some fly species have evolved to look similar to bees
and wasps, presumably gaining protection from their
similarity to aggressive insects.Müllerian Mimicry
Although Batesian mimicry is well represented in the
tropics, recall that most tropical plant species contain
defense compounds. Therefore, any caterpillar species
will likely have to cope with defense compounds
in adapting to a food source. Various degrees of
unpalatability among caterpillars should be expected
in the tropics, because so many of the food plants
encountered by the larval insects have defense
compounds that, if stored or metabolically modified
by the insect, could render the creature unpalatable.
Such secondary metabolites represent strong selection
pressures on lepidopterans.
In 1879, Fritz Müller suggested that two or more
unpalatable species would benefit in evolutionary fitness
by close resemblance. If two unpalatable species look
alike, the would- be predator needs to be educated only
once, not twice. The greater the resemblance, the greater
the advantage would be to each species. This concept
of convergent patterns among unpalatable species is
termed Müllerian mimicry. Müllerian mimicry is in
theory mutualistic, because individuals of both species
benefit from the mimicry. But there may be many cases
in which species show varying degrees of palatability.
Nonetheless, simulations have demonstrated that when
predators have an array of prey from which to choose,
any resemblance to one another among prey can induce
intense selection for mimicry.
Both Heliconius erato and H. melpomene are
unpalatable; both are brilliantly colored, and they look
remarkably alike. What is even more remarkable is that
there are 11 distinct morphs (or races) of H. melpomene
in the American tropics, ranging from Mexico to
southern Brazil. These morphs do not look the same.
John Turner learned that for every local morph of H.
melpomene, there is a virtually identical local morph of
H. erato. Both species have converged in morphological
variation throughout their ranges (fig. 11- 1). Only one
morph of H. erato, which is restricted to a small range
in northern South America, lacks a H. melpomene
counterpart.
Fieldwork in which wing patterns are manipulated
has demonstrated selection for Müllerian mimicry.
For example, in a study performed in western Ecuador
using local morphs of Heliconius erato, H. cydno, and
H. eleuchia, those butterfly morphs that were most
distinctive, whose wing coloration or pattern did not
match the predominant model, consistently suffered
higher rates of predation.
More recent genetic studies on heliconid butterflies
by the Heliconius Genome Consortium (2012) included
the sequencing of the genome of Heliconius melpomene.
Using the genome sequences as a baseline, researchers
compared gene sequences and chromosomal evolution
among other heliconid species with those of H.
melpomene. What was learned is that gene exchange
through hybridization is clearly implicated among H.
melpomene, H. timareta, and H. elevatus, which are
all co- mimics. Furthermore, the hybridization is most
apparent in the gene sequences controlling the mimicry
patterning. The study concluded that hybridization
among the various species, which has promoted
exchange of gene sequences, has had a profound effect
on rapid evolution of the present mimicry patterns.
Given the level of potential protection afforded
by mimicry, it is likely that any negative effects of
hybridization are offset.Aposematic Insects as Guides for
BioprospectingThe presence of numerous plant defense compounds
in sweeping numbers of tropical plant species has
led to the concept of bioprospecting, the attempt to
discover compounds of potential medical use in plants.
It has long been appreciated that tropical peoples are
often keenly aware of the medicinal and other uses of
plants (ethnobotany); this topic is discussed in greater
detail in chapter 17. Bioprospecting attempts to screen
plant species for potential efficacy as drugs for use
in treating a wide range of human diseases. Thus far
bioprospecting has yielded some promising drugs for
use in cancer treatment as well as for treating severe
parasitic agents such as those that cause leishmaniasis
(see Appendix: Words of Caution).
In a unique approach to bioprospecting, Julie Helson
and colleagues looked at the presence of aposematic
insects as potential indicators of plants with active
chemicals that could prove useful against disease.
The logic was straightforward applied evolutionary
theory. Plants with potent compounds exert a selection
pressure such that only a few insect types evolve to202 chapter 11 evolutionary arms races: more coevolution, more complexity