288 CHAPTER 11Specialists and Generalists
As noted in the previous section, understanding the evolution of life history varia-
tion generally focuses on survival and reproduction components of fitness. But a
broader conception of an organism’s life history includes many other aspects of
its life, such as dispersal (see Chapter 8) and its use of habitats, food, and other
resources. For example, if you were to join the growing crowd of moth enthusiasts
in eastern North America, you might want to add the juniper geometer (Patalene
olyzonaria) to your “life list.” The best way to do that would be to find caterpillars
(FIGURE 11.15A) and rear them until they become moths. The only way to find the
caterpillars is to search the foliage of juniper trees, because the juniper geometer
eats nothing else. In the same habitat, you might find larvae of the fall canker-
worm (Alsophila pometaria; FIGURE 11.15B), a member of the same moth family,
on oaks, maples, cherries, elms, hickories, and many other trees. With respect to
diet, one species is a specialist and the other a generalist. Such a distinction can
be made with reference to many aspects of organisms’ ecology. For example, some
species of crabs and other marine animals are restricted to full-salinity waters,
while related species may be found both in seawater and brackish estuaries (FIG-
URE 11.15C,D). In both of these examples, one species is more specialized than the
other; it is said to have a narrower ecological niche.
The range of environments that a species can tolerate is often matched by the
amount of variation in its habitat. Many tropical species experience less variation
in temperature than do species at higher latitudes, where seasonal change is much
more pronounced [28]. As expected, temperate-zone species of Drosophila, frogs,
and other ectothermic animals tend to have broader temperature tolerance than do
tropical species (FIGURE 11.16) [24, 42, 56]. However, this pattern is based mostly
on the greater cold tolerance of high-latitude species; the critical thermal maxi-
mum (CTmax), above which the animal cannot function, is much the same, regard-
less of the species’ geographic distribution. This has led to the suggestion that
CTmax is near or at its upper evolutionary limit. If so, tropical species that already
live near the upper limit may be especially endangered by global warming [16].
Why have species evolved differences in niche width? We might think that
having a broad and versatile niche would usually be advantageous, because envi-
ronmental factors vary in space and time: it should be advantageous to tolerate
changes in salinity or temperature, or to switch to a different host plant if the usual
or best host becomes rare. Even if the environment is constant, individuals in everyFIGURE 11.15 pecialists and general-S
ists. (A) The larva of the juniper geometer
(Patalene olyzonaria) feeds only on juniper
trees, and closely resembles juniper foli-
age. (B) The larva of the fall cankerworm
(Alsophila pometaria) is a generalist that
feeds on diverse species of trees. (C) The
blue crab (Callinectes sapidus) is found in
waters with a broader range of salinity than
is (D) the less tolerant lady crab (Ovalipes
ocellatus).Futuyma Kirkpatrick Evolution, 4e
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