Researchers: This system is exceptional in terms of the rates of evolutionary change and adaptive divergence, and it is likely that this may be due to the uncommon circumstances posed by the isolated and fragmented Galápagos landscape.
Abstract: Beak shape in Darwin’s ground finches (Geospiza) is emblematic of natural selection and adaptive radiation, yet our understanding of the genetic basis of beak shape variation, and thus the genetic target of natural selection, is still evolving. Here we reveal the genomic architecture of beak shape variation using genomewide comparisons of four closely related and hybridizing species across 13 islands subject to parallel natural selection. Pairwise contrasts among species were used to identify a large number of genomic loci that are consistently related to species differences across a complex landscape. These loci are associated with hundreds of genes that have enriched GO categories significantly associated with development. One genomic region of particular interest is a section of Chromosome 1A with many candidate genes and increased linkage. The distinct, pointed beak shape of the cactus finch is linked to an excess of intermediate frequency alleles and increased heterozygosity in significant SNPs, but not across the rest of the genome. Alleles associated with pointier beaks among species were associated with pointier-beaked populations within each species, thus establishing a common basis for natural selection, species divergence and adaptive radiation. The adaptive genomic landscape for Darwin’s finches mirrors theoretical expectations based on morphological variation. The implication that a large number of genes are actively maintained to facilitate beak variation across parallel populations with documented interspecies admixture challenges our understanding of evolutionary processes in the wild. (paywall) – Lawson LP, Petren K. The adaptive genomic landscape of beak morphology in Darwin’s finches. Mol Ecol. 2017;00:1–12. https://doi.org/10.1111/mec.14166 More.
In Darwin’s finches, hundreds of loci appear to be involved in maintaining distinct phenotypic differences between species, and these loci are highly associated with developmental genes. This number of loci poses several challenges for advancing our understanding of evolutionary processes. First, further functional verification of the role played by each of hundreds of implicated genes poses a large obstacle using traditional mis-expression techniques. Beginning by validating those genes in common between MLM, GLM and Fst analyses before proceeding to the entire data set may help prioritize the most likely candidates for craniofacial change. Second, how so many genetic differences can become co-aligned over time in the face of ongoing gene exchange challenges our understanding of evolutionary processes. This system is exceptional in terms of the rates of evolutionary change and adaptive divergence, and it is likely that this may be due to the uncommon circumstances posed by the isolated and fragmented Galápagos landscape.
So the textbook Darwin’s finches, revered icons of textbook evolution, are not a good example? Never mind. They’ll still be in the books a decade from now anyway. As Zombie Science shows, these icons just keep coming back.
See also: Darwin’s finches not a good example of Darwinian evolution, but of hybridization