From: CurtAdams@aol.com
Date: Wed Apr 03 2002 - 17:38:43 MST
In a message dated 4/3/02 15:44:54, bradbury@aeiveos.com writes:
>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.
Until recently, more children meant fewer resources for the rest and
a consequent decreased survival. While data on historical humans is
lacking, it's a good rule for any creature that its fecundity is near
optimal. So alleles for increased fecundity had both benefits and costs.
Now survival is almost 100% regardless of how many kids you have.
Increased fecundity is now just gravy for the genes.
>> 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).
No, high population growth rates also select for rapid reproduction. Alleles
for increased fecundity are experiencing growth relative to the rest of the
population and so are alos selected to move reproduction forward.
[snip some suggestions for how agriculture may have changed human nature]
Testing these would be interesting.
>> The shift is that there's no limit on reproduction, which is quite recent.
>Huh? Humans have never had a limit on reproduction.
Well, call it a limit on successful reproduction. You could birth more, but
they'd just starve.
>> 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.
Definitely not China; they had major famines into this century.
I'll look for data on Japan, but weather variation is the primary
traditional cause of famine everywhere and Japan gets some
pretty wild weather. (modern famines are primarily political)
>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.)
Anthropological research shows all the hunter-gatherer societies
have periodic severe food shortages. Africa and the Fertile Crescent
have *huge* rainfall variation. Periodic famines are part of what's
killing traditional societies today - famine forces them into the
better-prepared modern societies. Such problems only occur every
few decades, perhaps, but that's plenty often in the long term.
>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).
It wasn't a change in genes that caused the transition, but a change
in culture. However, the lower reproduction due to those
shift is going to be partly caused by pre-existing genes, (virtually
anything about humans has both genetic and cultural components) and
those genes now face huge negative selection pressure. When they
go away people will go back to having lots of babies.
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