Environmental Engineering Reference
In-Depth Information
Whenever we want to derive an ultimate answer to a functional ques-
tion we need to appeal to an evolutionary process. The one used in this
topic is the principle of adaptive evolution by natural selection. Let's now
apply it to see how different species might have come to tolerate certain en-
vironmental conditions.
We begin with a rabbit species that lives in semi-desert shrub land
ecosystem at middle to southern latitudes. Hot temperatures and high lev-
els of direct solar radiation typically characterize this life zone. Let's assume
that individuals of this species live there because this environment offers op-
timal climatic conditions for that species. Individuals of this rabbit species
tend to be very large (about the size of a beagle dog); they have long ears
and long, gangly legs and a very thin fur coat.
Suppose that the region providing optimal conditions is crowded so that
individuals residing there compete fairly intensely for resources that are im-
portant to fitness (e.g., vegetation). In such a case, some individuals (less
competitive ones) may be preempted from getting enough resources to sup-
port their survival needs. Suppose that these individuals are forced to relo-
cate to more northerly, cooler conditions—otherwise they would die. In the
new locations, they live under less than optimal conditions.These individ-
uals may survive and reproduce at these new locations, but not to the same
extent as their counterparts that live under optimal conditions. Indeed, in-
dividuals that live under extreme temperature and solar radiation conditions
that push their tolerance limits may not survive at all. One reason that those
individuals might not live is that their large bodies and thin coats result in
a high surface area to body volume ratio with limited insulation. In addi-
tion, traits—long ears and gangly legs—that allow this species to cope well
in hot conditions by rapidly dissipating heat now become a penalty because
they exacerbate heat loss in an environment where it would be better to
conserve heat.
Now, individuals in a population do not have identical body structure.
There is variety in populations. Suppose that, as part of the variety, there
were individuals living at the cooler margin that had a somewhat smaller
body size and smaller ears and legs and a slightly thicker fur coat.These in-
dividuals would not lose body heat as rapidly as their larger, gangly coun-
terparts. Essentially, we now have two strategies in the same population
vying for existence at the cooler margin. However, individuals with the
smaller body plan have a distinct advantage because they are able to con-
serve heat better than larger individuals. Suppose that this energy saving can
