Something borrowed: Ancestral variation and introgression drive rapid evolution in Darwin’s Finches

Genomic study identifies 28 ancestral genetic modules that predate the radiation.

Despite the overwhelming evidence for evolution, many misconceptions still persist (often actively promoted by creationists). One example is the so-called “waiting time problem” which claims that populations must wait for the right mutation to appear before they can adapt to a new environment. This view grossly misrepresents how evolution actually works. In addition to novel mutations, populations can draw on ancestral genetic variation and even DNA introgressed from closely related species. What remains an open question, however, is the relative contribution of new mutations, ancestral variants, and introgressive hybridization to evolutionary change.

A recent study in Science Advances offers a compelling example of how ancestral variation and introgression have fueled the adaptive radiation of Darwin’s finches. The researchers found that multiple ancestral genetic modules have been exchanged among species, contributing to the remarkable phenotypic diversity that characterizes this iconic group of birds.

Twenty-eight Loci

Carl-Johan Rubin, Erik Enbody and their colleagues compared the genomes of three Geospiza species that differ in their beak size: Small Ground Finch (G. fuliginosa), Medium Ground Finch (G. fortis) and Large Ground Finch (G. magnirostris). Using admixture mapping, they identified 28 genomic regions significantly associated with variation in beak morphology. These regions were clustered on macrochromosomes and included the previously characterized ALX1 and HMGA2 genes, which are known to influence beak shape.

Next, the researchers explored the evolutionary origins of these 28 genomic regions by comparing their genetic make-up across the phylogeny of Darwin’s finches. This approach revealed that these regions predate the divergence of the genera Geospiza and Camarhynchus. Further analyses, along with previous studies, also provided evidence of introgression among several species.

Genome-wide admixture mapping uncovered 28 regions (highlighted in red) that are associated with beak morphology in three Geospiza species. From: Rubin et al. (2022).

Rapid Radiation

Taken together, these findings provide a clear illustration of how ancestral variation and introgression can fuel rapid adaptive radiations. The authors nicely summarize the main message at the end of the introduction.

We show that the origin of these haplotype blocks linked to phenotypic divergence predates speciation events. These genetic modules have been reused over the past million years, were exchanged by gene flow, and contributed to the rapid phenotypic evolution and speciation among Darwin’s finches.

There’s no need to wait for new mutations when evolution can work with what already exists.

References

Rubin, C. J., Enbody, E. D., Dobreva, M. P., Abzhanov, A., Davis, B. W., Lamichhaney, S., et al. (2022). Rapid adaptive radiation of Darwin’s finches depends on ancestral genetic modules. Science Advances, 8(27), eabm5982.

Featured image: Medium Ground Finch (Geospiza fortis) © Judy Gallagher | Wikimedia Commons