From: Robert J. Bradbury (bradbury@aeiveos.com)
Date: Wed Apr 03 2002 - 16:41:47 MST
On Wed, 3 Apr 2002 CurtAdams@aol.com wrote:
> Current selection pressure on increased fecundity is astronomical.
Huh? Human fecundity hasn't changed at all in a long time. What
has changed is the fraction of infants (and mothers) surviving
the childbirth and early infancy stages.
> If say, the standard deviation of offspring number is 1 and the heritability
> 1/3, offspring # goes up 1/3 every generation and you have pressure
> to speed up generation time.
Wrong! (sorry Curt) -- the evolutionary pressure that speeds up generation
time (and generation size) is your environmental hazard function. The greater
the hazard function, the more pressure there is to reproduce sooner with
more offspring (insects being a case in point).
> Going to agriculture is not the relevant shift, although I expect that has
> changed human nature too, albeit less significantly.
Going to agriculture allowed people to live in cities which in turn
required a diversification of immune systems to resist microorganisms
that are transmitted by close proximity. This, I suspect would promote
mate selection on the basis of different immune systems. There may also
be some "socialization" selection that may go beyond that one finds in
very small groups of primates.
> The shift is that there's no limit on reproduction, which is quite recent.
Huh? Humans have never had a limit on reproduction. It is thought
that one of the primary reasons women get breast cancer is that
in primitive eras women spent most of their time pregnant or lactating
reducing the number of ovulation cycles. The human (female) genome
is not designed to promote longevity for non-pregnant females.
However -- it doesn't do you much good to be reproducing continually
if most of those children fail to reach reproductive maturity.
> Freedom from famine is only 300 years old even in England (Holland
> might be longer, I don't have data; certainly nowhere else) and in
> most of the world it's only this generation.
I think you ought to go research China or Japan. I suspect that
other than brief instances of harsh conditions induced by weather
variants they have had sufficient food for thousands of years.
Going back even further -- I think it is believed that most hunter
gatherer "tribes" had no shortage of food supplies -- at least
in Africa where fruit was abundant or in the fertile crescent.
(Of course you can make a case that in glacier covered Europe
that was not the case.)
> The demographic transition is more recent than that
> and so alleles which permit the transition have been shielded from
> detrimental selection.
I don't believe for a second that evolution of the human species
has produced the "demographic transition". Its a combination
of better medical practices, vaccinations, better nutrition
(not always dictated by absolute caloric intake...) and
sanitation (reducing the risks to which we are exposed).
Robert
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