Fstis unity if there is no migration and very small if there is much migration. Thus,
Fstis a measure of isolation. For example, statistics from DNA fingerprinting and mtDNA
indicated little genetic differentiation between populations of mule deer on the north
and south rims of the Grand Canyon, Arizona (Travis and Keim 1995). Genetic dif-
ferences between populations can be of three forms:
1 There are no differences with distance between populations, suggesting high gene
flow and random mating (panmixia).
2 There is a positive linear relationship between genetic divergence and distance. This
suggests that the species distribution is sufficiently wide that individuals do not move
across the whole range, gene flow is restricted, and local adaptation occurs. This is
called isolation-by-distance. For example, the genetic distance of coconut crabs (Birgus
latro) on the Pacific islands shows such a relationship (Moritz and Lavery 1996).
The greenish warbler (Phyloscopus trochiloides) in Asia also shows a clear trend in
genetic divergence with distance (see below) (Irwin et al. 2005).
3 A population may have a genetic divergence greater than that predicted by dis-
tance, suggesting that it has been cut off from the rest of the population for a long
period. It is a genetic island, it reflects the effects of time (i.e. history), and so this
is called the island model. Thus, coconut crabs on Christmas Island in the Indian
Ocean show a greater genetic divergence than expected by distance relative to crabs
on the Pacific islands (Moritz and Lavery 1996).
Populations whose individuals interbreed freely (a panmicticpopulation) will have
similar allele frequencies throughout their range. Individuals should be adapted to
the average conditions of the range, ideally those of the center. At the edge of the
range where conditions are extreme, the individuals should be less well adapted; they
cannot develop local adaptations to these edge conditions because any tendency towards
genetic divergence is swamped by gene flow from the center. However, if gene flow
through a population is slow relative to the rate of local genetic adaptation, the
individuals at one end of the range can differ from those at the other end, with
intermediate forms between (the isolation-by-distance model). This gradual trend in
appearance or behavior is called a cline. A good example of this is provided byCervus
elaphus(Fig. 3.6). In Europe it is called red deer and is a relatively small dark
animal. Males produce a deep-throated roar during the rut. At the other end of its
range in North America the same species is called elk. Here it is larger, lighter
colored, and the mating call of males is a high-pitched whistle or “bugle.” Forms
intermediate in both morphology and behavior occur across Asia.
The range of some species extends around the world. Where the two ends meet
the animals have diverged sufficiently such that individuals no longer interbreed and
they behave as separate species. These are called ring species. The classic example is
the black-backed gull / herring gull pair. In Europe the herring gull (Larus argentatus)
is light gray on the upper surface of its wings and back, but these parts gradually
become darker through Asia and North America so that they are entirely black on
the eastern seaboard of the USA. This form has crossed the Atlantic and lives in Europe
as the black-backed gull (L.fuscus) without interbreeding with the herring gull: it
behaves as a separate species. An elegantly documented example of a ring species
by Irwin and co-workers is that of the greenish warbler, a small insectivorous bird
of forests in central Asia (Irwin 2000; Irwin et al. 2001a). The parent population
exists near the Himalayan mountains. Two branches of this population have spread
around the Tibetan plateau, one northwest in western Russia and northern Europe,
brent
(Brent)
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