Biology Reference
In-Depth Information
Yellow-throated males
look like receptive females (which also have yellow throats). They
do not defend territories. Instead, they attempt to sneak matings.
Blue-throated males
are less aggressive than orange-throated males. They defend small
territories in which they guard a single female.
Frequency
dependent cycles
in three colour
morphs of a male
lizard
Comparison of throat colours of fathers and sons indicates that these differences are
genetic and involve one locus with three alleles: the
o
allele is dominant and the
b
allele
is recessive to the
y
allele. Therefore, orange males have genotype
oo
,
ob
or
oy
; blue males
are
bb
and yellow males are
yy
or
by
.
During the years 1990-1999, the frequencies of the three morphs changed along a
250 m sandstone outcrop (Fig. 5.19b). Detailed studies of behaviour, combined with
parentage analysis using DNA profiles, revealed that these frequency changes were driven
by a game in which each strategy had a strength, which enabled it to outcompete neighbours
of one morph, but also a weakness which left it vulnerable to neighbours of another morph.
Thus, blue-throated males mate-guard their females and so avoid cuckoldry by yellow-
throated sneaker males, but they can become overpowered by aggressive orange-throated
males. Orange-throated males, on the other hand, cannot guard all their females at once
and so they are vulnerable to cuckoldry by yellow-throated males. Finally, yellow-throated
males are outcompeted by mate-guarding blue males. The game is, therefore:
rare orange beats common blue
rare yellow beats common orange
rare blue beats common yellow
and so on, as we repeat the cycle again.
This is just like the rock-paper-scissors game that children play: paper covers rock,
scissors cuts paper, rock crushes scissors, and so on.
We hope the reader is now convinced that the strategy-tactic distinction is crucial.
Where alternative tactics are involved, as part of one conditional strategy, it is usual for
one tactic to have a higher pay-off (callers in natterjack toads, horned male dung
beetles). However, where the alternatives are genetic strategies then for them to coexist
we would expect them to have equal average pay-offs at a stable equilibrium frequency,
or to exhibit frequency dependent cycles.
ESS thinking
In this chapter, we have used 'ESS thinking' to examine how competitors search for
resources and have seen that individuals tend to adopt stable distributions in response to
resources varying in space and time. We have also shown how the stable outcome of
competition is often for there to be variability in behaviour. This can occur within individuals
in the form of conditional strategies such as: 'play tactic A below threshold x, and tactic B
above threshold x'. In this case, we need to analyse which threshold forms the ESS. Or, the
variability could be between individuals, with some playing A and others playing B as
alternative strategies. In this case there may be an evolutionarily stable polymorphic state
or cycles involving frequency dependent replacement of strategies over time.
