Survival of the "fit enough"
There are many reasons why natural
selection may not produce a "perfectly-engineered" trait. For
example, you might imagine that cheetahs would be more fit (produce
more offspring) if they could run just a little faster and catch
more prey. Here are a few reasons why natural selection might not
produce perfection or faster cheetahs:
- Lack of necessary genetic variation
Selection can only operate on the available genetic variation.
A cheetah might run faster if it had "faster genes" but
if "faster" genes are not in the population from mutation
or gene flow, evolution in
this direction will not happen. A "faster" cheetah might
evolve if the fastest cheetah in this population passed its "faster" genes
to its offspring.
- Constraints due to history
Perhaps a different arrangement of leg muscles and bones would
produce cheetahs that run faster however, the basic body
form of mammals is already laid out in their genes
and development in
such a mutually constrained way, that it is unlikely to be altered.
There really may be "no way to get there from here."
Changing one feature for the better might change another for the
worse. Perhaps "faster genes" are in the population but
there is a trade-off associated with them: running faster for
short distances means the cheetah's metabolism requires even
more energy or that the cheetah's legs must become hazardously
delicate. Although longer limb bones increase stride, their chances of failing
due to bending loads increases as well. In this case, perhaps
it would get no net increase in fitness as
a result of the "faster genes."