Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!pacific.mps.ohio-state.edu!linac!midway!quads.uchicago.edu!chi9
From: chi9@quads.uchicago.edu (Lucius Chiaraviglio)
Newsgroups: sci.bio
Subject: Re: human lifespan and evolution
Message-ID: <1990Nov3.034400.25121@midway.uchicago.edu>
Date: 3 Nov 90 03:44:00 GMT
References: <1990Oct28.120050.7521@newcastle.ac.uk> <1990Oct29.000540.6280@midway.uchicago.edu> <90304.100534JAHAYES@MIAMIU.BITNET>
Sender: news@midway.uchicago.edu (News Administrator)
Organization: Department of Biology at University of Chicago
Lines: 76

In article <90304.100534JAHAYES@MIAMIU.BITNET> JAHAYES@MIAMIU.BITNET (Josh
Hayes) writes:
>In article <1990Oct29.000540.6280@midway.uchicago.edu>, chi9@quads.uchicago.edu
>(Lucius Chiaraviglio) says:
>>
>>In article <1990Oct28.120050.7521@newcastle.ac.uk> W.P.Coyne@newcastle.ac.uk
>>writes:
>>>Suppose the people in a communtiy delated having chlidren until as late
>>>in life as possible (but not so late that the population declined because
>>>of too few births).
>>>Could this cause the average lifespan to increase by several decades, if
>>>they continued this over tens of generations?
>>
>>        A similar experiment has been done in the fruit fly Drosophila
>>melanogaster.  The conceptual difference between this experiment and what you
>>propose is that instead of directly getting the flies to delay reproduction,
>>those flies capable of reproducing late were selected by transferring to new
>>vials only those offspring produced late in the life of the flies.  After
>>several generations of this, the selected flies were noticeably slower to
>>reproduce and lived substantially longer -- I think both effects were in the
>>lower tens of percent, but I can't remember the exact numbers.  Lower tens of
>>percent would correspond to a couple of decades longer life for us.
> 
>But this presupposes that there exists genetic variation for both
>delayed reproduction and delayed senescence, and that the two traits
>are linked. There probably is no such variance.

	Actually, no such presupposition was necessary to try this experiment,
although I have no idea whether the experimenters had any such presupposition.
It is not clear from the experiment whether the delayed reproduction and
delayed senescence traits are linked or were merely selected together.  I
forgot to mention that these traits were indeed genetically transmissible,
although I don't think a detailed linkage analysis came out of this (maybe in
the works?).

	In order for the experiment to have worked, either the initial
population must have had significant genetic variation for these traits, or
else these traits must be subject to fairly rapid modification by mutation
(which, however, would operate even prior to the selection, thus generating
significant variation by the time the experiment started).

>And do you refer to males and females?

	The selection was applied to both genders of the flies.

>                                       How long do males have to
>wait; after all, some men long past 70 can still father children;
>few women past 45 can carry a pregnancy to term successfully.

	Nevertheless, the human population does have some variation with
respect to this.  If any of this variation is of genetic basis, as is most
likely the case, it will be subject to the kind of selection described by the
original poster.

>                                                              The
>fact is, people are usually long done with reproduction before
>senescence becomes an issue. If we force people to wait until, say,
>40 to have children, they'll still be well done with child-bearing
>by the time they're 50, and expected life span these days in the
>U.S. is what? About 75 years? You posit a linkage that delayed
>reproduction that is still successful is a genetic trait that is
>linked to longer lifespan. Forcing everyone to delay vitiates the
>genetic component of the former, and the linkage is completely
>unsupported. Even as an intellectual argument, I don't think it works.

	You put words into my keyboard that I did not type.  What I said was
that in the Drosophila experiment, both traits were selected for.  As I said
above, they may have been linked, but they may have merely been co-selected.
If such a selection was performed upon humans, and the traits were not
linked, one would expect that maximum reproductive age would go up until it
approached maximum lifespan sufficiently closely that the adverse effects of
declining health associated with senescence became a limiting factor upon
reproduction, and then maximum lifespan would start to go up as well.

--
|   Lucius Chiaraviglio    |    Internet:  chi9@midway.uchicago.edu