Evolution, 4th Edition

70 CHAPTER 3

predators in many families commonly have a streamlined shape, a forked tail fin,

and a slender tail base (caudal peduncle), whereas fishes that live in complicated

environments, such as among corals or vegetation, have a deep, compressed body

that enables them to change direction rapidly (FIGURE 3.19).

Biologists often predict such correlations by postulating, perhaps on the basis

of a model, the adaptive features we would expect to evolve repeatedly in response

to a given selective factor. For example, in species in which a female mates with

multiple males, the several males’ sperm compete to fertilize eggs. Males that pro-

duce more abundant sperm should therefore have a reproductive advantage. In

primates, the quantity of sperm produced is correlated with the size of the testes,

so large testes should be expected to provide a greater reproductive advantage in

polygamous than in monogamous species. Paul Harvey and Mark Pagel compiled

data from prior publications on the mating behavior and testes size of various pri-

mates [25]. They confirmed that, as predicted, the weight of the testes, relative to

body weight, is significantly higher among polygamous than monogamous taxa

(FIGURE 3.20).

An important aspect of this example is that although all the data needed to test

this hypothesis already existed, the relationship between the two variables was

not known until Harvey and Pagel compiled the data, because no one had had any

reason to do so until an adaptive hypothesis had been formulated. Hypotheses

about adaptation can be fruitful because they suggest investigations that would

not otherwise occur to us.

Also, notice that because the consistent relationship between testes size and

mating system was not known a priori, the hypothesis generated a prediction. The

Futuyma Kirkpatrick Evolution, 4e

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Evolution4e_03.18.ai Date 12-01-2016

Penstemon strictus

P. barbatus

(B) Normal ower

(C) Lower lip removed

(D) Projecting anthers

(E) Narrow corolla

(A)

FIGURE 3.18 Experimental test of a hypothesis of

adaptation. (A) Bee-pollinated Penstemon strictus

(top) and hummingbird-pollinated P. barbatus

(bottom). (B–E) Experimental modifications of

flowers of a bee-pollinated species (Penstemon

strictus) to mimic features of hummingbird-

pollinated species of Penstemon. (B) The normal

flower of P. strictus. Modifications included (C)

removal of the lower lip “landing platform,” (D) re-

attaching stamens so that the anthers project from

the flower, and (E) constriction to form a narrower

corolla tube. (B–E after [7].)

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