Evolutionarily Stable Strategies (ESSs)

From: Katherine Lyne (kml295@soton.ac.uk)
Date: Thu Oct 02 1997 - 12:15:07 BST

This is the way that I understand the principle of ESS, as explained in
"The Selfish Gene"

The principle of the Evoltionary stable strategy is to do with the
question of agression among species. All memebrs of any species are in
fact in competition for the same resources. Whilst it might seem that
it would be advantageous for one individual to be agressive to the
other members to minimise competition, this would not actually beshuch
a good idea. By killing one member, that agressor would have not only
helped himself, by killing a competator, he would also have helped the
other remaining members. This would actually be bad for his own

The ESS is the principle by which different characteristics are
balanced among a species. In a hypothetical situation green spotted
people are agressive and prone to fighting till the bitter end. Red
blobbed people are only mildly aggressive and will retreat if attacked.
Green spotted people will always win if fighting reds. If however
greens fight each other one will win and one will die. Greens will also
sustain injuries if they engage in combat. In this situation, Greens
will always win in a species of reds and greens but will sustain more
injuries than the reds and so the net gain for the aggressive greens
will be less than for the peaceloving reds. (Dawkins explains this

So, at one extreme the population will consist mostly of reds. However,
in this situation the few greens around would profit by winning all
combat with the peaceful reds and greens would increase in number. They
would increase until they started killing each other again and the reds
began to increase in number Thus a equilibrium of reds and greens is
established and is "Stable". The population consists of both reds and
greens. The priciple is a strategy because it is a "pre-programmed
behavioural policy" ie it works within an individual without their

This strategy also has room for "Cheats" which dawkins discusses later
in the book

Katy Lyne

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