Evolution, 4th Edition

328 CHAPTER 13

nestlings like their own young, but some host species do recognize parasite eggs

and either eject them or desert the nest and start a new nest and clutch. Many

brood parasites have counteradapted by laying mimetic eggs [65]. Each popula-

tion of the common cuckoo (Cuculus canorus) contains several different genotypes,

which prefer different hosts and lay eggs closely resembling those of their pre-

ferred hosts (FIGURE 13.9B). Nick Davies and Michael Brooke traced the fate of

artificial cuckoo eggs placed in the nests of various host species [16]. Bird species

that are not parasitized by cuckoos tend not to eject cuckoo eggs. But among the

cuckoos’ preferred hosts, those species whose eggs are mimicked by cuckoos reject

artificial eggs more often than those whose eggs are not mimicked. These species

have adapted to brood parasitism by evolving greater discrimination. Moreover,

populations of two host species that reject artificial cuckoo eggs in Britain accept

them in Iceland, where cuckoos are absent. Thus two evolving traits, host discrimi-

nation and cuckoo egg pattern, shape the evolution of this interaction.

Aposematism and mimicry

Diverse animals, such as bees and coral snakes, have evolved warning, or apo-

sematic, coloration: bright colors that signal to a potential predator that they are

distasteful or dangerous. Predators learn to avoid the color pattern, and so both

the predator and the aposematically colored prey benefit. The warning pattern is

subject to positive frequency-dependent selection because individuals that deviate

from the common pattern, which predators have learned, are likely to be attacked

(see Figure 5.24). Thus, a mutation that confers a new aposematic pattern is likely

to be disadvantageous. How new aposematic phenotypes evolve is therefore a

puzzle. They might be caused by genetic drift in places or at times when selection

by predators is relaxed (see p. 130 in Chapter 5) [48].

Mimicry is a form of convergent evolution in which resemblance between dif-

ferent species has evolved because it is advantageous for members of one species

to resemble another. The species, then, do not owe their resemblance to common

ancestry, but in some cases are so similar that experts have to look very carefully to

distinguish them. The most common kind of mimicry is defensive mimicry, which

often is based on the aposematic coloration of other species [49]. Two common

forms of defensive mimicry are named for the naturalists who first recognized

them. In Batesian mimicry, a palatable species (a mimic) resembles an unpalat-

able species (a model). Selection on a mimetic phenotype can depend on both its

density, relative to that of a model species, and the degree of unpalatability of the

model. A predator that can learn is more likely to avoid eating a butterfly that looks

like an unpalatable model if it has had a recent reinforcing experience (e.g., vomit-

ing after eating a butterfly with that pattern). If the predator has recently swal-

lowed a tasty butterfly, however, it will be more, not less, inclined to eat the next

butterfly with the same phenotype. Thus the rarer a palatable Batesian mimic is

relative to an unpalatable model, the more likely predators are to associate its color

pattern with unpalatability, and so the greater the advantage of resembling the

model will be. If a rare new phenotype arises that mimics a different model spe-

cies, it will have higher fitness, and so a mimetic polymorphism can be maintained

by negative frequency-dependent selection, as is seen in the African swallowtail

Papilio dardanus (FIGURE 13.10).

The other major form of defensive mimicry is Müllerian mimicry, in which two

or more unpalatable species are co-mimics (or co-models) and jointly reinforce

aversion learning by predators. This hypothesis was proposed by Fritz Müller in

1879 and has been confirmed by experiments in which the survival of distaste-

ful mimetic butterflies was shown to be higher if they closely matched an abun-

dant co-mimic species (FIGURE 13.11). This form of mimicry causes positive

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