Encyclopedia of Evolution.pdf - Online Reading Center

0 extinction
past (a genetic bottleneck; see founder effect) can condemn
a population to reduced survival ability even if circumstances
should become favorable to its growth.
An example of a species that is near extinction because
of a genetic bottleneck is the nene, or Hawaiian goose (Nesochen
sandiviensis). Due to the efforts of conservation workers
and public cooperation, populations of the nene (the Hawaiian
state bird) are growing, following its near extinction in the first
half of the 20th century. DNA studies that compare modern
birds with birds preserved in museums indicate that modern
populations of the nene have much lower genetic diversity than
did populations in the 19th century. Many goslings, whether
hatched in captivity or in the wild, have birth defects, and many
of them die. Small, isolated human populations also exhibit
traits that remain hidden in larger populations, for example
extra fingers and toes (polydactyly) and metabolic disorders.
Saving species from extinction can provide a tremendous
benefit to science and to human well-being. Plant
breeders, for example, wish to introduce genes for drought
resistance into maize (Zea mays; also called corn) raised in
the United States. Drought resistance genes are available
in Tripsacum grass, but Tripsacum cannot crossbreed with
maize. A wild species of perennial maize, Zea diploperennis,
can crossbreed with both Tripsacum and Zea mays and can
thereby act as the bridge for introducing Tripsacum genes
into agricultural maize populations. Zea diploperennis was
almost extinct when botanist Hugh Iltis discovered it in the
20th century.
Conservationists frequently say that “extinction is forever.”
This statement is obviously true for a species—once it
is gone it cannot come back—but is also true of the genetic
diversity within a species.
Further Reading
Cardillo, Marcel, et al. “Multiple causes of high extinction risk in
large mammal species.” Science 309 (2005): 1,239–1,241.