Unnatural selection
Some scientists argue that we are in the middle of a mass, human-induced extinction. What implications does this have for the future of evolution? Sanjida O’Connell reports
Salit Kark is named after a small bird; Salit means wheatear in Hebrew. It is an appropriate name for a woman who talks passionately about her chosen subject, another desert-living bird, the chukar partridge.

Dr Kark, an evolutionary ecologist at Stanford University in California, studies the chukar in the Negev desert. The bird is found scattered across the globe in many diverse habitats. Dr Kark wants to use it as a test case — her aim is to discover the kind of changes these birds are undergoing, but her grand ambition is nothing less than to decipher the future of evolution.

Since Darwin figured out how natural selection works, how a species changes through time and may eventually develop into another species, evolution has been ticking along nicely. Or has it? Is it putting on the brakes or going into fast-forward? Some scientists argue that we are in the middle of a great extinction. Evidence from the fossil record suggests that there have been five major mass extinctions; this, the sixth, is human-induced. This time around, they say, more species will be lost than in any of the previous extinctions; it could take at least five million years before animals and plants recover in numbers and variety.

Could this destruction of species and habitats alter the future of evolution? According to Dr Norman Myers, a fellow of Green College, Oxford, who is working on a book about the future of evolution, “the current crisis will eliminate large numbers of species and reduce the ability of the remaining species to recover and generate new species”.

Dr Kark believes that the future of evolution is linked tightly to biodiversity — the numbers and types of species found in different areas. Her study site, the northern Negev, is an area of contrasts, ranging from lush to arid; sometimes there is a mean annual rainfall of half a metre, other years it dwindles to a few centimetres. Ecologists call such an area an ecotone.

Dr Kark caught partridges living there and studied their genes and appearance. She found that the variation in this one area was as great as the variation between chukar partridges across the whole region. Other bird species living in the area also showed an incredible diversity.

“Even if the overall numbers are not high, this is a region in which speciation (the creation of new species) could take place,” says Dr Kark. The birds, it seems, are evolving in response to the stresses and variability seen in their habitat. This is how new species are born.

The biodiversity in areas such as ecotones may be high, but is the rate of evolution actually changing? “I don’t know if I can say that evolution is speeding up,” says Dr Kark. “I think it’s probably happening at the same rate, and it’s just that there is more opportunity for diversity.” Dr Myers thinks that evolution might speed up in certain areas; as habitats are destroyed and the animals die with them, there will be opportunities for new species to colonise these environments.

As a result, evolution will speed up, but this increase in new species will not match the rate of extinction. Research by a team of ecologists headed by Dr Raymond Huey, a zoologist at the University of Washington in Seattle, confirms that evolution can speed up dramatically.

About 20 years ago the European fruit fly was introduced accidentally to California. In Europe, flies in the south have larger wings than those in the north. Dr Huey’s research team found a 4 per cent increase in wing size in the immigrant flies collected between the south and north of California. The flies had, in less than two decades, replicated the type of wing change seen in Europe — an incredibly fast rate of change.

Evolution can even go into reverse. Professor Michael Rose, from the University of California at Irvine, has been breeding fruit flies for the past 25 years, selecting populations for their ability to reproduce either late in life, in mid-life, early in life, or to survive for a long period without food. When Professor Rose returned these genetically altered populations of flies to their original environment, it took fewer than 20 fly generations for most flies to revert to standard fruit flies.

If evolution can rewind or fast-forward and if, as Dr Kark has discovered, areas of change are more conducive to this evolutionary variability, might places that have been disturbed by humans actually create an increase in species diversity? Dr Kark says: “When an ecotone is created by people, such as a city that encroaches on a natural environment, then no. That is because when the animals reach that city, it is a dead end. They become extinct.”

The prognosis looks grim. According to Dr Myers, any increase in the number of species produced as evolution speeds up will be counterbalanced by the number of species lost. With such a decrease in genetic variability, it will be difficult if not impossible for new species to arise. Large vertebrates will almost certainly be lost and pest and weed species will proliferate: rats and ragweed, cockroaches and dandelions. In addition, what he terms “the powerhouses of evolutionary change” — wetlands, rainforests and coral reefs — are being decimated. It is these areas that provide much of the world’s biodiversity, and without them the planet will become considerably impoverished. “People think evolution is about the past,” says Dr Kark, “but it’s a live thing, it’s going on all the time. And we must think about the future of evolution now.”

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January 19, 2001