Female choice drives asymmetrical introgression in Tree Finches.
On Floreana Island (Galapagos archipelago) two species of Tree Finch (genus Camarhynchus) are interbreeding. The Medium Tree Finch (C. pauper) is critically endangered while the Small Tree Finch (C. parvulus) is more common on the island.
The Desperation Hypothesis
This situation – hybridization between a common and a rare species – sets the scene to test an interesting hypothesis known as Hubb’s Principle. This idea – which was coined by ichthyologist Carl Leavitt Hubbs in 1955 – predicts that mating with an individual from another species is more likely when the chances of finding a mate of your own species are low. In other words, when a bird cannot find a proper partner, it becomes so ‘desperate’ that it chooses a partner from another species (hence the other term for this idea: ‘Desperation Hypothesis’). From a gene flow perspective, you would expect genetic material to flow from the rare into the common species. With regard to the Tree Finches, this means gene flow from Medium into Small Tree Finch.
And this is exactly what Katharina Peters and her colleagues found. Using microsatellites, they showed asymmetric introgression (i.e. interspecific gene flow) from the rare Medium Tree Finch into the more common Small Tree Finch. Further analyses revealed that female choice is probably the main driver of this pattern. Small Tree Finch females never paired with a Medium Tree Finch male, whereas Medium Tree Finch females often choose a Small Tree Finch as their partner.
A previous study (which you can read about here) already indicated that Small Tree Finches and hybrids sing similar songs. Darwin’s Finches learn songs from their fathers, which in the case of hybrids is a Small Tree Finch. So, Small Tree Finch females – which prefer males that sing the songs of their species – are more likely to pair with hybrids. Backcrossing is thus biased towards Small Tree Finches, resulting in gene flow into this species. Sounds logical, right? You might have to read this paragraph a couple of times to get the mechanism, so I made this scheme to visualize the process.
Peters, KJ, SA Myers, RY Dudaniec, JA O’Connor, S Kleindorfer. (2017). Females drive asymmetrical introgression from rare to common species in Darwin’s tree finches. Journal of Evolutionary Biology. 30:1940-1952.
This paper has been added to the Thraupidae page.