Biology Reference
In-Depth Information
No. species of
caterpillars
Table 4.2
Brightly
coloured species
of caterpillars of
British butterflies
are more likely to
be aggregated in
family groups
than cryptic
species (Harvey
et al
., 1983).
Dispersion
Aposematic
Cryptic
Family
groups
9
0
Solitary
11
44
coloration get through to the next generation. This process, where traits can be favoured
because they benefit relatives, who share copies of the same genes, is termed kin
selection; it will be explained in more detail in Chapter 11. Mathematical models have
shown that with family grouping bright coloration can evolve in a distasteful species
provided the brighter forms are not too conspicuous and provided the predator needs to
sample fewer of them, compared to cryptic prey, in order to learn that they are distasteful
(Harvey
et al
., 1982).
Although Fisher's solution is ingenious, recent work has challenged two of its
assumptions. Firstly, the assumption that the sampled individuals always perish
may be wrong. Many brightly coloured insects have tough integuments which
protect them against attacks by naive predators and they are released unharmed.
Indeed, in some cases the defences themselves (e.g. hairs on a caterpillar) are
brightly coloured. Thus, in some cases, there may be a direct advantage to the
individual in being conspicuous; so long as the bright colouring is more easily
remembered, a bright distasteful caterpillar is better protected than a cryptic one in
subsequent encounters with the same predator (Sillén-Tullberg, 1985). Many
predators are reluctant to attack a novel prey item ('neophobia') which would also
promote the survival chances of a mutant, more conspicuous prey type (Marples &
Kelly, 1999). Secondly, Fisher assumed that family grouping sets the stage for
the evolution of warning coloration but a phylogenetic analysis (Chapter 2) of
butterflies suggests that warning coloration evolved
before
gregariousness (Sillén-
Tullberg, 1988). It seems likely, therefore, that warning coloration sometimes
evolved because of the direct advantage it brought to individuals in decreasing their
likelihood of attack, and grouping evolved afterwards through diluting predator
attacks per individual (Chapter 6). Thus, grouping is not always critical for the
evolution of bright colours. Note that many of the brightly coloured species in
Table 4.2 are solitary.
This discussion highlights the central problem in aposematism, namely the trade-off
between the costs of conspicuousness in increasing the probability of attack by naïve
predators, and the benefits from increased protection against experienced predators
through more memorable and detectable signalling. It seems likely that this balance of
selection pressures will vary between different aposematic organisms, so one
evolutionary pathway is unlikely to explain all cases.
But, individuals
may benefit
directly … and
grouping may
have evolved
after warning
colours
