skip to main content

Title: Dewlap colour variation in Anolis sagrei is maintained among habitats within islands of the West Indies

Animal signals evolve in an ecological context. Locally adapting animal sexual signals can be especially important for initiating or reinforcing reproductive isolation during the early stages of speciation. Previous studies have demonstrated that dewlap colour inAnolislizards can be highly variable between populations in relation to both biotic and abiotic adaptive drivers at relatively large geographical scales. Here, we investigated differentiation of dewlap colouration among habitat types at a small spatial scale, within multiple islands of the West Indies, to test the hypothesis that similar local adaptive processes occur over smaller spatial scales. We explored variation in dewlap colouration in the most widespread species of anole,Anolis sagrei, across three characteristic habitats spanning the Bahamas and the Cayman Islands, namely beach scrub, primary coppice forest and mangrove forest. Using reflectance spectrometry paired with supervised machine learning, we found significant differences in spectral properties of the dewlap between habitats within small islands, sometimes over very short distances. Passive divergence in dewlap phenotype associated with isolation‐by‐distance did not seem to explain our results. On the other hand, these habitat‐specific dewlap differences varied in magnitude and direction across islands, and thus, our primary test for adaptation—parallel responses across islands—was not supported. We suggest that neutral processes or selection could be involved in several ways, including sexual selection. Our results shed new light on the scale at which signal colour polymorphism can be maintained in the presence of gene flow, and the relative role of local adaptation and other processes in driving these patterns of dewlap colour variation across islands.

more » « less
Award ID(s):
Author(s) / Creator(s):
 ;  ;  ;  ;  
Publisher / Repository:
Oxford University Press
Date Published:
Journal Name:
Journal of Evolutionary Biology
Medium: X Size: p. 680-692
["p. 680-692"]
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Disentangling the effects of neutral and adaptive processes in maintaining phenotypic variation across environmental gradients is challenging in natural populations. Song sparrows (Melospiza melodia) on the California Channel Islands occupy a pronounced east‐west climate gradient within a small spatial scale, providing a unique opportunity to examine the interaction of genetic isolation (reduced gene flow) and the environment (selection) in driving variation. We used reduced representation genomic libraries to infer the role of neutral processes (drift and restricted gene flow) and divergent selection in driving variation in thermoregulatory traits with an emphasis on the mechanisms that maintain bill divergence among islands. Analyses of 22,029 neutral SNPs confirm distinct population structure by island with restricted gene flow and relatively large effective population sizes, suggesting bill differences are probably not a product of genetic drift. Instead, we found strong support for local adaptation using 3294 SNPs in differentiation‐based and environmental association analyses coupled with genome‐wide association tests. Specifically, we identified several putatively adaptive and candidate loci in or near genes involved in bill development pathways (e.g.,BMP,CaM,Wnt), confirming the highly complex and polygenic architecture underlying bill morphology. Furthermore, we found divergence in genes associated with other thermoregulatory traits (i.e., feather structure, plumage colour, and physiology). Collectively, these results suggest strong divergent selection across an island archipelago results in genomic changes in a suite of traits associated with climate adaptation over small spatial scales. Future research should move beyond studying univariate traits to better understand multidimensional responses to complex environmental conditions.

    more » « less
  2. Abstract

    Colours relay information to conspecifics and predators unique to an environment and are shaped by natural selection favouring colours that enable higher fitness. For decades, ecologists have grappled with various methods of quantifying colour. Spectrophotometers offer precise and accurate data, but their high price limits accessibility. Here we test the validity of an accessible method of quantifying colour. We analysed photographs from four species of Anolis lizards from urban and forest habitats. We compared dewlap colour in order to determine if photographic analysis can detect inter- and intraspecific differences with the same power as a spectrophotometer. We hypothesized that photographs would capture colour data comparable to a spectrophotometer within the visible light range, and that habitat divergence would be associated with intraspecific differences in dewlap. We demonstrate, as hypothesized, that photographic colour data are consistent with spectrophotometer data and capable of differentiating dewlap variation within the visible light spectrum. Differences in colour between urban and forest populations were significant for some but not all species, and the part of the colour spectrum that shifted was not consistent across species. Our results support photographic analysis as an alternative for quantifying colour to study both inter- and intraspecific variation in visible colour.

