I would like to suggest that Darwin's greatest theoretical contribution in
science was the insight that the naturally-occurring modifications from one
generation to another of a species were sufficient, if extended over a
sufficiently long time, to account for all species that lived or were known
to have lived (in Darwin's day). What he saw was that the modifications
that were going on on a daily basis were part of a larger pattern of
modification, rather than merely isolated species-specific events as many
of today's creationists and other ID-supporters claim.
Selection is often mistakenly regarded as the driving force in evolution
because it provides conditions that make specific types of variations
advantageous and thus encourages those variations to become statistically
dominant in a population. But selection does not create the variations in
the first place. Selection also will tend to penalize many variations, so
it can be regarded not only as a driving force of evolution but the
thwarting force of evolution. Why? Because, if a new variation in a
population is *disadvantageous*, it gets culled out quickly.
Variation, left to itself, would tend to fill all available genetic
"space." That is, it would tend to produce some examples of every possible
genetic sequence, as modification was piled onto modification over time.
Though Darwin did not know about genes specifically, he did see that
modification of the organism from generation to generation could not be
limited without outside intervention, and that, therefore, it could
account, in principle, for all species.
Though plant breeders cannot make plums (so far) the size of watermelons,
this has more to do with internal biological viability and functionality
than it does with the ultimate limits of genetic variation. Given time and
a means of ensuring that the entire plum could receive water and nutrients,
and that it could keep from crushing itself under its own weight, we could
probably grow plums the size of even the largest watermelons. But, this is
not something that will happen in just a few generations (usually), because
it requires the modification of much more genetic material than merely
expanding a plum to the limits supportable by the existing genetic context.
Of course there *are* limits on organisms that can survive, because physics
will not be denied (regardless of how much ID supporters would like to
think it's irrelevant).
But the limits of physics do not mean that there is any barrier that would
prevent some population of apes from evolving into hominids, and that would
then prevent a population of hominids from evolving into today's humans. As
long as biological viability can be maintained in such a process, there is
no ultimate genetic barrier to such transitions.
What Darwin saw (as Mayr recently pointed out in a Scientific American
essay) was the fluidity of evolution, the *lack* of the Platonic types that
had been so much a standard aspect of how people thought about such things
(and that deeply permeates creationist/ID thinking to this day).
But, as in many other issues, Plato was wrong, and this incredibly
simplistic way of thinking simply does not work in the real, physical
world, where species boundaries (if any) are fluid and shifting and
overlapping. If species A can interbreed with species B, are they really
separate species? If so, what if species B can interbreed with species C,
but species A *cannot* interbreed with species C? If only A and C existed,
we'd call them separate species, but if B exists and can interbreed with A,
we would normally regard them as the *same* species. But, this reasoning
would make B and C also the *same* species, and thus make all three the
same species. But then, what are we to make of the fact that A and C cannot
interbreed?
What this shows is not that the concept of species is invalid, but that it
is as much a concept of cognitive method as it is of real organisms in the
world. We can salvage the concept of species in this case, but *not* the
Platonic concept of species; *that* has to go.
Creationists and other ID supporters have not yet grasped that their
fundamental way of conceptualizing such issues is simply fundamentally
*wrong*. The underlying epistemological premise of rigidly fixed types is
*false*, and therefore it simply does not work in the real world.
Creationism and most other forms of ID are therefore not merely wrong
scientifically. They are wrong, and irreparably wrong, in a fundamental
*philosophical* way, a way that *prevents* them from *ever* being
scientifically right, because the world is *blatantly* incompatible with
such a way of thinking about life and species and evolution.
This is true even if some kind of ID *is* found to be true; today's
mainstream ID theorists will be the greatest *hindrance* to finding it
because they are wearing Plato's blinders and can't think clearly otherwise.
Helping to get biological scientists to think outside this incredibly
mind-crippling epistemology was Darwin's greatest *general contribution* to
science and to human civilization generally. Sadly, typical ID theorists
still thinking in just such empirically falsified Platonistic terms, and
they show no signs of correcting their epistemological errors any time soon.
--Chris
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