Biology Reference
In-Depth Information
CHAPTER 5
Competing
for Resources
Photo © Douglas Emlen
Our discussion in Chapter 3 of how individuals exploit resources omitted a crucial factor:
competition. When many individuals exploit the same limited resources, they become
competitors and the best way for one individual to behave often depends on what
its competitors are doing. In other words, the pay-offs for various strategies are frequency
dependent. We need, therefore, to consider what might be the stable outcome of
competition. John Maynard Smith and George Price (1973) introduced the concept of the
Evolutionarily Stable Strategy or ESS , namely a strategy that, if all members of a population
adopt it, cannot be bettered by an alternative strategy. They originally introduced the idea
to model the evolution of fighting strategies, but it is widely applicable to all cases where
individuals interact. The key question to ask is: could a mutant strategy do better?
A human example helps to explain this idea. Imagine a crowd of people sitting on the
floor to watch a concert. Someone stands to get a better view. Those behind now have to
stand in order to see and so a wave, from sitting to standing, begins to spread through
the crowd. Eventually everyone is standing, with the end result that no-one can see any
better than before! In this example, sitting is not an ESS (in a crowd of sitters, someone
standing does better) whereas standing is an ESS (once everyone is standing, it then
doesn't pay anyone to sit). This example also makes the point that the ESS is often not
what would be best for everyone; if only everyone had agreed to sit, they would all have
been more comfortable.
Evolutionarily
stable strategies
The Hawk-Dove game
To illustrate 'ESS thinking' more formally, we shall consider the Hawk-Dove game
(Maynard Smith, 1982). Assume contestants meet randomly to compete for a resource.
Imagine a simple world in which there are just two possible strategies. Hawks always
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