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 genesbut
if faster genes are
not in the population
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 fasterhowever, 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 populationbut
there is a trade-off associated with them: running faster for
short distances means the cheetahs metabolism requires even
more energy or that the cheetah's legs must become hazardously
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.
Natural selection may not produce perfection, but youd at least
expect it to get rid of obviously deleterious genes, wouldnt you?
Find out more about maladapted organisms.