question from Re: the definitions of evolution

From: bivalve (
Date: Wed Oct 10 2001 - 12:24:11 EDT

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    >One thing that does puzzle me is how the chromosomes
    of related species can be so different in there arrangement
    in the respective genomes. For example, morphologically,
    the mouse and the rat look quite similar. Yet the
    arrangement of the genes on the respective organisms
    seems very different. Likewise, often the introns also
    seem to have almost no homology. <

    This is quite variable. Even within mice, there can be
    substantial chromosomal fusion; I believe this was
    examined in mouse populations from isolated valleys in
    Italy. Chromosomal rearrangement produces a barrier to
    reproduction, but has little effect on the individual unless
    there is associated duplication or deletion. Thus, it can
    become established readily in small populations such as
    are presumed typical of speciation events. On the other
    hand, oysters show very little karyotype variation, though the
    location of genes does vary.

    Introns likewise vary in their variability. ITS shows higher
    intercopy variation within individuals of certain plants than
    the interspecies variation in this group, to the annoyance of
    plant systematists. However, the intraspecific variability in
    oysters is quite low, and ITS is widely used for phylogenetic
    studies at low taxonomic levels.

    Both of these being relatively unconstrained by selective
    pressures, they should be expected to vary rapidly in many

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