A team of scientists including Texas A&M University and Uppsala University professor Leif Andersson has discovered the origin of a new species of finch living among Darwin’s finches in the Galápagos archipelago.
Published on Thursday, Nov. 23, in the journal Science, their study reports that the new lineage was formed by the hybridization of two different species of Darwin’s finches.
Darwin’s finches provide an iconic model for the evolution of biodiversity on earth due to natural selection.
While conducting their field work on the small island of Daphne Major in 1981, Princeton University researchers Rosemary and Peter Grant observed an immigrant male that sang an unusual song and differed in size from all resident species on the island. Throughout their 40 consecutive years of direct observation, the Grants found that a new lineage, which they named the Big Bird lineage, was initiated when that male bred with a resident medium ground finch female.
The couple followed the new putative lineage for six generations, over 30 years. DNA sequence data now have revealed that the immigrant male was a large cactus finch, which, remarkably, must have flown to Daphne from Española island, more than 60 miles to the southeast.
The identification of the Big Bird lineage is significant because while a critical step in speciation is the establishment of reproductive isolation and the process of speciation is usually assumed to take a very long time, in the Big Bird lineage, it happened in just two generations, according to the Grants.
“The interesting aspect of this study is that a hybridization between two distinct species led to the development of a new lineage that after only two generations behaved as any other species of Darwin’s finches,” said Andersson, a visiting professor in the Texas A&M College of Veterinary Medicine & Biomedical Sciences’ (CVM) Department of Veterinary Integrative Biosciences (VIBS) and an author on the study. “If a naturalist had come to Daphne Major island without knowing that this lineage arose very recently it would have been recognized as one of the four species on the island. This clearly demonstrates the value of long-running field studies.”
Traditionally, good species respect species boundaries and cannot produce fully fertile progeny if hybridization happens, as is the case for the horse and the donkey, for example; however, in recent years, it has become clear that some closely related species that normally avoid breeding with each other exchange genes by hybridization surprisingly often.
The study’s authors have previously reported that there has been a considerable amount of gene flow going on among the 18 species of Darwin’s finches for thousands of years. All 18 species have been derived from a single ancestral species that colonized the Galápagos 1-2 million years ago.
“It is very likely that new lineages, like the Big Birds, have originated many times during the evolution of Darwin’s finches,” Andersson said. “The majority of these have gone extinct, but some may have led to the evolution of contemporary species. We have no idea about the long-term survival of the Big Bird lineage, but it has the potential to become a success and it provides a beautiful example of one way in which speciation occurs. Charles Darwin would have been excited to read this paper.”
The Big Bird lineage was identified because of the unique song of the original immigrant male, since sons learn the song of their father and females mate with males that sing like their fathers, according to the Grants, who are also listed as co-authors of the study.
The new lineage also differed from the resident species in beak morphology—often occurring in order for a lineage to be ecologically competitive—which is also a major cue for mate choice. In the case of the Big Bird lineage, its beak morphology evolved from that of the immigrant finch to allow the species to utilize different food sources on the Galápagos.
“It is very striking that when we compare the size and shape of the Big Bird beaks with the beak morphologies of the other three species inhabiting Daphne Major island, the Big Birds occupy their own niche in the beak morphology space,” said Sangeet Lamichhaney, a post-doctoral fellow at Harvard University and co-author on the study. “Thus, the combination of gene variants contributed from the two interbreeding species in combination with natural selection led to the evolution of a beak morphology that was competitive and unique.”
The study was supported by the Galápagos National Parks Service, The Charles Darwin Foundation, the National Science Foundation, the Knut and Alice Wallenberg Foundation, and The Swedish Research Council.