Biology Reference
In-Depth Information
to some central questions: Why do species evolve to become habitat specialists in the first
place? How widespread is this condition of extreme habitat specialization in nature, and is it
more commonly observed in plants than in animals?
The answer to all three questions invokes evolution by natural selection. Imagine a species
with a broad habitat tolerance. Some of its members will be breeding in widely distributed
habitat types, while others will be reproducing in rare habitat types. If those individuals that
reproduce in the rare habitat are to evolve into a new species specifically adapted to that hab-
itat, two conditions must be met. First, they must become isolated from the rest of the pop-
ulation so that they breed only with one another, as we saw in the Foja Mountains of New
Guinea. Interbreeding with individuals reproducing in other habitats prevents the evolution
of habitat specialization. The second condition is that the reproductively isolated population
must persist long enough for evolutionary processes to yield a considerably different organ-
ism, that is, a new species.
For several reasons, these conditions are much more easily met with plants than with an-
imals. First, among plants whose flowers are animal pollinated and whose seeds also are dis-
seminated by animals, genes are generally transferred over only short distances. In contrast
with most animal populations, a plant population does not need to be far from others of its
species to achieve the grist of speciation—reproductive isolation. Second, whereas few ver-
tebrates can reproduce asexually, many plants can self-pollinate. This enables them to repro-
duce successfully in small, isolated areas. Third, because plants are intimately tied to the soil
for their nutrients, they more readily evolve adaptations to specific soil types. Few animals
have intimate relationships with particular soil types, and for those that do, the relationship
is with structure (ease of burrowing, chances of becoming waterlogged), not chemistry. Thus
the world abounds with plants that are restricted to specific soil types, often ones found in
only a few places. Southwestern Australia, which features rocky outcrops separated from one
another by only short distances, for example, has thousands of plant species found on only
one or a few outcrops. Even those short distances are too wide for pollen or seeds to be trans-
ferred between them.
As I rested, my gaze turned to a cluster of lovely pink lady's slipper orchids growing in the
sandy soil. Among the 250,000-425,000 vascular plants on Earth, perhaps as many as 20-30
percent can be considered extreme habitat specialists. To find many of them, though, I was
sprawled out in the wrong part of the world. Because the spot where I was reclining, together
with much of the northern forest zone, had been under a glacier 14,000 years ago, the habitats
of this region are now broadly similar and the majority of temperate northeastern US forest
species are relatively widespread.
Globally, extreme habitat specialization among plants is more prevalent in Mediterranean-
like climate regions than in, say, the temperate forest of Michigan. Aside from one such re-
gion mentioned earlier, southwestern Australia south of the city of Perth, there are four others
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