The arrival of an 'alien' bird to a remote island in the Galápagos archipelago 36 years ago has provided direct genetic evidence of a novel way in which new species arise.
The newcomer belonged to one species, but mated with a member of another species resident on the island, giving rise to a new species that today consists of about 30 individuals, effectively within two generations.
The study – published in the journal Science – comes from work conducted on Darwin's finches, which live on the Galápagos Islands, in the Pacific Ocean off the coast of Ecuador. The remote location has enabled researchers to study the evolution of biodiversity under natural selection.
The direct observation of the origin of this new species occurred during field work carried out over the last four decades by B Rosemary and Peter Grant, two scientists from Princeton University, New Jersey, USA, on the small island of Daphne Major.
"The novelty of this study is that we can follow the emergence of new species in the wild," said B Rosemary Grant. "Through our work on Daphne Major, we were able to observe the pairing up of two birds from different species and then follow what happened, to see how speciation occurred."
In 1981, a graduate student working with the Grants on Daphne Major noticed the newcomer, a male that sang an unusual song and was much larger in body and beak size than the three resident species of bird on the island.
"We didn't see him fly in from over the sea, but we noticed him shortly after he arrived. He was so different from the other birds that we knew he did not hatch from an egg on Daphne Major," said Peter Grant, professor of zoology, ecology and evolutionary biology.
Medium Ground Finch is resident on Daphne Major, and became one of the parents of the new species (putneymark/commons.wikimedia.org).
The researchers took a blood sample and released the bird, which later bred with a resident Medium Ground Finch, initiating a new lineage. The Grants and their research team followed the new ‘Big Bird lineage’ for six generations, taking blood samples for use in genetic analysis.
In the current study, researchers from Uppsala University analysed DNA collected from the parent birds and their offspring over the years. The investigators discovered that the original male parent was an Española Cactus Finch from Española island, situated more than 62 miles to the south-east in the archipelago.
The other parent of the 'Big Bird lineage' was a vagrant Española Cactus Finch which wandered over from its nominal island, 62 miles to the south-east of Daphne Major (Harvey Barrison/commons.wikimedia.org).
The remarkable distance meant that the male finch was not able to return home to mate with a member of his own species and so chose a mate from among the three species already on Daphne Major. This reproductive isolation is considered a critical step in the development of a new species when two separate species interbreed.
The offspring were also reproductively isolated because their song, which is used to attract mates, was unusual and failed to attract females from the resident species. The offspring also differed from the resident species in beak size and shape, which is a major cue for mate choice. As a result, the offspring mated with members of their own lineage, strengthening the development of the new species.
Researchers previously assumed that the formation of a new species takes a very long time, but in the Big Bird lineage it happened in just two generations, according to observations made by the Grants in the field in combination with the genetic studies.
All 18 species of Darwin's finches derived from a single ancestral species of grassquit or seedeater (members of the tanager family) that colonised the Galápagos 1-2 million years ago. The finches have since diversified into different species, and changes in beak shape and size have allowed different species to utilise different food sources on the Galápagos. A critical requirement for speciation to occur through hybridisation of two distinct species is that the new lineage must be ecologically competitive – that is, good at competing for food and other resources with the other species – and this has been the case for the Big Bird lineage.
"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, the Big Birds occupy their own niche in the beak morphology space," said Sangeet Lamichhaney, a postdoctoral fellow at Harvard University and the first 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 definition of a species has traditionally included the inability to produce fully fertile progeny from interbreeding species, as is the case in Horse and Donkey, for example. However, in recent years it has become clear that some closely related species, which normally avoid breeding with each other, do indeed produce offspring that can pass genes to subsequent generations. The authors of the study have previously reported that there has been a considerable amount of gene flow among species of Darwin's finches over the last several thousand years.
"One of the most striking aspects of this study is that hybridisation 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", explained Leif Andersson, a professor at Uppsala University who is also affiliated with the Swedish University of Agricultural Sciences and Texas A&M University. "A naturalist who came to Daphne Major without knowing that this lineage arose very recently would have recognised this lineage as one of the four species on the island. This clearly demonstrates the value of long-running field studies," he said.
It is likely that new lineages like the Big Birds have originated many times during the evolution of Darwin's finches, according to the authors. The majority of these lineages have gone extinct but some may have led to the evolution of contemporary species. "We have no indication 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," said Andersson. "Charles Darwin would have been excited to read this paper."
Reference
Lamichhaney, S, Han, F, Webster, M T, Andersson, L, Grant, B R, and Grant, P R. 2017. Rapid hybrid speciation in Darwin’s finches. Science 23 November 2017 : eaao4593. DOI: 10.1126/science.aao4593.
