鈥淕HOST鈥� genes found in old museum specimens have provided the first clear
proof from wild populations that genetic diversity dwindles when a once-common
organism becomes rare.
Conservationists have always assumed that past population crashes are the
reason why some species鈥攕uch as cheetahs and northern elephant
seals鈥攕how little genetic variation among individuals. This genetic
uniformity may put species at greater risk of extinction.
However, 鈥渋t鈥檚 one thing to assume, and another thing to have the direct
evidence,鈥� says Juan Bouzat of the University of Illinois at Urbana-Champaign.
Bouzat and his colleagues studied the greater prairie chicken, a large
prairie-dwelling grouse whose population has plummeted as farmers convert native
prairie to cornfields. In Illinois, its numbers have fallen from 10 million a
century ago to less than 50 today.
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The researchers extracted DNA from 15 museum specimens of prairie chickens
collected during the 1930s and 1960s鈥攂efore the worst of the population
bottleneck鈥攁nd from preserved tissue samples of 32 birds collected from an
Illinois nature reserve since 1974. They then looked in each specimen at short
stretches of DNA, called microsatellites, that tend to vary greatly in the
sequence of bases they contain.
The present-day prairie chickens averaged only 3.67 different versions, or
alleles, at each microsatellite. In contrast, the pre-bottleneck museum
specimens contained 5.12 alleles per microsatellite.
鈥淭he museum birds are showing that there was something there that is already
lost, and you will never be able to recover it,鈥� says Bouzat. Other ecological
geneticists say they believe the study is the first ever to track the loss of
genetic diversity as a population crashes.