The ‘human’ effect on origin of new species
NYT News Service
Disturbing the natural habitat might even adversely
affect the evolution of new species, if the new research study is any indication.
Humans can threaten species with extinction in many ways, including overfishing, pollution and deforestation. Now a pair of studies points to a new danger to the world's biodiversity: Humans may be blocking new species from evolving.
New species evolve when old species split apart. Animals living on a peninsula might become cut off from the mainland by rising sea levels, for example. They would adapt to life on the island and acquire mutations not shared by the rest of their species. If the sea level should drop and the animals could mix together again, the mutations might make it less likely that the two populations could interbreed. They would be on their way to becoming separate species. A few places provide
scientists with a good view of these forces at work. In western Canada, many lakes formed when the glaciers retreated at the end of the last ice age 11,000 years ago.
Three-spine
sticklebacks Fish invaded the lakes from the ocean and were cut off from their marine home. Some of those fish have evolved into new species. The most striking of the invading fish are three-spine sticklebacks. They have followed the same evolutionary path in six lakes. In each case, some of the sticklebacks evolved into slender, fast swimmers that hunt in the open water for small invertebrates. The others became bulky bottom feeders living in the shallows. These pairs of species have split so recently that they can still produce viable offspring. But less than one percent of each new generation are hybrids. “There’s never really any ambiguity,” said Eric Taylor, who is a biologist at the University of British Columbia. Taylor and his colleagues suspect that hybrids are rare because the fish prefer to mate with fish that look like themselves. Natural selection may favour that preference, because hybrid sticklebacks cannot hunt for food as well as their specialised parents. In 1999, Taylor and other stickleback specialists paid a visit to one of the lakes, Enos, on Victoria Island. When they caught the sticklebacks, they found far more intermediate sticklebacks than had previously been reported. He and his colleagues set out to test their impression. They began by taking measurements of 887 sticklebacks scientists had collected from Enos Lake from 1977 to 2002. In the early years, the scientists found, almost all the sticklebacks had one of the two typical body shapes. But from 1994 to 1997 the groups blurred into one broad continuum. Studies of the DNA of the sticklebacks produced a similar picture. In 1994, big and small sticklebacks could be distinguished by genetic markers. Since then, the markers have become mixed within the entire stickleback population.
Taylor was very suspicious about introduction of the American signal crayfish into the lake shortly before the sticklebacks collapsed. By disturbing the habitats of the sticklebacks, the crayfish may be driving them into more contact, leading to more
interbreeding.
Compelling studies
Another scientific team has found signs of a similar transformation on the Galapagos Islands. Instead of fish, the scientists have been studying Darwin's finches, named for Charles Darwin, who described them after his visit to the islands in 1835. Studies of Darwin’s finch DNA show that birds descended from immigrants that arrived a few million years ago. Since then they have evolved into 14 species found nowhere else. The beaks of Darwin’s finches have evolved allowing them to feed on different food.
The small ground finch has a delicate beak it uses to quickly eat small, soft seeds. The large ground finch uses its heavier beak to crush big, hard seeds. Between those extremes is the medium ground finch, which eats both kinds of seeds. Some populations of the birdsappear to be diverging much like the sticklebacks. On Santa Cruz Island, the birds tend to have either large or small beaks, which they use to specialise on large or small seeds. Studies of their genes suggest that the two groups tend to mate with birds like themselves. Andrew Hendry of McGill University in Montreal noticed that the finches on a part of the island known as Academy Bay seemed to lack the striking difference in the size of their beaks. He and his team measured the birds’ beaks and compared them with measurements taken since 1964.