Biology Reference
In-Depth Information
strong and evidently genetic. Studies of the seasonal emergence of the
three races also showed that the plum race appeared nearly 2 wk earlier
in the season than the apple and walnut races because plum orchards are
about 2-3 wk ahead of apple and walnut orchards in their seasonal devel-
opment.To achieve the earlier emergence requires genetic adaptations in
the mechanism of breaking diapause and in developmental heat require-
ments. Thus, adjustments in several genetic systems were probably
involved in the evolution of these races. Circumstantial evidence suggests
that the walnut race differentiated from the apple race, and the plum race
from the walnut race.
The codling moth is just one of many alien herbivores that have been
introduced to new regions, both inadvertently and deliberately. Arthro-
pod pests of crops, forages, horticultural plants, commercial forest trees,
and even weeds tend to track the introductions of these plants to new
world regions. Often, they arrive without their own competitors, preda-
tors, parasites, and diseases.Vertebrate herbivores have likewise been intro-
duced accidentally and deliberately, the latter for real or imagined bene-
fits. Increasingly, in addition, deliberate introductions are being made for
anticipated biological control benefits. All of these species are subject to
selective pressures that may lead to evolutionary shifts in their patterns of
plant feeding.
Coevolution of Plants and Herbivores
Although in this chapter we focus on evolutionary changes by alien her-
bivores, we should note that these patterns represent only one side of a
coevolutionary process (Thompson 1994, 1999a).Techniques of molecu-
lar biology and chemical ecology are revealing that plant-herbivore inter-
actions lead to evolutionary shifts by both members. Plants evolve protec-
tive chemicals in response to herbivore damage, and herbivores in turn
evolve ways to overcome these defenses (Cornell and Hawkins 2003). In
the case of many interactions involving plants and alien herbivores, these
interactions tend to occur in patchy environments, in which details of the
evolutionary setting vary. In some locations, which represent coevolution-
ary “hot spots,” particular plants and herbivores may show very tight pat-
terns of mutual evolutionary adjustment (Thompson 1999b). In other sit-
uations, corresponding to evolutionary “cold spots,” the degree of mutual
adjustment may be weak due to disruptive influences of other evolution-
ary pressures.Thus, a geographic mosaic of strongly and weakly coevolved
populations of plants and herbivores is likely to exist.
