Biology Reference
In-Depth Information
of interbreeding. In still other cases, they may constitute distinct, but per-
haps unrecognized, biological species that essentially do not interbreed.
For forms capable of interbreeding, some differences may exist because of
genetic drift, but consistent differences must be maintained by natural
selection in the face of the tendency of gene flow to maintain genetic
homogeneity.
Ecological, and ultimately, evolutionary shifts of host plants by herbiv-
orous invertebrates do occur, however. Even species with relatively
restricted host preferences tend to show some genetic variability in host
preference, and selection can often alter such preferences rapidly. Under
experimental conditions, for example, oviposition preferences of some
herbivorous arthropods could be changed in as few as 10-16 generations
(Bernays and Graham 1988). Under artificial selection, the rice planthop-
per (
Nilaparvata lugens
) was converted to a biotype capable of attacking a
formerly resistant rice variety in only 10 generations (Claridge and den
Hollander 1980). The ability of such species for adaptation to new hosts
or hosts with altered characteristics is essentially universal. In most cases,
however, the new plant hosts are likely to be close relatives of former
hosts (Futuyma 2000).
In some cases, the shifts are encouraged when alternate plant hosts
provide “enemy-free space” that compensates for any reduction in growth
and survival that the herbivore may experience on the new host plant
(see, e.g., Gratton and Welter 1999).That is, a new host plant may provide
a reduction in predator or parasitoid pressure on the herbivore, reducing
its mortality due to these agents. If this benefit exceeds the growth and
survival detriments due to feeding on a new plant host, natural selection
will favor the host plant shift. In time, selection may also improve the
growth and survival performance of the herbivore on the new host.
In other cases, the availability of alternate food plants with low herbi-
vore utilization may permit host plant shifts or expansion. Funk and
Bernays (2001), for example, suggested that the ragweed aphid,
Uroleucon
ambrosiae
, which primarily uses a giant ragweed (
Ambrosia trifida
) in east-
ern North America, has recently expanded its range into the southwest-
ern United States and Mexico, where it utilizes a wide range of host
plants of the same family. Although aphids from southwestern populations
still show the highest preference for giant ragweed, they accept other host
plants more readily than do eastern aphids. Bernays and Funk (2000)
found that olfactory organs of the antennae of ragweed aphids from the
southwestern United States were fewer in number and possessed a smaller
