Evolution__3rd_Edition

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The tiger swallowtail butterfly
exists in two forms
Another swallowtail species exists
in many forms
CHAPTER 8 / Two-locus and Multilocus Population Genetics 195
8.1 Mimicry in Papilio is controlled by more than one
genetic locus
The characters we dealt with in earlier chapters have been characters controlled by
single genetic loci. Enzymes, such as alcohol dehydrogenase, are encoded by a single
gene, and it is not much of a simplification to treat the polymorphism in the peppered
moth as a set of genotypes at one locus. We now move on to consider evolutionary
changes at more than one locus.
The first example concerns a multilocus polymorphism. We can lead into it via a
similar polymorphism that is controlled by a single locus. Both examples come from
the same attractive group of butterflies called swallowtails. The swallowtails have a
global distribution, and Papilio is the largest genus of them; their most striking
characteristic is a “tail” on the hindwing. Swallowtail butterflies come in many colors a
gorgeous greens, subtle shades of reds and orange, and marbled patterns in white and
gray a but the commonest type has stripes of black and yellow. The North American
tiger swallowtail Papilio glaucus is easy to recognize by its tiger stripes, as it flutters
through woodland lanes or humid valleys. Or rather, most tiger swallowtails are easy to
recognize in this way. In part of the species’ range (roughly, to the southeast of a line
from Massachusetts to south Minnesota and from east Colorado to the Gulf Coast) the
standard form of the tiger swallowtail lives alongside another form of the same species.
This second form is black, with red spots on its hindwings, and is called nigra; it is only
found in females. The nigra form is not poisonous, but mimics another species, the
pipevine swallowtail Battus philenor, which is poisonous. The nigra form’s geographic
distribution fits that of the pipevine swallowtail. The nigra form is well protected there
from predatory birds that have learned by stomach-churning experience not to eat
butterflies looking like pipevine swallowtails. The tiger swallowtail, therefore, has a
mimetic polymorphism. It has both the standard non-mimetic tiger morph of yellow
and black stripes, and a black mimetic morph.
The tiger swallowtail P. glaucus comes to look almost simple when compared with
the amazing array of females in the species P. memnon (see Plate 3, between pp. 68
and 69). P. memnon lives in the Malay Archipelago and Indonesia; its male is again
non-mimetic, though its color is deep blue rather than yellow and black stripes.
However, instead of one mimetic female morph, P. memnon females come in almost
numberless variety. Their forewings show different geometric patterns of black and
white; their hindwings, as well as varying in shape, can be colored in yellow, orange,
or blood red, and may or may not have a bright white spot; some have tails, others do
not; the abdomen varies in color; and a spot at the butterfly’s “shoulder” (i.e., at the
base of the forewing near the head) called the epaulette, may be present in various
shades of red.
Clarke & Sheppard (1969, also Clarke et al. 1968) suggested that each female form
(or “morph”) mimics a different model (Plate 3 shows six examples: notice that three of
them have tails and three do not). Their evidence is not strong, as it comes only from
the geographic ranges of mimic and model, and from superficial similarity of appearance
(which is not exact in all cases). Good evidence for mimicry requires experimentally
showing that birds that have learned to avoid the model will also then avoid the