Conservation genetics of an island-endemic lizard: low Ne and the critical role of intermediate temperatures for genetic connectivity

Forest cover and geographical distance influence fine-scale genetic structure of leaf-toed geckos in the tropical dry forests of western Mexico

The biodiversity within tropical dry forests (TDFs) is astounding and yet poorly catalogued due to inadequate sampling and the presence of cryptic species. In the Mexican TDF, endemic species are common, and the landscape has been continually altered by geological and anthropogenic changes. To understand how landscape and environmental variables have shaped the population structure of endemic species, we studied the recently described species of leaf-toed gecko, Phyllodactylus benedettii, in coastal western Mexico. Using double-digest restriction site-associated DNA sequencing data, we first explore population structure and estimate the number of ancestral populations. The results indicate a high degree of genetic structure with little admixture, and patterns corresponding to both latitudinal and altitudinal gradients. We find that genetic structure cannot be explained purely by geographical distance, and that ecological corridors may facilitate dispersal and gene flow. We then model the spatial distribution of P. benedettii in the TDF through time and find that the coastline has been climatically suitable for the species since the Last Glacial Maximum. Landscape genetic analyses suggest that the combination of isolation by distance (IBD) and isolation by resistance (IBR; forest cover) has influenced the spatial genetic structure of the species. Overall, our genomic data demonstrate fine-scale population structure in TDF habitat, a complex colonization history, and spatial patterns consistent with both IBD and other ecological factors. These results further highlight the Mexican TDF as a diversity hotspot and suggest that continued anthropogenic changes are likely to affect native fauna.

Habitat-linked genetic variation supports microgeographic adaptive divergence in an island-endemic bird species

We investigated the potential mechanisms driving habitat-linked genetic divergence within a bird species endemic to a single 250 km² island. The island scrub-jay (Aphelocoma insularis) exhibits microgeographic divergence in bill morphology across pine-oak ecotones on Santa Cruz Island, California (USA) similar to adaptive differences described in mainland congeners over much larger geographic scales. To test whether individuals exhibit genetic differentiation related to habitat type and divergence in bill length, we genotyped over 3,000 single nucleotide polymorphisms (SNPs) in 123 adult island scrub-jay males from across Santa Cruz Island using restriction site-associated DNA sequencing (RADseq). Neutral landscape genomic analyses revealed that genome-wide genetic differentiation was primarily related to geographic distance and differences in habitat composition. We also found 168 putatively adaptive loci associated with habitat type using multivariate redundancy analysis (RDA) while controlling for spatial effects. Finally, two genome-wide association analyses revealed a polygenic basis to variation in bill length with multiple loci detected in or near genes known to affect bill morphology in other birds. Our findings support the hypothesis that divergent selection at microgeographic scales can cause adaptive divergence in the presence of ongoing gene flow.

Adaptive divergence in bill morphology and other thermoregulatory traits is facilitated by restricted gene flow in song sparrows on the California Channel Islands

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.