Evolution, 4th Edition

NATuRAl SElECTioN ANd AdAPTATioN 71

predictions made by evolutionary theory, like those in

many other scientific disciplines, are usually predictions

of what we will find when we collect data. (Prediction in

evolutionary theory does not usually mean that we predict

the future course of evolution of a species.) Predictions of

what we will find, deduced from hypotheses, constitute

the hypothetico-deductive method, of which Darwin

was one of the first effective exponents [16, 45].

imperfections and Constraints

Darwin noted that “natural selection will not produce

absolute perfection, nor do we always meet, as far as we

can judge, with this high standard in nature” (On the Origin

of Species, chapter 6). Selection can fix only those genetic

variants with a higher fitness than other genetic variants in

a particular population at a particular time. It cannot fix the

best of all conceivable variants if they do not arise, or have

not yet arisen, and the best possible variants often fall short

of perfection because of various constraints [14, 49]. Among

these constraints are trade-offs (following the maxim

“There is no such thing as a free lunch”). For example, with

Futuyma Kirkpatrick Evolution, 4e

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Evolution4e_03.20.ai Date 11-28-2016

200

Testes weight (g)

100

10

(^1110)
Body weight (kg)
100 200
Polygamous
Monogamous
FIGURE 3.20 Relationship between weight of the testes and body
weight in polygamous and monogamous primate taxa. The data
support the prediction, based on the theory of natural selection,
that males in polygamous species have relatively larger testes, which
produce more sperm, than do males in monogamous species. The
photos show polygamous mating bonobos (Pan paniscus) and a
monogamous pair of yellow-cheeked gibbons (Nomaseus gabriel-
lae). (After [25].)
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Evolution4e_0319.ai Date 11-02-2016
(A) (B)
(C) (D)
FIGURE 3.19 Body form of fishes is adapted for different modes of swimming. Open-
water, fast-moving predators such as (A) jacks (Carangidae) and (B) swordfishes (Xiphi-
idae) have a slender body, narrow caudal peduncle, and narrow, forked tail fin. Fishes
that maneuver in small spaces have a deep body, as in (C) angelfishes (Pomacanthidae)
and (D) the Moorish idol (Zanclidae), both inhabitants of coral reefs. (C and D courtesy
of Michael D. Bryant.)
03_EVOL4E_CH03.indd 71 3/22/17 1:19 PM