Evolution__3rd_Edition

388 PART 4 / Evolution and Diversity
Pollen fertility (%)
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Geographically distant populations
have often evolved isolating
barriers
Ring species are striking examples
Distance between parental strains (miles)
Figure 14.4
Postzygotic isolation between populations of the Californian
flower Streptanthus glandulosus. Flowers taken from different
populations were crossed, and produced hybrids. The amount of
good pollen produced by the hybrid offspring was measured and
expressed as a percentage of the pollen produced by the parents.
Hybrids from more distant crosses tended to be less fertile.
(50 miles ≈ 80 km.) Redrawn, by permission of the publisher,
from Kruckeberg (1957).
results show that crosses between members of nearby populations are usually fertile,
but fertility decreases for crosses between more distant populations.
Research of this kind has been mainly done with plants, and has measured
postzygotic isolation. The usual result is that some crosses show postzygotic isolation
while others do not, and that more distant populations are more strongly isolated.
Some research of this kind has also been done on animals, and on prezygotic isolation.
Korol et al. (2000), for example, showed that fruitflies from different slopes of Mount
Carmel in Israel preferentially mated with flies from their own locality. In all, we have
extensive evidence that isolating barriers tend to evolve between geographically distant
populations of a species in nature.
The members of distant populations normally have to be brought together in the lab
by an experimenter in order to measure the amount of reproductive isolation between
them. They do not usually naturally meet up. However, there is one exceptional circumstance
in which members of “distant” populations come together in nature: this is
the phenomenon of ring species (Section 3.5, p. 50). In California, the salamander
Ensatina eschscholtzii spread down from the north, and sent separate colonizing populations
down the coast and inland to the east of the central valley (Jackman & Wake
1994). These two lines of populations meet up in the south, in San Diego County,
where they are reproductively isolated and effectively two species (Wake et al. 1986).
The two reproductively isolated populations, or species, in the south are connected by a
continuously interbreeding series of populations looping round north California and
Oregon.
Ring species provide dramatic evidence that normal genetic divergence within one
species can build up to a sufficient level to generate two species. We do not see it, in
most cases, because the genetic extremes within a species are living far apart. But in a
ring species the extremes exist side by side and the resulting reproductive isolation is
directly observable in nature. Examples are rare because the populations of a species are
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