|
The central argument of the theory of natural selection is
summarized by Darwin in The Origin of Species as follows:
As more individuals are produced than can possibly survive,
there must in every case be a struggle for existence, either
one individual with another of the same species, or with the
individuals of distinct species, or with the physical conditions
of life. ... Can it, then, be thought improbable, seeing that
variations useful to man have undoubtedly occurred, that other
variations useful in some way to each being in the great and
complex battle of life, should sometimes occur in the course
of thousands of generations? If such do occur, can we doubt (remembering
that more individuals are born than can possibly survive) that
individuals having any advantage, however slight, over others,
would have the best chance of surviving and of procreating their
kind? On the other hand, we may feel sure that any variation
in the least degree injurious would be rigidly destroyed. This
preservation of favorable variation and the rejection of injurious
variations, I call Natural Selection.
Darwin's argument addresses the problem of explaining the adaptive
character of organisms. Darwin argues that adaptive variations
("variations useful in some way to each being") occasionally
appear, and that these are likely to increase the reproductive
chances of their carriers. Over the generations favorable variations
will be preserved, injurious ones will be eliminated. In one place,
Darwin adds: "I can see no limit to this power [natural selection]
in slowly and beautifully adapting each form to the most complex
relations of life." Natural selection was proposed by Darwin
primarily to account for the adaptive organization, or "design,"
of living beings; it is a process that promotes or maintains adaptation.
Evolutionary change through time and evolutionary diversification
(multiplication of species) are not directly promoted by natural
selection (hence, the so-called "evolutionary stasis,"
the numerous examples of organisms with morphology that has changed
little, if at all, for millions of years, as pointed out by the
proponents of the theory of punctuated equilibrium). But change
and diversification often ensue as by-products of natural selection
fostering adaptation.
Darwin formulated natural selection primarily as differential
survival. The modern understanding of the principle of natural
selection is formulated in genetic and statistical terms as differential
reproduction. Natural selection implies that some genes and genetic
combinations are transmitted to the following generations on the
average more frequently than their alternates. Such genetic units
will become more common in every subsequent generation and their
alternates less common. Natural selection is a statistical bias
in the relative rate of reproduction of alternative genetic units.
Natural selection has been compared to a sieve which retains
the rarely arising useful genes and lets go the more frequently
arising harmful mutants. Natural selection acts in that way, but
it is much more than a purely negative process, for it is able
to generate novelty by increasing the probability of otherwise
extremely improbable genetic combinations. Natural selection is
thus creative in a way. It does not "create" the entities
upon which it operates, but it produces adaptive genetic combinations
which would not have existed otherwise.
The creative role of natural selection must not be understood
in the sense of the "absolute" creation that traditional
Christian theology predicates of the Divine act by which the universe
was brought into being ex nihilo. Natural selection may rather
be compared to a painter which creates a picture by mixing and
distributing pigments in various ways over the canvas. The canvas
and the pigments are not created by the artist but the painting
is. It is conceivable that a random combination of the pigments
might result in the orderly whole which is the final work of art.
But the probability of Leonardo's Mona Lisa resulting from a random
combination of pigments, or St. Peter's Basilica resulting from
a random association of marble, bricks and other materials, is
infinitely small. In the same way, the combination of genetic
units which carries the hereditary information responsible for
the formation of the vertebrate eye could have never been produced
by a random process like mutation. Not even if we allow for the
three billion years plus during which life has existed on earth.
The complicated anatomy of the eye like the exact functioning
of the kidney are the result of a nonrandom process—natural
selection.
Email
link | Printer-friendly | Feedback
| Contributed by: Dr. Francisco Ayala
|