    more » « less
  3. Abstract

    We identified a geographic mosaic ofEurosta solidaginisfly traits produced by coevolution of the stem gall‐forming fly with both its natural enemies and its host plants at small geographic scales in the presence of gene flow. These tritrophic interactions between the fly with its natural enemies and with its host‐plantSolidago altissimaproduced what has been termed a small‐tiled geographic mosaic of coevolution. Selection on gall diameter and length varies between prairie and forest habitats due to differences in host plants and natural‐enemy communities. At the prairie–forest ecotone where prairie and forest habitats are intermixed, we found that geographic selection mosaics on gall diameter and length varied on a scale of a few kilometers. Gall diameter variation among sites correlated with selection on gall diameters, indicating local adaptation. In contrast, gall lengths did not correlate with selection, indicating that gene flow may have prevented local adaptation of this trait. Eastern (forest) and western (prairie) subspecies ofE. solidaginishave been proposed based on fly wing patterns, and these had intermediate forms in the ecotone indicating gene flow between these subspecies. Variation in wing patterns correlated with gall diameter, indicating that gene flow between prairie and forest fly populations may influence the distribution of gall traits. The ratio of forest to prairie vegetation increases with latitude, but there was no indication of latitudinal clines in gall or wing traits. Our results indicate that selection for differentiation in coevolved traits is strong enough to overcome gene flow in small tiles of habitat. The result is that ecological forces produce a dynamic mosaic of genetically differentiated locally adapted populations. It also indicates that prairie and forest host races ofE. solidaginisform a mosaic hybrid zone in this region.

    more » « less
  4. Abstract

    Adaptation to environmental change requires that populations harbor the necessary genetic variation to respond to selection. However, dispersal‐limited species with fragmented populations and reduced genetic diversity may lack this variation and are at an increased risk of local extinction. In freshwater fish species, environmental change in the form of increased stream temperatures places many cold‐water species at‐risk. We present a study of rainbow darters (Etheostoma caeruleum) in which we evaluated the importance of genetic variation on adaptive potential and determined responses to extreme thermal stress. We compared fine‐scale patterns of morphological and thermal tolerance differentiation across eight sites, including a unique lake habitat. We also inferred contemporary population structure using genomic data and characterized the relationship between individual genetic diversity and stress tolerance. We found site‐specific variation in thermal tolerance that generally matched local conditions and morphological differences associated with lake‐stream divergence. We detected patterns of population structure on a highly local spatial scale that could not be explained by isolation by distance or stream connectivity. Finally, we showed that individual thermal tolerance was positively correlated with genetic variation, suggesting that sites with increased genetic diversity may be better at tolerating novel stress. Our results highlight the importance of considering intraspecific variation in understanding population vulnerability and stress response.

    more » « less
  5. Premise

    The drivers of isolation between sympatric populations of long‐lived and highly dispersible conspecific plants are not well understood. In the Hawaiian Islands, the landscape‐dominant tree,Metrosideros polymorpha, displays extraordinary phenotypic differences among sympatric varieties despite high dispersibility of its pollen and seeds, thereby presenting a unique opportunity to investigate how disruptive selection alone can maintain incipient forms. StenophyllousM. polymorphavar.newelliiis a recently evolved tree endemic to the waterways of eastern Hawai'i Island that shows striking neutral genetic differentiation from its ancestor, wet‐forestM. polymorphavar.glaberrima, despite sympatry of these forms. We looked for evidence for, and drivers of, differential local adaptation of these varieties across the range ofM. polymorphavar.newellii.


    For paired populations of these varieties, we compared seedling performance under contrasting light conditions and a strong water current characteristic of the riparian zone. We also conducted a reciprocal transplant experiment and contrasted adult leaf anatomy.


    Results suggest that the riparian zone is harsh and that selection involving the mechanical stress of rushing water, and secondarily, light, led to significant reciprocal immigrant inviability in adjacent forest and riparian environments. The strongest adaptive divergence between varieties was seen in leaves and seedlings from the site with the sharpest ecotone, coincident with the strongest genetic isolation ofM. polymorphavar.newelliiobserved previously.


    These findings suggest that disruptive selection across a sharp ecotone contributes to the maintenance of an incipient riparian ecotype from within a continuous population of a long‐lived and highly dispersible tree species.

    more » « less