Evolution__3rd_Edition

270 PART 3 / Adaptation and Natural Selection
“Whether” and “how” a character
is adapted are different questions
Adaptation can be studied ...
. . . by engineering models, ...
10.5.4 Conclusion: the genetics of adaptation
We have met four theories about the genetic changes that occur during adaptive evolution.
The “Goldschmidt” theory, that adaptations evolve by macromutations, has been
rejected because of its theoretical implausibility. Macromutations will almost always
reduce the quality of adaptation. Wright’s theory, that adaptations evolve by the shifting
balance process, has not been the topic of this section; but should be included for
completeness. We saw in Section 8.13 (p. 216) that the shifting balance theory continues
to inspire research, but no one has yet shown it to be important in evolution.
Fisher’s original theory suggested that adaptive evolution proceeds only by many
mutational steps each of small effect. This theory has never been ruled out (or ruled in),
and has been highly influential. However, modern research is looking at an expanded
theory, that builds additional factors onto Fisher’s basic model. Experimental work
may be able to test what mix of large or small mutations contribute to adaptive evolution,
depending on the ecological conditions.
10.6 Three main methods are used to study adaptation
We should distinguish two questions about any character of an organism. One is
whether it is adaptive. The other is (if the character is an adaptation) how it is an adaptation.
The first question is complicated, because the answer will depend on what
definition of adaptation is used. Several definitions exist, and the methods of recognizing
adaptations vary from definition to definition. We shall return to the question in
Section 10.8 below. Here we can look at the methods used to study adaptations, to work
out how the attribute in question is adaptive.
The study of adaptation proceeds in three conceptual stages. The first is to identify,
or postulate, what kinds of of genetic variant the character could have. Sometimes, as
in peppered moths (Section 5.7, p. 108) for instance, this is done empirically. Other
characters do not vary genetically and for them it is necessary to postulate appropriate
theoretical mutant forms. For example, when we come in Chapter 12 to look at why
sex exists, we shall postulate mutant forms that reproduce clonally, or asexually.
The second stage is to develop a hypothesis, or a model, of the organ or character’s
function. The original hypothesis for peppered moths was that coloration functioned
as camouflage. Hypotheses are of varying quality, but they can be improved on as work
proceeds. As we saw in Section 5.7 (p. 108), melanic coloration in peppered moths
seems to have some other advantage in addition to camouflage in polluted areas.
Another example comes from beak shape in birds. In this book, we shall often consider
beak shape as an adaptation to the food supply. Larger beaks are better at eating larger
and tougher food items, as we saw from the Grant’s research on Darwin’s finches
(Section 9.1, p. 223). However, beaks have other functions too, including lice preening,
and beak shape matters for those other functions (Clayton & Walther 2001).
A good hypothesis is one that predicts the features of an organ exactly, and makes
testable predictions. In morphology, these predictions are often derived from an engineering
model. For example, hydrodynamics is used to understand fish shape, while
..