image: Schematic illustration of the evolution of the Big Bird lineage on the Daphne Major island in the Galápagos archipelago. Initially an immigrant large cactus finch male (Geospiza conirostris) bred with a medium ground finch female (Geospiza fortis). Their offspring bred with each other and established the Big Bird lineage. view more
Credit: Photos © K. Thalia Grant for <i>G. conirostris</i> and Peter R. Grant for the remainder. Reproduced with permission from K. Thalia Grant, and Princeton University Press, which first published the remaining images in 40 Years of Evolution (P. R. Grant & B. R. Grant, 2014).
Darwin's finches in the Galápagos archipelago provide an iconic model for the evolution of biodiversity on earth due to natural selection. A team of scientists from Princeton University and Uppsala University now reports that they have observed the origin of a new species. A new lineage was formed by the hybridization of two different species of Darwin's finches. The study is published in Science.
Direct observation of the origin of a new putative species occurred during field work carried out by Rosemary and Peter Grant (Princeton University), on the small island of Daphne Major for 40 consecutive years. In 1981 they observed an immigrant male that sang an unusual song and differed in size from all resident species on the island. The male bred with a resident medium ground finch female and thereby initiated a new lineage which they named the Big Bird lineage. They followed the new lineage for 6 generations over 30 years. DNA sequence data now reveal that the immigrant male was a large cactus finch. Remarkably, it must have flown to Daphne from Española Island, which is more than 100 km to the southeast.
'A critical step in speciation is the establishment of reproductive isolation. It is usually assumed that this process takes a very long time but in the Big Bird lineage it happened in just two generations, say Rosemary and Peter Grant.'
'One important reason for this is the unique song of the immigrant male, since sons learn the song of their father and females mate with males that sing like their fathers, continue Peter and Rosemary Grant. A second reason is the new lineage differed from the resident species in beak morphology, which is also a major cue for mate choice.'
All 18 species of Darwin's finches have been derived from a single ancestral species that colonized the Galápagos 1-2 million years ago. They have diversified into different species, and changes in beak morphology in particular have allowed different species to utilize different food sources on the Galápagos. Thus, another critical requirement for speciation to occur through hybridization is the new lineage must be ecologically competitive, 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 island the Big Birds occupy their own niche in the beak morphology space', explains Sangeet Lamichhaney, currently post-doctoral 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.'
A classical definition is that 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, which normally avoid breeding with each other, exchange genes by hybridization surprisingly often. The authors of this study have previously reported that there has been a considerable amount of gene flow going on among species of Darwin's finches for thousands of years.
'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. 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', says Leif Andersson at Uppsala University, the Swedish University of Agricultural Sciences and Texas A&M University.
'It is very likely that new lineages like the Big Birds have originated many times during the evolution of Darwin's finches. 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', ends Leif Andersson.
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Journal
Science