Biology Reference
In-Depth Information
(b)
(a)
0.30
0.25
tactic A
0.20
0.15
tactic B
0.10
0.05
0.00
-0.05
Low
x
High
3.5
4.0
4.5
5.0
5.5
6.0
Competitive ability
Pronotum width (mm)
Fig. 5.15
(a) Gross's (1996) model for the threshold morphological switch between
alternative tactics within a conditional strategy. The fitness of each alternative tactic
A and B varies with an individual's competitive ability. On average tactic B has the lower
pay-off. However, below threshold x, B does best while above the threshold A does best.
With permission from Elsevier. (b) The reproductive success of horned (solid symbols)
and hornless (open symbols) male dung beetles
Onthophagus taurus
. Horned males
begin to do better at a body size of about 5 mm pronotum width, which corresponds
to the threshold for horn development in the population under study. Hunt and
Simmons (2001).
(a sigmoidal relationship; Fig. 5.17a). However, this threshold varies between isolated
island populations just 40 km apart. On islands where earwig population density is
higher, and where there are higher pay-offs for fighter morphs (more females can be
defended), the switch to long forceps occurs at smaller body sizes, so a greater proportion
of males in the population has long forceps (Fig. 5.17b).
Alternative strategies: equilibria and cycles
The examples we have discussed so far all involve one conditional strategy with
alternative tactics. Individuals employing tactics with poor pay-offs are doing the best
they can given their poor competitive ability. Tactic choice is a case of bad luck, not bad
genes. However, in theory, different competitive behaviours could result from alternative
genetic strategies, so there would be a genetic polymorphism in the population. In this
case, if one strategy always had poorer pay-offs then this
would
be a case of bad genes,
and we would expect natural selection to eliminate it from the population. Therefore, if
alternative genetic strategies persist in population we would expect them to have, on
average, equal success.
