Adrian wrote:
>> [most previous material sniped]
>
>As far as I know, error catastrophe occurs when the mutation rate gets
>above one harmful mutation per progeny - .5 harmful mutations per gamete
>per generation. Since Kimura (1983) estimated that amino-acid altering
>mutations are ten times more likely to be harmful than neutral, then the
>estimated rate of neutral mutation would be .05 per generation. That
>figure seems to be too small to explain every much. In ten millions
>years, using the example above, you get only about 25000 neutral
>substitutions.
>
Much of the genome does not code for anything actually used by the cell
and, so far as we know, could vary freely (spacers, exons, pseudogenes,
etc.). Also, the estimate of harmful to neutral mutations of Kimura sounds
high relative to the results of mutational studies. A recent (this year)
Nature (sorry for the vague reference, but it's as much as I remember)
reviewed a paper that found most mutations had little effect on efficiency
of the studied proteins.
>> >> In particular, a minor change affecting courtship behavior could
>> >> be enough to isolate two subpopulations. Also, one mutation may easily
>> >> involve more than one nucleotide. Gene duplications (e.g., unequal
>> >> crossing over) and retroviruses can produce large changes in a single
>>step.
>> >
>> >Aren't these events also exceedingly rare? Are they enough to solve the
>> >problem?
>>
>> These events are generally rare (although many plants can easily form
>> polyploid hybrids that act as genome duplication events, this is unusual in
>> animals), but the time spans involved are also quite long.
>
>Given the above example of ten million years, would it be long enough?
>For starters, how rare are these events anyway?
>
Another mailing list cited 2 million as the time since the most recent
incorporation of a recognized retrovirus into the human genome, but that
other mammals have much more recent ones. If this is representative, ten
million is enough. In animals, genome duplications seem to have been quite
rare, but gene duplication is not all that unusual. Several common genetic
diseases (e.g. Down's syndrome) represent duplication of sizable portions
to entire chromosomes; duplication on the scale of genes would rarely be
noticable unless it were looked for.