Genetic bridges: Medium ground finch functions as a conduit for gene flow between two other Darwin’s Finches

Recent study highlights the evolutionary potential of multispecies hybridization.

“Hybridization is not always limited to two species; often multiple species are interbreeding.” This is the opening sentence of my recent review paper on multispecies hybridization in birds. Throughout this paper, I explored different evolutionary and ecological consequences of hybridization between multiple species. One of my favorite ideas concerns “genetic bridges” where one species functions as a conduit for gene flow between two other species that are not interbreeding. You can imagine my excitement when a study by Peter and Rosemary Grant appeared in the journal PNAS where they provide some evidence for this scenario.

Cactus_Finch

A common cactus finch © Mike’s Birds | Wikimedia Commons

 

Geospiza

The extensive dataset of Darwin’s Finches from Daphne Major island (Galapagos Islands) allowed the Grants to retrace the interactions between three species: the medium ground finch (Geospiza fortis), the small ground finch (G. fuliginosa) and the common cactus finch (G. scandens). Between 1975 and 2011, the number of hybrids between these species was meticulously noted down. It turned out that the medium ground finch occasionally interbred with the resident cactus finch and the immigrant small ground finch. Hybrids between cactus finch and small ground finch were never observed on Daphne Major. However, genetic analyses did uncover some admixed individuals between these species. They might be the outcome of extra-pair copulations or backcrosses with other hybrids.

hybrids_DF

The frequency of hybrids on Daphne Major increased over time. The letters on the vertical axes indicate the different species combinations: medium ground finch (fortis, F), cactus finch (scandens, S) and small ground finch (fuliginosa, f). From: Grant & Grant (2020) PNAS

 

Hybrid Fitness

The production of hybrids between these three species is just the first step of the process. For interspecific gene flow to occur, these hybrids need to survive and continue breeding. Further observations showed that survival rates of hybrids were similar to those of pure individuals and confirmed the existence of three-way hybrids (referred to as trihybrids in the paper). Together, these results suggest that gene flow among these three species is occurring.

But how are the genes flowing? Is one species functioning as a genetic bridge? To answer this question we need to take a closer look at the hybrid patterns. The authors write that “secondary admixtures occur through interbreeding of G. scandens and Ff hybrids [i.e. G. fortis x fuliginosa] or backcrosses.” This indicates that fuliginosa genes are flowing into G. fortis (through the production of hybrids) and consequently to G. scandens (through interbreeding with these hybrids). In other words, the medium ground finch is acting as a genetic bridge between small ground finch and cactus finch.

suvival

Survival curves from different Darwin’s Finches and their hybrids. These findings indicate no obvious differences in survival between pure individuals and hybrids. From: Grant & Grant (2020) PNAS

 

An Evolutionary Stimulus

These patterns of genetic exchange also affect the morphology of the Darwin’s Finches. Both two-way and three-way hybrids showed increased variance in certain traits, such as beak morphology. The surplus in morphological and genetic variation provides the fuel for further evolutionary changes. This can be particularly important in current events with human-induced habitat changes and rapid climate change. Hybridization might be the key to cope with these challenges. In the 1950s, the botanists Anderson and Stebbins already argued that introgressive hybridization can act as an evolutionary stimulus. The Grants echo this conclusion and write that “Hybridizing species may therefore be disproportionately successful in coping with a changing environment in the future, as in the past”

finches

Patterns of gene flow between the three Darwin’s Finches. The medium ground finch (G. fortis) acts as a genetic bridge between the other two species. From: Grant & Grant (2020) PNAS

 

References

Grant, P. R., & Grant, B. R. (2020). Triad hybridization via a conduit species. Proceedings of the National Academy of Sciences117(14), 7888-7896.

This paper has been added to the Thraupidae page.

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