ECOSYSTEMS 155
selection could upset this balance—
hence why an ESS is “stable”—and
that these behaviour patterns are
genetically preprogrammed.
ESS has its roots in game theory:
a mathematical way of working out
the best strategy in a game. Many
examples of how animals behave
emerge as being evolutionarily
stable strategies, such as territorial
behavior and hierarchies. For
example, the genetically pre-
programmed “rules” of “if resident,
fight and defend” or “if visiting, give
in and retreat,” which would help
animals retain territory, combine
to make territorial behavior an ESS.Balancing strategies
The payoff that an individual
animal gains—or the price it risks
paying—by displaying a particularbehavior can be quantified,
so biologists can work out which
strategies are likely to be most
stable by using mathematical
models (see box). If the model
does not match the behavior
of animals in the real world, then
it suggests that stability has
not evolved.
In real rather than hypothetical
ecosystems, it is not a single
strategy that is stable, but theBehavior arising from conflicts
over space and territory might emerge
as evolutionarily stable strategies. Fruit
bats jostle for the best spots in the
trees, with alpha males driving weaker
bats down to lower branches.See also: Evolution by natural selection 24–31 ■ The selfish gene 38–39 ■ Predator–prey equations 44–49 ■ Ecological
niches 50–51 ■ Trophic cascades 140–143 ■ Biodiversity and ecosystem function 156–157The hawk-dove “game”
The simplest demonstration
of John Maynard Smith’s
evolutionarily stable strategy
(ESS) concerns a hypothetical
response to aggression known
as the hawk-dove “game.” In
this, individuals can either be
hawkish and fight until badly
injured, or dovish and posture,
but then retreat. Hawks will
outmatch doves, but could be
seriously harmed in a fight with
another hawk. Doves routinely
escape injury, but waste timein posturing. Which strategy
would be better for passing
on genes? Maynard Smith and
his collaborators devised a
mathematical model to provide
the answer, and—in this
instance—being more hawkish
than dovish emerged as the
ESS. It predicts a ratio of seven
hawks for every five doves,
which is equivalent to any one
individual being hawkish
seven-twelfths of the time, and
dovish five-twelfths of the time.balance between two or more
strategies within the system
as a whole. The overall balance
is therefore better called an
evolutionarily stable state, and
not a strategy. Such a balance
emerges when all individuals have
equal fitness: they pass on their
genes to the same extent. The
state remains stable, even when
there are minor changes in the
animal’s environment. ■US_154-155_Evolutionary_stable_state.indd 155 12/11/18 6:25 PM