Parallel evolution and convergent evolution are big themes within anole biology, so our lab was excited to discuss a new paper by Thorpe et al looking at these concepts in Lesser Antillean anoles. The paper focused on evidence for parallel evolution across seven small islands that contained both xeric and montane habitats with at least one species of anole split between the two habitats. Xeric habitats tend to occur along island coasts and are hotter, drier, and have less canopy cover, while montane habitats occur in the interior of islands and are cooler and wetter. There are many physical differences consistently found between the anoles associated with each type of habitat, even within a species; perhaps the most obvious examples are the repeated differences in skin color and pattern between habitats, beautifully illustrated in the first figure of the paper.
Figure 1 from Thorpe et al, showing the repeated evolution of characteristic xeric and montane color patterns in the Lesser Antilles
Thorpe et al. set out to conduct tests of parallel evolution among seven anole species using 18 phenotypic traits that vary between habitats, including both morphological and pattern measurements. In addition, they used mitochondrial DNA sequencing to produce a new phylogeny of these species and control for phylogenetic interference in their comparisons. The authors first used a principal components analysis to confirm that the major source of climatic variation is found within each island and between different habitats, rather than across different islands. The authors found convincing evidence for parallel morphological evolution in multiple phenotypic traits, especially those associated with skin pattern and hue: anole populations in xeric habitats consistently converge on a grey skin color and those in montane habitats converge on green. Thorpe et al. also go on to suggest that divergence in coloration may be the result of signal optimization in environments with different chromatic backgrounds (characterized by variance in background vegetation or sun exposure). The authors describe a possible evolutionary scenario in which an anole population first colonizes the coastal areas of each island after a dispersal event, and then rapidly expands into the interior montane areas of the island and adapts to new conditions there. Given the constant concern of climate change, repeated evolution in response to different climatic conditions may offer hope that anole populations can respond to rapid environmental change.
The most famous story of parallel evolution in anoles is the convergent evolution of ecomorphs across the islands of the Greater Antilles. This paper offers the tantalizing possibility of another type of convergent evolutionary pattern, this time within species but across habitat types. The smaller islands of the Lesser Antilles may be too constrained to allow for speciation driven by ecomorph specialization, but could still promote significant population divergence across habitats. More information on the adaptive differences between these xeric and montane populations, along with characterization of their genetic structure, could shed light on these possibilities. Based on these results in the Lesser Antillean populations, there is also the possibility that this type of xeric and montane divergence exists within species in the Greater Antilles, and fine-scale studies of population structure could reveal another level of convergent evolution in those species.
Thorpe, R. S., Barlow, A., Malhotra, A. and Surget-Groba, Y. (2015), Widespread parallel population adaptation to climate variation across a radiation: implications for adaptation to climate change. Molecular Ecology, 24: 1019–1030. doi: 10.1111/mec.13093