Evolution__3rd_Edition

..
. . . by experiment, ...
. . . and by the comparative method
CHAPTER 10 / Adaptive Explanation 271
Table 10.1
The wing stripe of some butterflies was painted over, and controls were painted with
transparent paint that did not affect their appearance. The number with intact wings at
different times after the treatment was measured. The frequency distributions are not
significantly different. From Silbergleid et al. (1980).
Age at Painted butterflies Controls
capture
(week) n % n %
0 81 83.5 88 90.7
1 14 14.4 6 6.2
2 2 2.1 2 2.1
3 0 0 1 1.0
4 0 0 0 0
5 0 0 0 0
construction engineering is used for shell thickness in a mollusk: the costs of building a
thicker shell have to be weighed against the benefits of reduced breakage, by wave
action or predators. This sort of research can be carried out at all levels, from the simple
and qualitative through to sophisticated algebraic modeling.
Stage three is to test the hypothesis’s predictions. Three main methods are available.
One is simply to see whether the actual form of an organ (or whatever character is
under investigation) matches the hypothetical prediction; if it does not, the hypothesis
is wrong somehow.
A second method is to do experiments. It is only useful if the organ, or behavior pattern,
can be altered experimentally. Almost any hypothesis about adaptation will predict
that some specified form of an organ will enable its bearer to survive better than
some other forms, but the alternatives are not always feasible. We cannot, for example,
make an experimental pig with wings to see whether flight would be advantageous.
When they are possible, experiments are a powerful means of testing ideas about adaptation.
Animal coloration, for instance, has been studied in this way. Color patterns in
some butterfly species are believed to act as camouflage by “breaking up” the butterfly’s
outline. Silberglied et al. (1980), working at the Smithsonian Tropical Research
Institution at Panama, experimentally painted out the wing stripes of the butterfly
Anartia fatima. The butterflies with their wing stripes painted out showed similar levels
of wing damage (which is produced by unsuccessful bird attacks) and survived equally
as well as control butterflies (Table 10.1); the wing stripes, therefore, are not in fact
adaptations to increase survival. They may have some other signaling or reproductive
function, though that would need to be tested by further experiments.
The comparative method is the third method of studying adaptation. It can be used if
the hypothesis predicts that some kinds of species should have different forms of an
adaptation from other kinds of species. Darwin’s classic study of the relation between
sexual dimorphism and mating system is an example we shall discuss below. Some