Evolution__3rd_Edition

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Mimicry requires the correct
combination of alleles at all the loci
concerned
CHAPTER 8 / Two-locus and Multilocus Population Genetics 197
T – W 2 F 2 E 2 B 2 /T – W 2 F 2 E 2 B 2 or T – W 2 F 2 E 2 B 2 /T + W 1 F 1 E 1 B 1 . The recombinant genotypes
such as T + W 1 F 1 E 2 B 2 do not exist naturally, except as very rare forms like anura.
The point to remember is that each of the morphs of P. memnon is thought to be
controlled by a multilocus genotype. The genetics of the mimetic polymorphism in
swallowtails differs from the camouflage polymorphism in the peppered moth (Section
5.7, p. 108), in which the different morphs are controlled by genotypes at one locus. A
whole set of one-locus genotypes is needed to produce each of the swallowtail butterfly
morphs.
8.2 Genotypes at different loci in Papilio memnon
are coadapted
How will natural selection act on a rare recombinant morph of Papilio memnon, such as
anura in Java? Successful mimicry requires as complete a resemblance as possible
between a mimic and its model. A potential mimic that mixes the patterns needed to
mimic two species will mimic neither as successfully as a mimic that resembles one
model in all respects. It will probably be selected against. Anura has the color pattern of
achates, but will not mimic the model species of achates because it lacks a tail on its
hindwings. The models of tailless morphs, in turn, have different color patterns, and
anura will not mimic them either.
In general, natural selection will act against any recombinants between the mimetic
five-locus genotypes. A five-locus genotype that mimics one model species in all five
respects will be favored. But a swallowtail collage, which mimics one model in three
aspects and another model in two other aspects, will look like neither and will be
selected against. The genes at the five loci in this situation are said to be coadapted, or to
show coadaptation. Coadaptation means that a gene (or genotype), such as T + (or
T + /T + ), is favored by selection if it is in the same body as a particular gene (or genotype),
such as W 1 (or W 1 /W 1 ), at another locus, but is selected against when combined
with other genes (or genotypes), such as W 2 (or W 2 /W 2 ), at that other locus. For example,
selection favors T + W 1 F 1 E 1 B 1 /T + W 1 F 1 E 1 B 1 and T – W 2 F 2 E 2 B 2 /T – W 2 F 2 E 2 B 2 individuals,
but (if the alleles with the 2 subscript are dominant) works against T + W 2 F 2 E 2 B 2 /
T + W 1 F 1 E 1 B 1 individuals. Selection has not been empirically confirmed to work against
the recombinant forms of P. memnon, but the argument is quite convincing.
8.3 Mimicry in Heliconius is controlled by more than one
gene, but they are not tightly linked
The passion flower butterflies of the genus Heliconius make an interesting comparison
with Papilio memnon. In South America, two species of Heliconius, H. melpomene and
H. erato, have multiple mimetic forms (Figure 8.1). The color patterns are again controlled
by many loci: 15 in H. erato and 12 in H. melpomene. However, in both species
the loci are scattered at random among the chromosomes rather than being tightly