Biology Reference
In-Depth Information
tics of prime importance are morphology and behavior that enable indi-
viduals to find, capture, and process prey organisms efficiently. Body size
of predators relative to that of their prey, for example, is a feature often
subject to strong natural selection.
For parasites and parasitoids, morphology and behavior likewise are
important, but physiological characteristics related to virulence—their
ability to infect and develop in host species—are also critical. Some fruit
flies of the genus
Drosophila
, for example, tend to defend against parasitoid
wasps by an immune response that leads to encapsulation of the wasp eggs
or larvae (Kraaijeveld and Godfray 1999). The parasitoids themselves
counter this response in various ways, including physiological mecha-
nisms and egg placement behavior. Kraaijeveld et al. (2001) showed
experimentally that selection can improve the ability of parasitoids to
infect
Drosophila
species that defend by encapsulation. In nature, the effec-
tiveness of these responses by parasitoids varies geographically with host
species in a complex manner. Nevertheless, it appears that host selection
by parasitoids can evolve quickly, given strong selective pressures (Rolff
and Kraaijeveld 2001). Distinct host races have been described for several
parasitoids of scale insects, for example (Diehl and Bush 1984).
Most predators, vertebrate or invertebrate, tend to be adapted to prey
of a particular size. For such predators, especially vertebrates, partitioning
of prey sizes is often the evolutionary outcome of food competition. Par-
titioning is often evident as character displacement when guilds of pred-
ator species differing in number of species are compared. Character dis-
placement is the increased difference in quantitative characters of two or
more species in areas of sympatry compared to areas of allopatry. For
predators, character displacement is usually seen in body size or in size of
jaws, teeth, or other elements of the feeding apparatus. In the context of
alien species, however, this phenomenon is often seen as character release,
a shift in morphology toward an intermediate condition when a species
is introduced to an area where it is freed from one or more competitors.
Robinson and Wilson (1994) summarized information on character
displacement and release in fish belonging to 52 genera in 17 families,
indicating that the phenomena are a frequent response to competition for
food. These phenomena are especially common in freshwater faunas of
northern North America, suggesting that many of the patterns are post-
glacial in origin. Dayan and Simberloff (1994) demonstrated that species
of mustelid mammals in Britain and Ireland show character displacement
in tooth and skull sizes, although the relation of this pattern to prey use
is not clear (McDonald 2002). These species are postglacial colonizers of
