At 06:47 2000-02-04 -0800, you wrote:
>Hi Gerry
>
>In reply to Tommy's point
>> No, and You won't. 1,7 % difference is quite enough to preclude
>> interbreeding between mammals (not for birds though). In any case
>chimps
>> have 48 chromosomes and humans 46, which would prevent interbreeding
>in
>> any
>> case.
>
>You wrote
>
>> Differences between Homo sapiens and "chimp" consist of less than 2%
>> similarity in DNA, 46 vs 48 chromosomes, and a substantial difference
>in
>> length and body weight. Are there other biological differences?
>> Gerry
>
>Surely you jest! Chimps have about 450cc of brain capacity, humans
>have about 1350cc. Chimps mature at 4 years, are sexually mature at 8
>years, and die about 40 years, humans are sexually mature at about 15
>years (with a dimorphism between male and female) have menopause 45-50
>and die at 78 years. How's that for starters. Most of these changes
>seem to have occurred as a result of changes in a few regulatory genes
>(humans resemble immature chimps much more so than they resemble adult
>ones - a case of neotony).
>
>Regards
>
>John
>
Actually everything You mention is in there in those 1,7% - there is
nowhere else it could be.
We're getting rather far off IE linguistics now but I think I will try to
clarify a bit what genetic change is.
Those 1,7 % is the gross difference between the average chimp and human
genomes, i. e. 17 out of 1000 base pairs are different if You match
homologous DNA. However most of that difference is really unimportant,
because it is either in non-coding parts of the genome, which (as far as we
know) has no function, or if in a functional gene it is a "neutral"
mutation, that is a change that has no effect on the working of the gene.
Those unimportant changes are however very handy for systematists, for
since they have no effect they are not selected either for or against and
therefore accumulate at a fairly constant rate. Therefore, at least between
reasonably closely related species, the amount of change tells You how long
it is since the lineages separated.
The genetic changes (mutations) that really affect an organism are much
fewer, and usually deleterious, and get selected against. A very few
mutations are advantageous and spread in a population, and it is these few
changes which are the important ones. It is perfectly possible that a
single mutation might isolate two species reproductively for example if it
affected some key element of their breeding behaviour.
So the gross amount of genetic difference is NOT a good measure of how
different two species are. Those 1,7 % is not a lot really, in many cases
the difference between species in the same genus are much larger. However,
Chimps and Humans are actually rather different, both in appearance and
behavior (completely different mating system just for starters), so
apparently an unusually large proportion are functional differences. As
John says many changes are probably in regulator genes that control the
action of other genes.
Tommy Tyrberg