How To BE FiT 289
species succumb to unfavorable conditions or to a limited sup-
ply of a resource; this was what inspired Darwin to conceive of
natural selection. So selection should always favor more versatile
genotypes. But no species can occupy all environments or eat all
possible food items, and some are exceedingly specialized. How
can we account for specialization [17, 22]?Advantages of specialization
The plasticity that often underlies broad tolerance can be disad-
vantageous because it has costs [2, 4, 40]. There may be costs
both to developing an altered phenotype (e.g., protein synthesis)
and to maintaining the ability to do so. Also, acclimation takes
time, and is triggered by cues such as temperature or day length.
There is a possibility of making a mistake if the cue is somewhat
unreliable, or of being in a maladaptive physiological state if the
environment quickly reverses.
Conversely, specialization may be advantageous for several
reasons. First, interactions with other species may favor special-
ization. We will see in Chapter 13 that when a species competes
with other species for resources such as food or habitat, geno-
types that choose or are adapted to a less used resource can have
higher fitness. In some cases, it is advantageous to evolve a pref-
erence for a safe space, an environment that is relatively free of
predators [10]. Herbivorous insects often suffer less predation and
parasitism if they reside on some host plant species rather than on others [54].
The most fully supported hypothesis for the advantage of specialization is based
on trade-offs, expressed by the aphorism “a jack of all trades is master of none.” A
specialist is likely to become more effective or efficient than a generalist, in which
performance of any one task is likely to be compromised by the characteristics—
behavioral, morphological, or physiological—needed to perform other tasks. Eliza-
beth Bernays proposed that trade-offs in cognitive processing may account for host
specialization in some herbivorous insects [8]. She tested this idea with a species of
aphid that is a host specialist in the eastern United States but a generalist that uses
diverse plants in the West. The eastern specialists were quicker to find a host plant
amid a bouquet of nonhost species, and tapped into the phloem for sap faster than
the western generalists [9]. Morphological trade-offs have been shown in many
organisms. Flowerpiercers are tropical birds, some of which feed on nectar in long,
tubular flowers that are adapted for pollination by hummingbirds. These species
have an unusual hooked bill with which they hold the flower and punch a hole in
its base (FIGURE 11.17A). When the hooked tip of the bill is clipped experimentally,
the birds are less efficient at obtaining nectar, but they become more proficient at
eating berries, which are the main diet of other species of flowerpiercers that have
a less developed hook (FIGURE 11.17B) [52]. A physiological trade-off has been
found between adaptation to salt water and fresh water in a small crustacean, the
copepod Eurytemora affinis, that has recently invaded the North American Great
Lakes from the ocean [34]. Copepods taken from freshwater and saline environ-
ments survived better in their “home” salinity (FIGURE 11.18), and both samples
revealed strong negative genetic correlations in survival at different salinities—
evidence of a trade-off.Specialization without trade-offs
Many studies, however, have not found evidence of trade-offs in performance
across different environments. For example, growth and survival of genotypes
of herbivorous insects, reared on two or more plants, are seldom negativelyFutuyma Kirkpatrick Evolution, 4e
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Evolution4e_11.16.ai Date 11-21-2016Temperature (°C)–10010304010 20 30 40 50 60
LatitudeCTmaxCTminFIGURE 11.16 The critical thermal maximum (CTmax) and
critical thermal minimum (CTmin) of species of frogs at dif-
ferent latitudes. CT is a temperature at which an important
function (often measured by locomotion) fails. Temperate-
zone (high-latitude) species are better able to function at
low temperatures than are tropical species, but there is little
difference among species in their thermal maximum. High-
latitude species have wider temperature tolerance, being
adapted to a more variable thermal environment. (After [24].)11_EVOL4E_CH11.indd 289 3/22/17 1:11 PM