are less than a million years old, all we have to work from are a few amber specimens from the Dominican Republic preserved over a narrow temporal window (15-20 mybp).  This lack of fossils is problematic because molecular genetic data suggest that that Anolis may be more than 50 million years old.  The fossil record for anole relatives isn’t much better, with only a handful of well-characterized fossils that are greater than 10 or 20 million years old existing across Iguania.  Although they are relatively uncommon, previous reports of iguanian fossils have occasionally been sloppy about assignment to extant clades.  Daza et al. (2012) have done a remarkable job of clarifying our understanding of fossil iguanians by providing not only a remarkably detailed and nuanced description of a new fossil iguanian from the Late Pliocene, but also by conducting quantitative phylogenetic analyses that place this fossil in a large tree that includes a diverse range of other fossil and extant iguanians.

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Anolis carolinensis is increasingly used as a genetic model organism, but we know suprisingly little about the distribution of geographic genetic variation in this species across its native range.  At this year’s evolution meetings, Marc Tollis presented his recently published work on phylogeography of Anolis carolinensis.  His work provides basic information on geographic genetic diversity within A. carolinensis, and permits tests of hypotheses about the contribution of riverine barriers, sea-level changes, and southern refugia to this diversity.  Tollis sampled 190 anoles from 9 states and obtained sequence data from mtDNA and 10 novel nuclear loci (4 introns and 6 anonymous loci).  Using phylogenetic analyses and the Bayesian clustering algorithms in Structurama, Tollis identifies four major clades that appear to have diverged from one another around 2 million years ago: North Carolina, Gulf-Atlantic, Suwannee, and Everglades.  Although these populations appear to have experienced range expansions, Tollis rejects the southern refugium hypothesis because expansion events predate the inter-glacial, genetic diversity is no greater in the south, and there is no consistent pattern of northern genotypes nested within southern genotypes.  Instead, Tollis’s data points to a rapid and recent westward expansion.    Tollis’s work also rejects the hypothesis that rivers are important barriers to Anolis carolinensis dispersal, a result that he suggests is not surprising given the group’s well-established overwater dispersal capabilities.  Because this phylogeographic work on Anolis carolinensis rejects both the riverine barrier and refugium hypotheses, it appears that the distribution of genetic diversity is somewhat unique and not widely shared with other taxa distributed across the same region.  Phylogeographic analyses of A. carolinensis are long overdue and Tollis’s presentation and associated publication are a most welcome contribution to the field.

Tollis M, Ausubel G, Ghimire D, & Boissinot S (2012). Multi-Locus Phylogeographic and Population Genetic Analysis of Anolis carolinensis: Historical Demography of a Genomic Model Species. PloS one, 7 (6) PMID: 22685573