rising@zoo.toronto.edu (Jim Rising) (11/17/90)
Since I posted some definitions a while back, I've received some
email questioning Darwin's equation of Sexual & Natural Selection.
I thought that I remembered a specific quote in his 1871 work, but
confess that I don't find it--so I withdraw that statement.
Doug Hemken sent me an interesting note that I'm certain he would
not mind my posting, so here it is:
Subject: Re: Sexual & Natural Selection
To: rising@zoo.utoronto.ca
X-Original-To: rising@zoo.utoronto.ca, doug.hemken@bbs.acs.unc.edu, HEMKEN
It's tough going to Darwin for definitions. Many of the terms
Darwin coined and used have not only long since been subtly
reinterpreted, but also the theoretical framework in which they
originally made sense has changed. For instance, Darwin's
"pangenesis" model of inheritance has no place in contemporary
thought. And the term 'natural selection' now has a broader
meaning than I think Darwin himself gave to it.
Darwin thought of natural selection and sexual selection as two
distinct phenomena. "Sexual Selection acts in a less rigorous
manner than Natural Selection. The latter produces its effects
by the life or death at all ages of the more or less successful
individuals" ([1877]:[226]). In other words, Darwin's 'natural
selection' is equivalent to survivability. Darwin understood
that both survival and finding a suitable mate produced more
offspring, but he viewed these as two distinct selection
mechanisms, and hence as two distinct mechanisms of evolution.
In contrast, today we think of sexual selection as just one more
type of natural selection.
Where Rising quotes Darwin as saying that in most cases "it is
scarcely possible to distinguish between the effects of natural
and sexual selection" ([1877]:[210]), Darwin was actually trying
to make the point that the phenotypic characteristics
('phenotype' being a post-Darwin concept) resulting from these
two selection mechanisms are hard to sort out. I.e., given a
characteristic distinguishing the male and the female of a
species, it may be difficult to determine whether that
characteristic is the result of natural or sexual selection.
Darwin was not claiming that natural and sexual selection are
nearly indistinguishable *as selection mechanisms*, but that
their *results* might often be nearly indistinguishable.
Because we today are the intellectual heirs of both the Mendelian
model of inheritance and the Evolutionary synthesis, it is
difficult to read Darwin casually without finding meanings Darwin
probably did not intend. (First there were the Darwinians, then
the Mendelians proved the Darwinians wrong! and then the
Synthesis proved them both compatible. What a convoluted
intellectual history!) That doesn't mean we shouldn't read
Darwin, or that we can't still learn from him. But it does make him
of limited use as a *contemporary* authority.
Darwin used the term sexual selection in his 1859 _Origin of
Species_, but he felt that the effect of sexual selection *on
speciation* was insignificant, and that natural selection played
the major explanatory role in 'the species question.' In
_Descent of Man, and Selection in Relation to Sex_ (originally
two separate books), Darwin attributes the term 'secondary sexual
characters' to someone named Hunter, someone apparently well
enough known no to require a footnote.
----------------------------------------------------------------
Darwin, Charles
[1877] 1989 _The Descent of Man, and Selection in Relation to
Sex_ (second edition), vols. 21 & 22 of _The Works of Charles
Darwin_, Paul H. Barrett and R.B. Freeman, editors, London:
William Pickering
--
Name: Jim Rising
Mail: Dept. Zoology, Univ. Toronto, Toronto, Ontario, Canada M5S 1A1
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