National Geographic Magazine
July 1992, pp 122-136
by Stephen J. O'Brien
Around the world, wildlife populations are shrinking into fragmented islands amid a sea of human expansion. Only 30 to 50 Florida Panthers cling to survival. Before a captive-breeding program began, the Black-Footed Ferret was down to 17 animals. In India fewer than 250 Asiatic Lions remain.
Before conducting genetic studies for Craig Packer's Lion project, I examined the genetic history of the Cheetah, whose range once spanned the globe. I was amazed to find that every one of today's 20,000 Cheetahs is genetically almost identical. They descend from survivors of a near-extinction catastrophe that resulted in generations of close inbreeding 10,000 years ago.
These and other species share something important with the Ngorongoro Lions -- a population bottleneck. It creates a shrinking gene pool that leaves fewer and fewer mating partners. What are the genetic implications?
The animals become part of a high stakes poker game -- with a crooked dealer. After
beginning with a 52-card deck, the players wind up with, say, five cards that they are
dealt over and over.
Ominous telltales, sperm from crater males (middle and right) show abnormalities when compared with a normal sample. Reproductive physiologist David Wildt and his colleagues at Washington's National Zoo found structural deformities in more than half the sperm of each male tested, strong evidence of inbreeding. The continuous decline of genetic diversity since 1969 is perhaps linked to a falling reproductive rate.
photo credits: David Wildt and Jo Gayle Howard,
As they begin to inbreed, congenital effects appear, both physical and reproductive.
Often abnomral sperm increase; infertility rises; the birthrate falls. Most perilous in the
long run, each animal's immune defense system is weakened.|
Thus, even if an endangered species in a bottleneck can withstand whatever human
development may be eating away at its habitat, it still faces the threat of an epidemic
that could well be fatal to the entire population.