New species evolve in just two generations
Darwin’s finches provide an unexpected insight into speciation, knocking accepted theories sideways. Andrew Masterson reports.
A new species can evolve in as few as two generations, researchers have found, shattering the orthodox position that speciation is a process that occurs slowly over a long time.
Ironically, the case study that led to this startling conclusion – detailed in a paper in the journal Science – concerns the finches of the Galapagos islands, the very collection of birds that helped Charles Darwin formulate his theory regarding the role of natural selection in evolution.
A team of researchers led by Leif Andersson from Uppsala University, in Sweden, report the emergence of a new species of finch, dubbed Big Bird, arising from an initial cross breeding between two species, the large cactus finch (Geospiza conirostris) and the medium ground finch (Geospiz fortis). From a first chance encounter, a new lineage which boasts a unique beak shape, unique vocalisations, and the inability to breed with other finch species emerged.
The Big Bird today comprises only about 30 individuals – all fiercely inbred, but meeting the definition of distinct species, nonetheless.
The case study is watertight because the set-up for the foundation mating between the two originator species was observed by a pair of scientists from Princeton University, who were visiting the Galapagos island of Daphne Major at the time.
The year was 1981 and evolutionary biologists Rosemary and Peter Grant had been studying the finches of the island group. When they noticed a strange bird with a largish beak and unusual song on Daphne Major, therefore, they knew immediately it was not one of the three finch species native to the place.
"We didn't see him fly in from over the sea, but we noticed him shortly after he arrived,” recalls Peter Grant. “He was so different from the other birds that we knew he did not hatch from an egg on Daphne Major.”
It turned out the intruder was from a species resident on Espanola Island, more than 100 kilometres away. Unable to return and thus find a mate from its own species, the finch somehow managed to mate successfully with a local girl.
Isolation is a critical step in speciation. The successful interbreeding would never have happened had not the male finch been somehow massively blown off course and – remarkably – found landfall on another tiny speck in the Pacific. Thus, if not for outrageous fortune, the cactus finch and the ground finch would not have challenged another fundamental definition of “species” – the inability to produce fertile offspring with a member of a different species.
For the resultant offspring, however, the results were potentially dire. The baby finches were neither one nor the other, and developed with beaks and calls that were unmatched among the resident species. Like isolated populations of humans have occasionally been known to do, therefore, and perhaps equally unwisely, they turned for attention to their own siblings.
The Grants, having taken an initial blood sample from the outsider, continued to monitor the little population of Big Birds, taking blood from the subsequent six generations.
Now, Andersson and his colleagues from Uppsala have analysed the DNA collected from each of those generations. They conclude that the Big Birds quickly developed unique structural characteristics with which they were able to forge an entirely new environmental niche that did not put them in competition with the more numerous resident finch species.
“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,” says co-author Sangeet Lamichhaney.
“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.”
He adds that a naturalist visiting Great Daphne today and unaware of the Big Birds’ history would have no reason to think the species was anything but ancient and firmly rooted on the island.
With only small numbers and a shallow genepool, of course, there is no guarantee of the new species’ robust and continued survival. Andersson notes that this type of emergence may have happened many times before, the results lost after a few generations to extinction.
“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,” he says.
“Charles Darwin would have been excited to read this paper.”