Encyclopedia of Evolution.pdf - Online Reading Center

would readily interbreed and produce fertile offspring, if they
were in contact, to be members of the same species. An isolating
mechanism therefore represents the first step in turning
separate populations into separate species (see hybridization;
speciation).
Geographic isolation can occur as continental drift
and the formation of new mountain ranges separate populations.
Geographic isolation can also occur from local geographical
features. In a large population that covers a wide area, all
of the individuals may be potentially able to interbreed, but do
not; by interbreeding with other individuals in the same locality,
they form subpopulations or demes that are imperfectly
isolated from one another. The effectiveness of geographic isolation
between populations or between demes depends on how
much dispersal occurs between them. The human species, after
its origin about 100,000 years ago, spread all over the world
and evolved geographic and racial differences. Travel was
slow and difficult, and populations remained largely separate.
However, this geographic isolation did not last long enough
to produce different human species; all human races are able
to interbreed, and frequently do. Today, geographic isolation
of human populations has largely broken down, and some
experts predict that distinctions between human races will disappear
a few centuries from now.
As long as populations continue to exchange genes, for
example through migration of individuals between them, they
cannot begin the process of speciation. Two geographically
isolated populations will almost inevitably evolve into different
species. This may occur because environmental conditions
(either climatic or biological) are different in the two populations,
thus natural selection favors different characteristics
in each population. But even if the environmental conditions
are essentially the same for both populations, they will still
diverge, because different mutations and gene combinations
will occur by chance in each of them. It is highly unlikely that
exactly the same history of adaptive events (genetic variation
and natural selection) would happen in both isolated populations.
If the two populations come back in contact, they do not
interbreed, because isolating mechanisms have evolved separately
in each.
In many cases, natural selection favors isolating mechanisms
in the absence of geographical isolation. These isolating
mechanisms prevent the populations from wasting their
reproductive resources on crosses that would produce fewer
or inferior offspring. Because natural selection occurs within
populations, it is important to recognize that natural selection
does not favor processes that drive populations apart. Instead
it favors processes that maximize reproductive success within
the resulting populations; reproductive isolation is a result,
rather than a cause, of this process.
Prezygotic isolating mechanisms prevent interbreeding
from occurring in the first place—that is, the zygote (fertilized
egg) does not form. Postzygotic isolating mechanisms operate
after the formation of the zygote.
Prezygotic isolating mechanisms include the following
examples:
Differences in pollination mechanisms. If a mutation
occurs in a population of flowering plants that changes the
isolating mechanisms
characteristics of the flowers, the new kind of flower may not
be recognized by the same pollinator that visits the old kind
of flower.
This is apparently what happened in California populations
of two closely related species of monkeyflowers (genus
Mimulus). M. lewisii has light purple flower that attract bees,
while M. cardinalis has yellowish-red flowers that attract
hummingbirds. Phylogenetic analysis (see cladistics) indicates
that the common ancestor of the two species was pollinated
by bees. Apparently, mutations occurred that increased
the production of red pigments (anthocyanins) and yellow
pigments (carotenoids) and increased the amount of nectar
production in the lineage that became M. cardinalis. The M.
cardinalis flowers, though side by side with M. lewisii flowers,
were reproductively isolated from them. This occurred
because bees cannot see red, ignored M. cardinalis, and preferred
M. lewisii, while hummingbirds can see red and preferred
M. cardinalis.
Subsequent to the reproductive isolation, further evolutionary
changes occurred in the two monkeyflower species.
M. cardinalis evolved a long, tubular flower shape that
matched the long break and tongue of the hummingbird,
and M. lewisii retained the short, wide tubular flower shape,
with a landing platform on the front, that the bees preferred.
The changes in flower shape reinforced the initial reproductive
isolation that had occurred because of a change in flower
color and nectar production. While the change in flower color
and nectar production was sufficient to separate one species
into two, natural selection subsequently favored changes in
flower shape that enhanced the success of each species with
respect to its own pollinator. What was once one species of
monkeyflower has evolved into two, side by side.
Although nobody was there to observe the original formation
of the two monkeyflower species, evolutionary biologists
Douglas Schemske and H. D. Bradshaw have experimentally
re-created the sequence of events. They crossed the two monkeyflower
species and produced a whole range of hybrids with
intermediate shapes, nectar volumes, and colors. They also
identified DNA markers that were associated with differences
in pigment production, nectar production, and flower shape
(see quantitative trait loci). They found that, among
these hybrids, the DNA sequences that were associated with
enhanced production of the two pigments and of nectar, not
the DNA sequences associated with flower shape, were most
effective in determining which pollinator visited each flower.
Pigment and nectar production had started the reproductive
isolation, and flower shape had reinforced it.
Another example of a change in pollination mechanism
involves Schiedea salicaria, a small shrub that is native to
Hawaii, investigated by evolutionary biologists Ann Sakai and
Steve Weller. A few relatively minor genetic mutations appear
to have caused some individuals in the population to rely more
on pollination by wind, and less on pollination by insects.
These two groups can cross-pollinate but, because they rely
largely on different methods of pollination, they do not frequently
do so. The individuals that have greater wind pollination
characteristics tend to live in drier locations, where insect
pollinators are less common. Though not completely isolated