Travel Reference
In-Depth Information
development or behavior, such that the hybrids may not survive or may be
sterile. This isolated group of lemurs, which by this time may look and behave
dif erently from other lemurs, has now achieved the status of a new species.
Biologists defi ne species as groups of organisms that carry sets of genes
that prevent them from exchanging genes with other groups. After its long
period of isolation, our group of lemurs has diverged so far from the others
that it fi ts the biological defi nition of a species.
This type of speciation is called
allopatric
or “other country,” because it
results from isolation. Allopatric speciation may take a long time. But, sur-
prisingly, speciation may speed up when the population that will become the
new species is not completely isolated, so that there is still some fraterniza-
tion between it and other groups.
Recall the Evolution Canyon in northern Israel that we explored in the
second chapter. There we saw that if selection is strong enough it can main-
tain genetic dif erences between members of a species on the two sides of the
canyon, even when there is substantial gene fl ow between them.
Something similar probably happened in the Ranomafana rainforest. As
two groups of lemurs began to specialize on dif erent food sources, hybrids
between them were poorly adapted to utilize either source. The hybrids would
not have survived, or would have had few of spring of their own. Hybrids
between diverging groups of bamboo lemurs in the Ranomafana forest were
at an especially great disadvantage, because the available food could be poten-
tially lethal to them. They might have been tempted to eat bits of bamboo that
carried a greater load of cyanide than their bodies could withstand.
When hybrids are unfi t there will be strong selection for genetic
isolating
mechanisms
that tend to prevent members of the two groups from hybridizing
in the fi rst place. For example, there may be selection for dif erent courting
and mating behaviors. Because of these dif erent behaviors, members of one
group will tend to ignore those of another. The lemurs that do not seek mates
across the boundary of their group will be at a great advantage, because none
of their of spring will be unfi t hybrids.
As alleles for these isolating mechanisms are selected for and accumu-
late in the groups' gene pools, the groups will no longer be able to exchange
genes, even though in theory they could. And, like our physically isolated
