|
|
Re: Evolution, "adaptation", and what's currently adaptive
Stephen Barnard (steve@megafauna.com)
Sat, 31 Aug 1996 02:12:47 -0800
Susan S. Chin wrote:
>
> Lenny, Bryant -
>
> I've been reading your discussion here with some interest, and I'd like to
> offer my point of view on this. Instead of two POVs, we'll likely end up
> with three when I'm done.
>
> I have not read the Gould & Lewontin article referred to in this thread,
> but am fairly familiar with Gould's work, and his ideas about evolution.
>
> My understanding of Gould's critique of adaptationists can be related to
> his concept, presented with Elisabeth Vrba in 1982, of "exaptations."
> Anyone familiar with this concept knows that it refers to characters which
> currently may serve an "adaptive" function, but did not arise and evolve
> via natural selection for the current utility. Their point being that
> just because something is functionally significant today, does not mean
> that the feature originated and was shaped by selective pressures, or
> "natural selection." Conversely, features which today may serve no
> apparent purpose or usefulness, and would therefore be expected to be
> lost due to selection pressures against retaining such traits, are
> somehow still around. Gould's view of exaptations nicely "solves" this
> problem by stating that characters which may appear useless can serve a
> function in the future, as a feature "co-opted" for a previously
> non-existant need. This concept of exaptations also works well for
> features which previously evolved for one purpose and now serves another,
> or several uses which optimize fitness. They would consider the original
> use the "adapted" feature, the new or added utilities of a feature as
> "exaptations."
I very much doubt that Bryant would find much, if anything, to quarrel about with
the "exaptation" notion. This is actually a pure adaptationist idea.
My problem with Gould and Lewontin is that their argument proceeds by carefully
setting up a straw man (The Panglossian adaptationist), and then knocking the
stuffing out it. Their paper has been seized upon by many people who, for one
reason or another, don't want to accept the notion of evolution through natural
selection. "Look, even the famous Stephen J. Gould says that evolution is bunk!"
>
> Gould & Vrba placed less emphasis on natural selection and the resultant
> adaptations to explain the evolution of features, and instead with their
> concept of exaptations, placed more emphasis on historical constraints of
> all organisms ("evolutionary baggage") and the evolution of characters
> from what is already "there" rather than producing something from scratch
> as an adaptationist approach might.
"Producing something from scratch" is *not* an explanation that a typical
adaptationist would make. It *may* be an explanation that a naive adaptationist
would make. The role of historical constraints fits very nicely within the
adpatationist framework.
>
> I don't know if that helps either side of the discussion, but the concept
> of exaptations does eliminate the need to explain every currently
> "adaptive" (functionally useful) feature as having historically originated
> and evolved due to selection pressures for that current use. Theirs is a more
> narrowly delineated concept of what an adaptation is. Just because
> something is adaptive today, doesn't necessarily mean it evolved via
> natural selection for that reason. When it comes to features, it's not
> necessary to explain all functionally significant characters as products
> of natural selection in Gould & Vrba's more narrowly defined
> adaptationist view.
>
First, exaptation is far from the universal paradigm of evolution, no matter how
much the self-aggrandizing Gould would like it to be. Second, Bryant is not
claiming that "just because something is adaptive today, [it must have] evolved
via natural selection for that reason." He has very specifically said that he is
not claiming that.
Steve Barnard
|