Encyclopedia of Evolution.pdf - Online Reading Center

founder effect
Further Reading
Fortey, Richard. Earth: An Intimate History. New York: Knopf,
2004.
Gould, Stephen Jay. “Magnolias from Moscow.” Chap. 31 in Dinosaur
in a Haystack: Reflections in Natural History. New York:
Harmony, 1995.
———. “The upwardly mobile fossils of Leonardo’s living earth.”
Chap 1. in Leonardo’s Mountain of Clams and the Diet of
Worms. New York: Three Rivers, 1998.
———. “The lying stones of Marrakech.” Chap. 1 in The Lying
Stones of Marrakech: Penultimate Reflections in Natural History.
New York: Harmony, 2000.
Niklas, Karl J., Robert M. Brown, Jr., and R. Santos. “Ultrastructural
states of preservation in Clarkia Angiosperm leaf tissues: Implications
on modes of fossilization in late Cenozoic history of the
Pacific.” Pages 143–159 in Smiley, Charles J., ed., Late Cenozoic
History of the Pacific. San Francisco: American Association for
the Advancement of Science, 1985.
Palmer, Douglas. Fossil Revolutions: The Finds That Changed Our
View of the Past. New York: Collins, 2004.
Prasad, Vandana, et al. “Dinosaur coprolites and the early evolution
of grasses and grazers.” Science 310 (2005): 1,177–1,180.
founder effect The founder effect may occur when a population
is founded (for example, on an island) by a small
number of emigrants that disperse from a larger, central
population. A small number of founders will not carry all of
the genetic diversity that was present in the central population.
Consider, for example, a central population that contains
individual plants with red and with white flowers. A few
seeds disperse to an island, but they happen to be seeds of redflowered
individuals. From the moment that the island population
is founded, it already has lower genetic diversity than
the population that produced it. It is unlikely that red flowers
confer any advantage on the island population; the fact that
the island population has no white-flowered plants is due to
chance. The founder effect can result in a population that,
even if it grows, remains depauperate in genetic diversity.
The founder effect is similar to the phenomenon of
genetic drift. In genetic drift, a population undergoes a “bottleneck”
event in which the population is reduced to a small
number, perhaps by a disaster. In a small population, just
by chance, some of the genes may be lost. Consider another
flower example in which the population, consisting both of
red-flowered and white-flowered individuals, experiences a
population crash. The red-flowered individuals may be the
only ones to survive, but their survival had nothing to do
with the color of their flowers. By chance, the genes for white
flowers have been lost. Genetic drift can almost permanently
reduce the genetic diversity in a population. When the population
begins to grow, it remains depauperate in genetic diversity.
Both the founder effect and genetic drift reduce genetic
diversity in a population: the founder effect, when a new
population is formed, and genetic drift, when the old population
suffers a bottleneck event. Both the founder effect and
genetic drift were investigated by geneticist Sewall Wright
(see Wright, Sewall).
Genetic drift has been experimentally confirmed. In one
experiment with fruit flies, geneticist Peter Buri investigated
a gene with two alleles (see Mendelian genetics). At the
beginning of each experiment, all of the flies were heterozygous,
therefore both of the alleles were equally common.
Every generation, Buri chose a small sample of males and
females, from which he raised the next generation. Every generation
of flies experienced a genetic bottleneck. By the 19th
generation, in over half of the populations, one or the other
of the two alleles became more and more common until it
was the only allele that was present.
Genetic drift can leave its mark on a population for
many generations afterward (see population genetics). One
in 20 humans who live on Pingelap Atoll in the Pacific Ocean
have a mutation that causes color blindness and extreme light
sensitivity. The usual population average for this mutation is
one in 20,000 people. The reason the mutation is so common
on Pingelap Atoll is that it was carried by one of the few survivors
of a typhoon and famine that struck in 1776.
Genetic drift can affect entire species. Cheetah populations
have a reduced genetic diversity due to a bottleneck
event in the past. The Hawaiian goose, or nene, also experienced
a genetic bottleneck which greatly reduced the genetic
variability in its populations. Both species, principally for this
reason, face extinction. Even when a species consists of a
large number of individuals, the fragmentation of the species
into many small populations can cause genetic drift to occur
in each of the small populations, resulting in the catastrophic
loss of genetic diversity. This is occurring in many natural
habitats, which are being set aside as only small, disconnected
nature preserves.
Humans provide one of the best examples of a genetic
bottleneck. The human species has a remarkably low genetic
diversity. Geneticists estimate that the entire human species
experienced a genetic bottleneck event about 70,000 years
ago, at which time the entire human species consisted of only
a few thousand individuals.
Further Reading
Buri, Peter. “Gene frequencies in small populations of mutant Drosophila.”
Evolution 10 (1956): 367–402.
Templeton, Alan, et al. “The genetic consequences of habitat fragmentation.”
Annals of the Missouri Botanical Garden 77 (1990):
13–27.
Young, Andrew, et al. “The population genetic consequences of habitat
fragmentation for plants.” Trends in Ecology and Evolution
11 (1996): 413–418.
fungi See eukaryotes, evolution of.