Re: Dawkins & Convergent Evolution- the final word (?)

From: Dace (
Date: Wed Aug 29 2001 - 07:22:27 BST

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    Subject: Re: Dawkins & Convergent Evolution- the final word (?)
    Date: Tue, 28 Aug 2001 23:22:27 -0700
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    > <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
    > 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
    > 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.


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