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

Date: Wed Aug 29 2001 - 10:36:03 BST

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    Subject: Re: Dawkins & Convergent Evolution- the final word (?)
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    On 28 Aug 2001, at 23:22, Dace wrote:

    > 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.
    Precisely due to the similarity of the selectional environmental
    pressures and opportunities with which they are presented.
    > 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.
    A correct way to phrase this would be divergence followed by
    convergence, both environmentally selectionally shaped.
    > 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.
    Except for superficial changes, such as pigmentation and hair
    texture, not much has changed, or had much time to, evolutionarily
    speaking. Here we are most definitely talking (geologically recent)
    common ancestor.
    > 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.
    Actually, natural, selection is that which renders it reasonable, and
    ther are NO examples in which naturale selection can be conceived
    to play no role, since it is as universally ubiquitous a process as is
    random mutation.
    > > <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.)
    Perhaps his rejected explanation was not the right one, yet a right
    one existed, such as minimizing evaporation water loss.
    > 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.)
    The wind does tend to carry butterflies all over, away from where
    such coloration evolved and was useful. Give 'em a couple of
    million years of the coloration being of no use, and more useful
    coloration will more likely emerge.
    > 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'd bet that these lakes are close to each other and that they either
    were once connected or that humans or animals have transported
    the cichlids, or their eggs, between lakes.
    > I'm looking for documentation of more examples.
    > Ted
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