why Cain killed Abel

[Bill Adlam <wa105@mead.anglia.ac.uk>]

Matt replied to Phil:

>>More specifically, it is probably related to maintaining a proper
>>ballance in the predator/prey ratio.
>                                                            ...Usually 
>the oldest individual in a nest is the strongest and selective-wise the 
>strongest individual is the individual that you want to pass on its 
>genes...
>                   ...the species can get a stronger foothold in the 
>ecosystem; this can work the other way around if their is not a lot of 
>food and an excess of predators would cause a die-off related to 
>over-predation of food.  It is related somewhat to the predator-prey 
>ratio, but I think it is probably more closely related to the survival 
>of a species and also to the bablance of the ecosystem.  

Alright, that's quite enough population-level, species-level and ecosystem-
level selection.  Individuals reap no genetic benefit from limiting their 
reproduction.  If there are too many predators some will die, but those who 
restrict their breeding are just as likely to starve, and won't multiply 
fast enough to keep up with the death rate.  Breeding less than you can is 
only evolutionarily plausible if it's very difficult to disperse.  Almost 
all flying birds can disperse very effectively.

The first chick to hatch is stronger because it was laid earlier, not 
because its genes are better.  So why lay a second egg?  The junior chick 
is a vice-president*.  It has no function whatsoever except to take over if 
the elder chick fails to hatch, dies, falls sick or is injured.  In some 
birds the number of chicks that can be reared is not known, so it's 
possible that all the offspring will survive, but in many cases, especially 
eagles, it is never possible to fledge two chicks in a season.

Extra eggs might also be laid just for food.  Eagles can't store dead 
animals for very long, and can't fly well if they are carrying lots of fat.  
Other benefits are conceivable, like keeping Number One warm when both 
parents are away, or giving it something to practice fighting.

                                                        All the best,

                                                                        Bill

* This is a rough analogy.  I'm not trying to make any comment about the 
usefulness of particular vice-presidents of the United States Of America, 
or of anything else.