Re: Convergence

From: Chris Taylor (
Date: Thu Aug 09 2001 - 14:13:53 BST

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    From: Chris Taylor <>
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    Subject: Re: Convergence
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    > Random mutation (within the same range of genetic possibilities),
    > followed by selection by similar environments for similar niches
    > should just about do it. Notice the word 'similar'; they are not the
    > same (or they could interbreed).

    There's another part to this story; as well as convergent evolution,
    there are 'morphogenetic attractors' which species are morphologically
    drawn to. This covers all the stuff that convergent evolution can't get.
    Brian Goodwin (inter alia) did some good stuff on these morphogenetic
    constraints (although I don't agree that his work 'challenges' Darwinism
    - just dust cover blag methinks). To summarise - there are some aspects
    of the physical world that affect the paths open to evolution; stuff
    like how many peaks and troughs of a concentration 'wave' of some
    molecule can you get along a body axis or around the circimference of a
    structure (Turing-style) - e.g. some animals are spotted, but the tails
    are ringed, because you can't set up complex enough concentration
    gradients in such a narrow structure. Or the Fibonacci sequence of side
    branch (etc.) angles on plants would be another.

    Additionally, for the tree with similar leaves, this could
    also/alternatively be a side effect of one or more traits of the
    organism that *are* truly evolutionarily convergent (answering the same
    question with the 'obvious' answer); this is known as pleiotropy (for
    the non-biologists here who may not have heard the word, its the
    situation where genes affect multiple traits, necessitating trade-offs).

    We don't need no hoodoo here. We just have to look hard for a good
    explanation, rather than running for the nearest shaman.

     Chris Taylor ( »people»chris

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