IN 1981 PETER and Rosemary Grant, a husband-and-wife team of evolutionary biologists, spotted something odd on Daphne Major. Every year for the previous decade they had travelled from Princeton University to this island in the Galápagos, to study its three endemic tanager species, part of a group known colloquially as “Darwin’s finches”. On this occasion their eyes were drawn to an unusual male that sported dark feathers and sang a unique song. Genetic analysis later identified him as a large cactus finch, probably blown in from Española, another part of the archipelago that is over 100km away.
Intrigued, the Grants followed the castaway as he explored his new home. They watched him mate with a local female medium ground finch. That produced five fit, healthy offspring. Those offspring were also surprisingly sexually selective. A single male excepted, they and their descendants mated only among themselves—and they have continued to do so ever since.
Despite this heavy inbreeding, the hybrids (two of which are pictured above) have been successful. They have carved out a niche in which they use their size and their deep beaks to exploit the large woody fruits of the Jamaican feverplant, which grows locally. They have, to all intents and purposes, become another species of Darwin finch, of which 13 were previously recognised. Though they do not yet have a Latinised scientific name, they are known to all as the “Big Bird” lineage.
Heretical thinking
This story would once have been considered deeply implausible. Evolution’s orthodox narrative does not suggest that hybridisation is how new animal species emerge. But, as genetic testing has proliferated, biologists have been confronted with an unexpected fact. Hybrids are not an evolutionary bug. They are a feature.
That knowledge is changing the way people think about evolution. The neat family trees envisaged by Charles Darwin in one of his early notebooks (see picture below) are turning into webs, and the primacy of mutation in generating the variation which natural selection then winnows is being challenged. The influx of genes accompanying hybridisation creates such variation too—and the harder people look, the more important that seems to get. Hybridisation also offers shortcuts on the long march to speciation that do not depend on natural selection at all. As the example of the Big Bird lineage shows, instead of taking millennia to emerge, a new species can appear almost overnight.
In truth, all this had already been recognised for simple organisms like bacteria. These exchange genes promiscuously between both more and less related individuals. But bacteria were unknown when Darwin came up with natural selection, and, ever since then, the subject of speciation has been dominated by examples drawn from animals and plants. To recognise that what is true for bacteria is also true for these multicellular organisms has profound implications, not least for how human beings understand their own origins. It seems appropriate, then, that the birds whose diversity helped inspire Darwin still have evolutionary tales to tell.
The conventional view of evolution is that mutations happen at random. Maladaptive ones are then eliminated by competitive pressure while adaptive ones proliferate. The result, over long periods of time and assisted by populations sometimes being split up by external circumstances, is change which eventually crystallises into new and separate species.
That process does leave the door open to hybrids. The genomes of closely related species may remain sufficiently similar to produce viable offspring. But these genes often fit together less well than those of parents from the same species. As a consequence, even viable hybrids are frequently infertile (think mules) and are also at higher risk of developmental and other types of illnesses. In fact, infertility in male hybrids is so common that it has a…
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