Darwin’s finches “reversing” their famous process of speciation

In a paper published this week on the American Naturalist, Kleindorfer et al. report on how one of the subgroups of Darwin’s finches, the insectivorous tree finches, are collapsing back via hybridization, and also suggest the extinction of the large tree finch, Camarhynchus psittacula.

The Darwin finches are some of the most iconic examples of adaptive evolutionary radiation, and consequently, speciation. There are some curious facts about the history behind Darwin’s finches that I think are interesting to share. History that which obviously involves our beloved blog namesake, Mr. Darwin.

Darwin finches, from Wikipedia.

Darwin finches, from Wikipedia.

Charles Darwin was known for his likings of hunting and avidity in collecting, and perhaps for that reason I always pictured Darwin happily shooting all kinds of finches in Galapagos and instantly recognizing how that was a major find, and making all the intricate connections between adaptive morphology and speciation. However, it was another shipmate of the Beagle, Syms Covington, who did most of the bird collections in Galapagos.

As with almost all breakthroughs, the “eureka” moment of this famous Darwin episode was an afterthought. Darwin didn’t even discuss the finches in the diary of his voyage on the Beagle at much length. At the time, Darwin thought those were blackbirds and gross-beaks. Only after being back in England is when the famous ornithologist John Gould identified those Galapagos birds as “a series of ground finches which are so peculiar [as to form] an entirely new group, containing 12 species.” After Gould had made his findings public is when Darwin associated their incredible morphological adaptation to the species divergence concept, when he noted that “seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends”. Also interesting is that, nowadays, we know Darwin’s finches diverged from a group of Tiaris birds, which originated in the Caribbean islands and then spread to Central and South America, and finally to the Galapagos.

Now, to add to their glorified fame as teachers of the workings of evolution, Darwin’s finches are showing us a snapshot of the reverse process. The paper of Kleindorfer et al – just hot off the presses on the American Naturalist (Feb. 24th) –  looked at the three Camarhyncus species, known as tree finches, in one of the Galapagos islands, Floreana, to test the mechanisms and functions of annual patterns of hybridization in these sympatric species.

Images of three sympatric tree finches from Floreana Island in 2010. A, genetic population 1; B, hybrid tree finch; and C, genetic population 2. From Kleindorfer et al (2014).

Images of three sympatric tree finches from Floreana Island in 2010. A, genetic population 1; B, hybrid tree finch; and C, genetic population 2. From Kleindorfer et al (2014).

“The three Camarhyncus species on Floreana Island are of special interest because Lack (1947) singled them out as a paradigmatic example of successful speciation in Darwin’s finches. The medium tree finch probably originated from a “small morph” of the large tree finch from Isabela Island, which was either followed by (Lack 1947) or preceded by (Grant 1999) separate colonization events of “large morph” large tree finches from Santa Cruz Island and small tree finches from another island. […] Evidence that we present here, however, suggests that these three species may represent a case of evolution in reverse …”

They had birds collected at three different time periods, 1900s, 2005, and 2010.  Their morphological and genetic analyses suggest that through time, species composition started to move away from the three distinct clusters (small, medium, and large), and by 2010, there were two species left, the small and the medium tree finches, along if a population of hybrids between the two.

“The results presented here go to the heart of evolutionary biology: by what criteria do we denote species, and by what criteria do new species form or collapse? Here we present evidence that three sympatric species of Darwin’s tree finches in the 1900s have collapsed, under conditions of hybridization, into two species by the 2000s.”

They argue that their results show a case of disassortative mating, where the females of the “small tree finches” (Camarhynchus parvulus) are choosing among the larger of the “medium tree finches” (Camarhynchus pauper), creating a hybrid population of intermediate morphology. As for the “large tree finch” species, Camarhynchus psittacula, they don’t appear in any of their collections during the 2000s, and authors suggest there is a chance the species has gone extinct.

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