The process of speciation usually takes a long time. But for a new species of Darwin’s finches, it happened in just two generations. And scientists were able to watch.
The new lineage that lives in the Galápagos archipelago comes from 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 40 consecutive years of direct observation, the Grants found that the new lineage, which they named “Big Bird,” began when that male bred with a resident medium ground finch female.
The researchers followed the putative lineage for six generations, over 30 years. DNA sequence data now reveal that the immigrant male was a large cactus finch, which must have flown to Daphne from Española island, more than 60 miles to the southeast.
As reported in Science, 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, the Big Bird lineage happened in just two generations.
“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,” says Leif Andersson, a visiting professor in the Texas A&M College of Veterinary Medicine & Biomedical Sciences.
“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, it’s become clear in recent years that some closely related species that normally avoid breeding with each other exchange genes by hybridization surprisingly often.
“Charles Darwin would have been excited to read this paper.”
It’s been reported previously 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 derive 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 says. “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 coauthors of the current 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,” says coauthor Sangeet Lamichhaney, a postdoctoral fellow at Harvard University.
“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 Galápagos National Parks Service, the Charles Darwin Foundation, the National Science Foundation, the Knut and Alice Wallenberg Foundation, and the Swedish Research Council funded the work.
Source: Texas A&M University