Signals and Speciation: Anolis Dewlap Color as a Reproductive Barrier
The divergence of signals used in sexual selection and species recognition is thought to play an important role in speciation. As such, the striking diversity of dewlap color in Anolis lizards may have contributed to the diversification of this species-rich group. Whether the dewlap acts as a reproductive isolating barrier however remains unclear. We tested the prediction that the degree of dewlap divergence between the two closely related and highly polymorphic Hispaniolan trunk anoles Anolis distichus and Anolis brevirostris is correlated with genetic divergence and the frequency of hybridization in nature. We use integrative analyses of dewlap color variation and molecular genetics to investigate two pairs of localities where the species co-occur, including one pair of localities where the two species exhibit dissimilar dewlaps and a second pair of localities where they have similar dewlaps. At one site where species share similar dewlap coloration, we found evidence of hybridization and lower levels of genetic differentiation. At all other localities, however, including another site where species share a similar dewlap color, the species were genetically divergent with little evidence of mitochondrial and nuclear gene flow. Together, our results suggest that dewlap color is not consistently associated with reproductive isolation at the species level. Instead, site-specific factors may influence the dewlap's role in maintaining species boundaries.Abstract
Sampled localities on Hispaniola where Anolis distichus and A. brevirostris co-occur. Filled circles represent sampled sites as well as the dewlap colors present. The yellow-filled circle indicates that both species exhibit similarly pale yellow dewlaps. Circles that are half orange and half yellow represent sites in which the species exhibit different colored dewlaps. The circle with a quarter orange and three quarters yellow represents a site in which pale yellow-dewlapped A. brevirostris co-occurs with an A. distichus subspecies with a similarly colored dewlap and one with an orange-colored dewlap. Sites 402 and 643 overlap because they are only 0.86 km apart. Sample sizes and photographs of representative males of A. distichus (left) and A. brevirostris (right) from each site are shown. For site 643, we show photographs of only the orange-dewlapped A. distichus subspecies, whereas the yellow-dewlapped subspecies and A. brevirostris look similar to those shown for site 402. Accompanying data for each site are also shown, with each bar in the graph representing an individual and thick vertical lines separating data for the two species. (a) Dewlap color, whereby the proportion of yellow and orange in each bar represents the proportion of yellow and orange in the dewlap, respectively. Gray bars represent juveniles or females, which could not be scored for dewlap color. (b) Mitochondrial clade to which the individual was assigned in a phylogenetic analysis, with different colors representing a different clade. Microsatellite (c) and AFLP (d) data, with colors representing the genetic cluster to which the individual was assigned from STRUCTURE analyses. Gray bars indicate no data. The asterisk and double asterisk above the data for site 643, respectively, indicate data for a juvenile with evidence for mixed ancestry from microsatellite data but not with AFLP data, and an adult female that was likely to have been phenotypically misidentified as A. brevirostris.
Results of hierarchical STRUCTURE analysis of AFLP data.
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