>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
cases.
Dr. David Campbell
Old Seashells
46860 Hilton Dr #1113
Lexington Park MD 20653 USA
bivalve@mail.davidson.alumlink.com
That is Uncle Joe, taken in the masonic regalia of a Grand
Exalted Periwinkle of the Mystic Order of Whelks-P.G.
Wodehouse, Romance at Droigate Spa
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