Tim Tyler  wrote in
news:c73nog$27rn$1{at}darwin.ediacara.org: 

> William Morse  wrote or quoted:
> 
>> As far as I can see, there is nothing wrong with an explanation that
>> is "for the good of the species" (group, deme, etc.) so
long as it is
>> not harmful to the individual. Individuals tend to compete with
>> conspecifics, and in the case of a fixed carrying capacity any trait
>> that benefits an individual will subtract from the fitness of the
>> rest of the population. But what if the trait increases the carrying
>> capacity? In this case it will still benefit the individual, but it
>> will also benefit the species! In general such traits may face
>> stiffer odds of spreading than traits that benefit just the
>> individual, since the individual benefit is at first just a fraction
>> of the overall benefit. But once established at even a relatively
>> small frequency, they may survive better than traits that merely
>> benefit the individual, since they reduce the odds of loss by 
>> sampling error by increasing the population. 
> 
> This is all well put.  However at the moment species selection faces
> the problem of not having any good examples of species-selected
> traits. 
> 
> If trying to find evidence for species selection - to illustrate that
> is isn't totally impotent as a force in nature - examples where
> adaptations favour the species and the indiviual are not terribly
> useful - since the simple hypothesis of individual selection only
> explains their existence rather well - and quantitative arguments
> in this area are harder to make convincingly.
> 
> It is cases where the trait is either neutral to the individual - or
> slightly deleterious - that would represent the best evidence for
> species selection.
> 
> This is why modern group selectionists have congregated around
> phenomena such as programmed death.  Death looks as though it
> offers little individual-level benefit - but arguments can
> be made that it improves group-level dynamics - e.g. by
> systematically removing organisms with high parasite loads
> from the population and preventing them from infecting other,
> unrelated members of the same population.
> 
> So far I'm not convinced that these folks have a case - but
> in terms of getting some evidence for high level selection
> goes, this is the right /sort/ of example.

Agreed. Theory is nice but doesn't mean spit without examples that support 
it, and the convincing examples in this case are those where traits are 
difficult to explain by individual selection. 

One of the interesting cases of programmed death in this regard is salmon. 
Even though their chances of surviving a trip back to the sea and a return 
trip to spawn at a later date may be very slim, their death only benefits 
their own eggs very indirectly. So the explanation for this behavior would 
seem to have to include either kin or group selection mechanisms.

The release of chemicals such as methyl jasmonate by plants in response to 
insect attack is a similar example. The chemical helps other plants resist 
attack, but at a cost to the releasing plant.

Rats reducing birth rates in response to crowding conditions is an example 
cited by Wilson. The reduced rate helps stabilize the population as a 
whole, but at a cost to the individual rat. This behavior has been noted as 
one reason why rats are among the more successful invaders of islands - 
other invaders tend to suffer disastrous population boom and bust cycles.

The phenomenon of locusts is a case where there may be a clear benefit to 
the individual, but only when all the individuals behave the same way. 


But I was also thinking of those cases where organisms more directly and 
drastically affect their immediate environment. Coral reefs are one 
example. Boreal forests are another. Passenger pigeons were at one time so 
numerous as to affect their environment - in fact their extinction was 
probably because they relied on huge numbers for their behavior to be 
successful. In some of these situations the species affects its own niche 
so drastically that you can only really understand the selective forces on 
an individual in context of how that affects the group.


Yours,

Bill Morse
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