Received: by alpheratz.cpm.aca.mmu.ac.uk id HAA11910 (8.6.9/5.3[ref pg@gmsl.co.uk] for cpm.aca.mmu.ac.uk from fmb-bounces@mmu.ac.uk); Wed, 29 Aug 2001 07:24:08 +0100 Message-ID: <002d01c13052$fbdb7680$8fdab3d1@teddace> From: "Dace" <edace@earthlink.net> To: <memetics@mmu.ac.uk> References: <2D1C159B783DD211808A006008062D310174604E@inchna.stir.ac.uk> Subject: Re: Dawkins & Convergent Evolution- the final word (?) Date: Tue, 28 Aug 2001 23:22:27 -0700 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 5.50.4133.2400 X-MimeOLE: Produced By Microsoft MimeOLE V5.50.4133.2400 Sender: fmb-bounces@mmu.ac.uk Precedence: bulk Reply-To: memetics@mmu.ac.uk
Vincent,
> <Dawkins does concede that evolutionary convergence is "vanishingly
> > improbable" in the neo-Darwinian model. >
> >
> No, he doesn't. What he's saying is that the probability that two
> organisms should follow exactly the same evolutionary steps- i.e. the
exact
> same genetic mutations and selections- is vanishingly small. That is NOT
> what happens in convergent evolution (if it did happen it might be called
> something like parallel evolution- two organisms developing in isolation
> from each other and yet following exactly the same evolutionary
> development). In convergent evolution what happens is that consonant
> environmental pressures over a long period of time produce organisms that
> may look superficially similar, and/or exhibit similar patterns of
> behaviour, despite having followed DIFFERENT paths of evolutionary
> development. Even without looking at their genes, the fact that they've
> followed different paths is evident in their differences- the best
> collective example being marsupials.
Of course there are differences. Why shouldn't there be? They're different
species in different locations. But they're also profoundly similar. This
is perhaps a better example of parallel evolution (a term now in wide
currency), since the starting points for these ant-eaters, moles, flying
squirrels, cats, wolves, mice, jerboas, etc., were the same tree shrews that
diverged 110 million years ago into marsupials and placentals. Parallel
evolution is quite common and crops up in human origins. Witness the
incredibly similar development of Eurasian Homo sapiens (Neanderthal) and
the African model.
I do understand the point you're making about Dawkins. He applies the
phrase "vanishingly improbable" not to convergence as it actually occurs but
to some sort of imaginary, ideal convergence. In other words, he's saying
nothing, which is why the passage is so confusing. In fact, convergence as
it actually occurs is vanishingly improbable even when natural selection is
involved. And there are many examples in which natural selection plays no
conceivable role.
> <As I stated before, there are numerous
> > examples of convergence with no explanation according to natural
> > selection,
> > such as traits that come in handy in relation to a predator that's never
> > existed in other locations where it crops up.>
> >
> Well, I'd like some examples here, since most that we know of
> species moving from one environment to another is usually chaos as native
> species are unable to deal with the intruders (again, Australasia with
it's
> cane toad and rabbit plagues etc. etc.).
F. W. Went studied convergence among shrubs in New Zealand. He found about
50 species of shrubs that had independently developed the same pattern of
"interlaced, tortuous branches and reduced leaves." This would presumably
have protected them from herbivores. But there are no herbivores native to
New Zealand. Moreover, this pattern "occurs in so many shrubs from
different habitats, it does not seem to be an adaptation to the
environment." He provides several other examples of convergence which he
contends cannot be explained by natural selection. He argues that the
chromosomes associated with these traits must have somehow hopped across
species. (Went, "Parallel Evolution," Taxon 20:197-226, 1971.)
This unlikely explanation can't account for convergence across widely
separated areas. Bernhard Rensch studied butterflies with similar color
patterns on their wings. Some of these could be explained by mimicry of
butterflies avoided by predators. But often these similar patterns appear
on butterflies in completely different places in which such mimicry would be
useless. (Rensch, Evolution Above the Species Level, Methuen, 1959.)
In the February 1999 issue of Scientific American, Stiassny and Meyer
discuss the inexplicable similarity of color patterns on the scales of
cichlids in separate African lakes.
I'm looking for documentation of more examples.
Ted
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