Travel Reference
In-Depth Information
allopatric group of lemurs that gradually accumulated genetic dif erences
until they were unable to exchange genes with other groups, these groups
will also now fi t the biological defi nition of species. But unlike allopatric
groups these potentially interbreeding groups have been driven apart by
strong natural selection for isolating mechanisms.
Dif erent variants of this kind of speciation process have been given the
names
sympatric
or
parapatric
(“same” or “adjacent” country). This kind of spe-
ciation can happen relatively quickly, because the alleles that confer isolating
mechanisms are subject to strong natural selection.
Much of the Ranomafana forest has been destroyed by farmers and log-
gers. But before that recent wave of destruction the forest was extensive
enough to hold a wide variety of ecological niches, and each of those niches in
turn was large enough to support good-sized lemur populations. It was that
earlier forest, not the tiny remnant that exists today, that made the speciation
of the lemurs possible. When it comes to speciation, the size and complexity
of the ecosystem plays an essential role. We will explore in the next chapter
how the vast rainforest ecosystems of the tropical Americas and Southeast
Asia have been among the greatest generators of species diversity.
The adaptation of one species to an ecological niche can open up more
niches, which can be occupied by new species in turn. For example, nothing
is greeted with more enthusiasm by the roistering insects of a rainforest fl oor
than the arrival of a fresh animal turd. But golden lemur excreta, loaded with
cyanide, are of -limits to most insects. The droppings of the golden lemur are
eaten exclusively by a dung beetle that is also able to survive high levels of
cyanide. Their turds have become the private property of this highly special-
ized beetle.
3
These dung beetles have also been selected for isolating mechanisms, so
that they do not mate with beetles of other closely allied species. Hybrid of -
spring would be unable to eat the cyanide-rich droppings, and they would
not be able to utilize the droppings of other animals because other special-
ized beetle species would drive them of .
Such accumulations of nested ecological niches are characteristic of the
world's richest ecosystems. They explain these ecosystems' amazing diver-
sity, and also contribute to their fragility. If conditions change, so that a
