Biology Reference
In-Depth Information
at about 75% of the population, with the other two morphs
decreasing to about 12.5% each (Fig. 4.7). Therefore, a stable
polymorphism was produced because of the way the predators
focused on the most detectable morph, which depended on
both its abundance and crypsis. At the stable equilibrium each
morph had equal risks of detection. In further experiments, it
was shown that a stable equilibrium was not the inevitable
outcome; if a morph was too conspicuous it was eaten to
extinction, while if one morph was much more cryptic than
the others it could become the only morph in the population.
Bond and Kamil (2002) extended this 'virtual ecology'
experiment by allowing moths to evolve via a genetic algorithm
('genes' coded for pattern and brightness, and offspring were
subjected to mutation and recombination). The jays often
failed to detect atypical 'mutant' cryptic moths and so these
increased in frequency. Over successive generations the moths
evolved to become both harder to detect and showed greater
phenotypic variance. This 'virtual' experiment therefore
mimics an arms race in nature, where prey evolve improved
crypsis and polymorphism. It would be interesting to extend
this by allowing the predator to evolve too. This would require
both virtual predators and virtual prey.
Apostatic selection
1:1
Proportion of prey 1 in environment
Fig. 4.6
Apostatic selection occurs when a
predator eats more of the commoner prey
types than expected from their relative
frequency in the environment. This promotes
polymorphism in prey, because rarer morphs
are more likely to survive.
Brightly coloured hindwings and eyespots
What about the brightly coloured hindwings? Debra Schlenoff (1985) tested the
responses of blue jays to models which had variously patterned 'hindwings' concealed
behind cardboard 'forewings'. The model moths were attached to a board and the jays
were trained to remove them to get a food reward underneath. When the models were
removed, the hindwings suddenly expanded from behind the forewings to mimic the
reaction of the real moths. Jays which had been trained on models with grey hindwings
showed a startle response when they were exposed to the brightly patterned hindwings
typical of
Catocala,
whereas subjects trained on brightly patterned models did not startle
to a novel grey hindwing. After repeated presentations the birds habituated to a
particular
Catocala
pattern but a novel bright pattern elicited another startle response.
These results provide good evidence for the startle hypothesis, and the habituation
effect suggests an adaptive advantage for the great diversity in hindwing patterns of
different sympatric species of
Catocala
.
Other cryptic moths and butterflies have eyespots on their wings (Fig. 4.2), which
they expose when disturbed causing the predator to cease its approach. Experiments
with peacock butterflies
Inachis io
have shown that birds are more likely to attack and
eat butterflies whose eyespots have been painted over compared to controls (eyespots
intact but painted elsewhere on the wing). Peacock butterflies are not distasteful, so the
exposure of eyespots is a case of intimidating the predator by bluff (Vallin
et al
., 2005).
It has long been assumed that eyespots are effective deterrents because they mimic
the eyes of the predator's own enemies, such as owls or birds of prey. However,
Experimental
evidence for the
startle effect
Why are eyespots
on prey an
effective defence?
