Inbreeding's Kiss of Death

Scientists have found a strong link between inbreeding--mating with first cousins and other close kin--and whether a small, isolated butterfly population went extinct. The finding, reported in tomorrow's Nature, bolsters the idea that genetic diversity must be considered when drawing up plans to protect endangered species.

The issue of whether a meager gene pool can lead to extinction in already fragmented populations has provoked "a hell of a lot of controversy," says Richard Frankham of Macquarie University in Australia. Although some biologists have argued for the power of inbreeding, a persuasive argument of late, he says, has been that climatic events and random fluctuations in population size are far more important in the wild.

But that's not the conclusion suggested by new data from Finland's Aland Islands, home to a Glanville fritillary butterfly "metapopulation"--many small, fragmented populations transiently connected when individuals fly between them. To see whether genetic diversity plays a role in extinction, a team led by population biologists Ilik Saccheri and Ilkka Hanski of the University of Helsinki in 1996 collected adult females from 42 populations and analyzed seven of their enzymes and one genomic DNA section for variants. After watching seven populations wink out in the last year, the team found that inbreeding accounted for as much as 26% of the differences from population to population in extinction rates. This link held up after ecological factors that also influence extinction, such as weather and habitat size, were taken into account.

The study is "as close as you'll get to direct evidence" that inbreeding figures in extinction, Frankham says. The findings, he and others say, suggest that wildlife managers should focus scarce resources on those threatened populations with larger gene pools within a species. Says Frankham, "This is going to be absolutely critical as we deal with fragmented populations."