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 作者:颛孙贡     |      日期:2019-03-07 01:03:06
By Matt Walker CONSERVATION biologists may have to rethink the way they manage populations of rare species. Introducing new animals can sometimes reduce rather than increase a population’s genetic variability, as measured by a widely used index. Small groups of animals in the wild and in captivity may face extinction because of the problems of inbreeding. As a population’s genetic variability decreases, the animals become more susceptible to disease and may produce unhealthy or malformed offspring. Introducing “fresh blood” is considered one of the best ways to reinvigorate a population. For example, researchers in Sweden reported this month that a population of severely inbred adders in southern Sweden increased dramatically following the introduction of a number of unrelated males (Nature, vol 402, p 34). However, this approach may not always work, says Kenichiro Shimatani of Michigan State University in East Lansing. The most powerful method of measuring genetic diversity is known as the “nucleotide diversity index”. “This is almost always used if DNA sequences of about more than 20 individuals in a population are available,” says Shimatani. The nucleotide diversity index averages the differences between sequences of nucleotides—the four molecular building blocks that make up DNA—in the genomes of individuals within a given population. Scientists believed that adding new individuals to a group would always increase its nucleotide diversity, says Shimatani. “However, I have found examples where it may increase or decrease depending on the situation.” Shimatani set up a model of a population of animals living on an island. He showed that adding new animals with moderately different DNA sequences can reduce the overall nucleotide diversity. This happens in the same way that adding a few trees of medium height to an imaginary forest full of mature oaks and saplings may lower the average height of the trees. Shimatani accepts that new individuals generally increase the genetic health of a population. But he says the nucleotide diversity index has its limitations, and advises against using it as “a panacea for measuring genetic diversity”. And in some situations,