Re: common ancestry

From: David Campbell (
Date: Tue Aug 01 2000 - 16:34:20 EDT

  • Next message: James W Stark: "Common ancestry and Scientific Classification"

    >Thanks for the clarification, David. It is only the
    >polyphyly of the living state itself (e.g., as argued
    >for by Webster and Goodwin 1982, Schwabe 1994,
    >or Gordon 1999) that necessarily excludes more
    >distant common ancestry.

    Also, polyphyly or monophyly of life has no direct bearing on ID versus
    natural means.

    >Actually, Philippe et al. (1994) took Drosophila and Aedes
    >as representatives of Insecta, a class within the Arthropoda.
    >Thus in their 1994 18S rRNA phylogeny, they give Insecta and
    >Nematoda as sister taxa, and argue that this is artifactual,
    >because of long-branch attraction (see David's explanation
    >of what long-branch attraction is, below).

    Do they have any other arthropods? They happened to choose poorly if not
    (determinable mainly by hindsight, although an insect plus a spider,
    centipede, crustacean, etc. would obviously have been better if available,
    or even different orders of insects rather than two flies). At any rate,
    if you assume that phylogenetic analysis of the sequences is a meaningful
    activity and then look at the results, you will find that flies appear as a
    very long branch. Thus, the results of the method give reason to suspect
    that the assumptions of the method (that long branches are not a problem)
    have been violated, as do the morphological and paleontological
    phylogenetic information that suggests that two flies have much closer
    common ancestry than flies and nematodes, making a chronologically long

    In the interval between 1994 and today, the number of published 18S
    sequences for bivalves has gone from about 17, representing only 5
    families, to over 100, representing most living higher taxa. Similar
    increases in the amount of data available for other taxa, such as
    publication of sequences for tardigrades, onychophorans, and other
    arthropod relatives, greatly improves the consistency of analyses,
    increasing confidence that the associations are in fact valid. Improved
    computers and algorithms are also important in the changes.

    David C.

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