Newsgroups: comp.ai.alife
Path: cantaloupe.srv.cs.cmu.edu!das-news2.harvard.edu!news2.near.net!howland.reston.ans.net!news.sprintlink.net!pipex!sunic!news.chalmers.se!news.gu.se!gd-news!d6243.shv.hb.se!sa209
From: sa209@utb.shv.hb.se (Claes Andersson)
Subject: Re: Reason for Short Life Spans?
Message-ID: <sa209.95@utb.shv.hb.se>
Sender: usenet@gdunix.gd.chalmers.se (USENET News System)
Nntp-Posting-Host: d6243.shv.hb.se
Organization: Department of Scocial Science
References: <Pine.HPP.3.91.941209140830.1790A-100000@alnilam.krl.caltech <3cnf1u$qlm@scratchy.reed.edu> <3cnrek$ggt@saturn.haverford.edu>
Date: Thu, 15 Dec 1994 16:23:58 GMT
Lines: 50

In article <3cnrek$ggt@saturn.haverford.edu> bdickey@s30.northdorms.haverford.edu (Brad Dickey) writes:

>John Hopson (jhopson@reed.edu) wrote:
>:       I don't understand the claim that there is no pressure to remove
>: late-lethal genes from the population.  Why do we assume that having
>: post-reproductive-stage organisms die is beneficial to the population? 
>: Sure, they can't add to the population directly, but I can think of any
>: number of reasons why it would be a good idea to have them hang around.
>:  Here's a couple off the top of my head:

>:        Predator bait:  Predators prefer to take down slower, less fit
>: prey.  Older organisms could act to prevent the predators from taking
>: more fit, reproductively active organisms, and therefore increase the
>: likelihood of species survival.

>This assumes that there are age-deleterious (late lethal) genes in the
>population anyway, thus making the old less "fit".  Predators have good
>reasons, as you say, for going for the weaker prey, including very young
>and much older animals.  However, by giving away weaker animals to the
>predators, you are encouraging them to reproduce and then you have more
>predators...  so you dont do any better by sacrificing the old.

Why not? That is about finding an optimal proportion. It's not a question 
of whether it is worth it to give anything away to the predators or not. It 
is a question of HOW MUCH it is worth to give away.

>This idea of old-sacrifice is under the general theory of "group
>selection" - where what is good for the group is selected for, over or
>orthogonal to the benefit of the individual.  The theory is intuitively
>good, but has some basic flaws; if an individual can gain from perversion
>of the group cooperation, it will be more successful.  Also, there is
>usually very little selection for an individual to keep a trait that is
>not directly benefitial to them or their offspring (or kin).  Remember
>that the individual is not particularly interested in species survival.  
>It is very much interested in PERSONAL survival.  Thus when it is beyond
>reproduction age, it's "purpose" is essentially over - a gene that becomes
>active and kills it at that time will not be selected against, unless...

 That's correct but still we can observe cooperative behavior in nature and 
the need not to be explained with group selection. It can be explained with 
kin-selection. One specimen can perhaps know who its mother and brothers and 
sisters are. All males in the horde has equal chance to be its father and a 
certain chance to be its cousin etc. Time and evolution tunes the behavior 
to be proportional to the avergage genetic distance between itself and a 
horde-members of certain groups. Once this is estabilshed, other seemingly 
social structures can evolve.

Claes Andersson. University of Bors. Sweden


