Yes, we are discussing the icon of Darwinism that you heard about at school. They interbreed so much, it is hard to know how much they are separate species. From the BBC
The most extensive genetic study ever conducted of Darwin’s finches, from the Galapagos Islands, has revealed a messy family tree with a surprising level of interbreeding between species.
It also suggests that changes in one particular gene triggered the wide variation seen in their beak shapes.
The study also revealed a surprisingly large amount of “gene flow” between the branches of the family.
This indicates that the species have continued to interbreed or hybridise, after diversifying when they first arrived on the islands.
“It’s been observed that the species of Darwin’s finches sometimes hybridise – Peter and Rosemary Grant have seen that during their fieldwork,” Prof Andersson told the BBC.
“But it’s difficult to say what the long-term evolutionary significance of that is. What does it contribute?”
What it contributes is that one would be hard pressed to show that there is any evolution going on, in the face of this much hybridization. A friend sends along a key point from the Discussion of the paywalled Nature paper:
Evidence of introgressive hybridization, which has been documented as a contemporary process, is found throughout the radiation. Hybridization has given rise to species of mixed ancestry, in the past (this study) and the present . It has influenced the evolution of a key phenotypic trait: beak shape. Similar introgressive hybridization affecting an adaptive trait (mimicry) has been described in Heliconius butterflies . The degree of continuity between historical and contemporary evolution is unexpected because introgressive hybridization plays no part in traditional accounts of adaptive radiations of animals [1, 2]. For young radiations it complements the better-known role of natural selection.
In short, Darwin’s finches are not a very good schoolbook illustration of the neo-Darwinian synthesis (Darwinism). How does one sort out what is Darwinism (natural selection acting on random mutation) and what is hybridization? Here’s the abstract:
Darwin’s finches, inhabiting the Galápagos archipelago and Cocos Island, constitute an iconic model for studies of speciation and adaptive evolution. Here we report the results of whole-genome re-sequencing of 120 individuals representing all of the Darwin’s finch species and two close relatives. Phylogenetic analysis reveals important discrepancies with the phenotype-based taxonomy. We find extensive evidence for interspecific gene flow throughout the radiation. Hybridization has given rise to species of mixed ancestry. A 240 kilobase haplotype encompassing the ALX1 gene that encodes a transcription factor affecting craniofacial development is strongly associated with beak shape diversity across Darwin’s finch species as well as within the medium ground finch (Geospiza fortis), a species that has undergone rapid evolution of beak shape in response to environmental changes. The ALX1 haplotype has contributed to diversification of beak shapes among the Darwin’s finches and, thereby, to an expanded utilization of food resources. (paywall)
But we will probably see the finches in the schoolbooks anyway, because Darwin’s name is, like, a brand. There is Darwin Day, there isn’t Hybrid Day.
It’s a brand lots of people have invested lots of time and money in. They won’t let that go to waste. Let the spin begin!
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