Biology Reference
In-Depth Information
Mimicry
The association between bright colours and repellent defences has led to the evolution
of various forms of mimicry.
Müllerian mimicry: repellent species look alike
Fritz Müller (1878) was the first to notice the similarity in colour patterns between
different repellent species (Fig. 4.14). For example, stinging wasps and distasteful
beetles, bugs and moths living in one area may all share the same yellow and black
colouration. Müller's idea was that if these repellent species look alike, then it will be
easier for a predator to learn to avoid them all; it just has to learn the one pattern and
then all the prey will benefit.
Müller suggested that colour patterns will evolve by a rarer morph converging on the
colour of a commoner morph. Imagine two distasteful species with different colours;
species A has a population of 10 000 individuals while species B is rarer with just 100
individuals. Assume a predator needs ten attacks to learn that a colour pattern is
associated with distastefulness. A rare mutant in species A which resembles species B
will suffer a disadvantage because it is more likely to be sampled in a population of 100.
By contrast, a rare mutant is species B that resembles species A will be at an advantage
because it is hidden in a larger population. Therefore, rarer morphs should evolve to
match commoner morphs because they have a 'greater umbrella of protection'.
A famous case of Müllerian mimicry involves the remarkable similarity between
distasteful species of
Heliconius
butterflies in central and South America. Up to a dozen
species living in the same area may share the same colour pattern, but colour patterns
vary geographically; in some areas the butterflies are all blue, in other areas orange, red
or of a tiger pattern (Fig. 4.14; Mallet & Gilbert, 1995). Two species,
H. melpomene
and
H. erato
, show parallel variation in pattern across south America and were the subject
of one of the first experimental tests of Müllerian mimicry by Benson (1972). In Costa
Rica, he painted some
erato
individuals so they were non-mimetic (in fact, they now
resembled another race of
erato
in Colombia). These survived less well than controls,
which were also painted but whose pattern remained mimetic. Reciprocal transfers
of different colour morphs across a morph boundary produced the same result; non-
mimetic forms survived less well and survivors had more beak marks on their wings,
suggesting increased predation by birds such as jacamas (Mallet & Barton, 1989).
Crossing experiments within both
melpomene
and
erato
have identified the regions of
the genome involved in controlling wing patterns in each species. Analysis of DNA
sequences has revealed that producing convergent patterns in the two species involves
changes in the same genes (Baxter
et al
., 2010). It is not yet known whether the genes
controlling colour patterns in mimicry rings involving different taxa (e.g. wasps and
butterflies) are also the same, but they may well be.
Müllerian mimicry still needs more experimental work. How do predators learn from
sampling? (Müller's idea of a fixed number of attacks to learn seems simplistic.) Why do
different colour forms occur in different geographical regions? These may reflect
differences in the signalling environment (different colours show best in different
habitats) or perhaps simply chance differences in initial conditions (which prey species
was the most abundant).
In theory, rarer
morphs evolve to
mimic commoner
morphs
Heliconius
butterflies
Convergent
patterns may
involve changes
in the same genes
