In general, as pointed out above, those features and
behaviors that are considered adaptations are explained teleologically.
This is simply because adaptations are features that come about
by natural selection.
Among alternative genetic variants that may arise by mutation
or recombination, the ones that become established in a population
are those that contribute more to the reproductive success of
their carriers. "Fitness" is the measure used by evolutionists
to quantify reproductive success. But reproductive success is
usually mediated by some function or property. Wings and hands
acquired their present configuration through long-term accumulation
of genetic variants adaptive to their carriers. How natural selection
yields adaptive features may be explained with examples where
the adaptation arises as a consequence of a single gene mutation.
One example is the presence of normal hemoglobin rather than
hemoglobin S in humans. One amino acid substitution in the beta
chain in humans results in hemoglobin molecules less efficient
for oxygen transport. The general occurrence in human populations
of normal rather than S hemoglobin is explained teleologically
by the contribution of hemoglobin to effective oxygen transport
and thus to reproductive success.
A second example, the difference between peppered-gray moths
and melanic moths is also due to one or only a few genes. The
replacement of gray moths by melanics in polluted regions is explained
teleologically by the fact that in such regions melanism decreases
the probability that a moth be eaten by a bird. The predominance
of peppered forms in nonpolluted regions is similarly explained.
Not all features of organisms need to be explained teleologically,
since not all come about as a direct result of natural selection.
Some features may become established by random genetic drift,
by chance association with adaptive traits, or in general by processes
other than natural selection. Proponents of the neutrality theory
of protein evolution argue that many alternative protein variants
are adaptively equivalent. Most evolutionists would admit that
at least in certain cases the selective differences between alternative
amino acids at a certain site in a protein must be virtually nil,
particularly when population size is very small. The presence
in a population of one amino acid sequence rather than another,
adaptively equivalent to the first, would not then be explained
teleologically. Needless to say, in such cases there would be
amino acid sequences that would not be adaptive. The presence
of an adaptive protein rather than a nonadaptive one would be
explained teleologically; but the presence of one protein rather
than another among those adaptively equivalent would not require
a teleological explanation.
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