dubois@uwmacc.UUCP (Paul DuBois) (07/31/85)
I got this message from Paul Dolber in the mail, and he said I
could post it if I wished. I think it is worthwhile, so here it
is:
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Dear Paul,
From: Wilson, Edward 0. 1978. On Human Nature. Harvard U Press,
Cambridge, MA, pp. 32-33.
"Adaptiveness means simply that if an individual displayed the traits
[never mind which ones] he stood a greater chance of having his genes
represented in the next generation than if he did not display the
traits. The differential advantage among individuals in this
strictest sense is called genetic fitness. There are three basic
components of genetic fitness: increased personal survival, increased
personal reproduction, and the enhanced survival and reproduction of
close relatives who share the same genes by common descent. An
improvement in any one of the factors or in any combination of them
results in greater genetic fitness. The process, which Darwin called
natural selection, describes a tight circle of causation. If the
possession of certain genes predisposes individuals toward a particular
trait... and the trait in turn conveys superior [genetic] fitness,
the genes will gain an increased representation in the next generation.
If natural selection is continued over many generations, the favored
genes will spread throughout the population, and the trait will become
characteristic of the species."
Note that, except in one instance where I made the insertion, the
expression is "genetic fitness," not fitness.
I agree with you that Rich's comment, as nearly as I can recall it,
was rather obvious: "Natural selection predicts that organisms
best able to survive will survive; only survivors can procreate."
There's no prediction there. Mike's comment, as nearly as I can
recall it, in essence finished the thought: "Under any given set
of conditions, the population as a whole will come more and more
to resemble those individuals which reproduce the most." Not
profound, but at least not silly. "Fitness" is somewhat confusing,
particularly in combination with "survive;" one ought to speak
instead of "genetic fitness," i.e., the ability to survive long
enough to produce many offspring who will also have the ability
to survive long enough to produce many offspring, and so on. In
your hypothetical example, the slow-reproducing population may have
been fitter in some scheme -- e.g., they may have better education,
better jobs, more money, and an appreciation for Honegger's
symphonies -- but the simple fact is that, if they reproduce less,
their *genetic fitness* is lower and the future population will
resemble them less and less.
One may now ask whether natural selection can make more interesting
predictions. Dawkins (in "The Selfish Gene") covers some of this
ground vis-a-vis reproductive strategies in insect societies, but
it was so complex that I can't shoot it back to you sensibly in less
than a megaline. So, let's try the following example from Wilson's
book, dealing with work done by Trivers and Willard:
1) In vertebrates (especially birds and mammals), large, healthy
males mate at a relatively high frequency while many smaller,
weaker males do not mate at all.
2) On the other hand, nearly all females mate successfully.
3) Females in the best physical condition produce the healthiest
infants, and these offspring usually grow up to be the
largest, most vigorous adults.
Now, suppose we are considering the females. According to evolution,
natural selection, genetic fitness, et al., what is the best
reproductive strategy for females to get the greatest number of
offspring who will successfully reproduce, thus maximizing the
contribution of their genes to the population? Well, if the
offspring of the female are decreasingly healthy as the female's
physical condition deteriorates, then it would seem best to have
the good-times offspring (which are healthiest) to be males more
often than bad-times offspring (since these less healthy offspring
would be equally likely as good-times offspring to mate successfully
if female, less likely to mate successfully if male).
"According to natural-selection theory, genes that induce this
reproductive strategy will spread through the population at the
expense of genes that promote alternative strategies. It works.
In deer and human beings, two of the species investigated with
reference to this particular question, environmental conditions
adverse for pregnant females are associated with a disproportionate
increase in the birth of daughters. Data from mink, pigs, sheep
and seals also appear to be consistent with the Trivers-Willard
prediction. The most likely direct mechanism is the selectively
greater mortality of male fetuses under adversity, a phenomenon
that has been documented in numerous species of mammals."
I'm only just now reading this book, and I'm not entirely
comfortable with it yet; such scenarios are sufficiently complicated
that it becomes hard to separate the antecedents from the
consequences, to tell whether the outcome is being predicted from
the givens, or the givens from the outcome -- or whether this makes
any difference. Perhaps this will pass with further reading/thought;
in any case, I recommend the book to you. Dawkins's examples involving
reproductive strategies in insects are even more complicated, but
in both presentations it is noteworthy that the investigators whose
work is being discussed first made predictions based on natural
selection, *then* went and checked on whether or not the predictions
were accurate. And again, note that it is "genetic fitness" which
is under discussion: In the above, the progeny in good times were
males more often than in bad times not because females are "fitter"
than males, but because the genes producing this effect ensure their
greater representation in the gene pool.
If you find this particularly enlightening, despite being written on
the fly, feel free to forward it to net.origins. Otherwise, don't
bother, and I'll try to cover the Trivers-Willard business myself
later in a more cogent form. While some material which is not in
quotation marks is more-or-less quoted, I take full responsibility
for any errors or misrepresentations in those parts not in quotation
marks.
Regards and best wishes, Paul.
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Concerning the last paragraph: even though I *have* posted this,
I (and I suspect others) would be interested in more on Trivers-Willard.
--
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Paul DuBois {allegra,ihnp4,seismo}!uwvax!uwmacc!dubois --+--
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"Why are you standing on one leg?" |
"I'm trying to see if I'm a stork."