PATTERNS OF DIVERSIFICATION IN PHYTOPHAGOUS INSECTS

Not only are host shifts (usually to related plants) probably an important driver of
speciation for plant-feeding insects (Berlocher and Feder 2002), but even rare
colonizations of unrelated plants may serve to open up new “adaptive zones” in which
adaptive radiation can occur.
Because many host associations are historically stable, phylogenies are especially
important in documenting and explaining patterns of host use in phytophagous insects
(Mitter and Farrell 1991, Farrell et al. 1992). Much of the literature in this area has
centered around the influential idea of coevolution (Ehrlich and Raven 1964), which, as
originally formulated, postulates that insects and plants have been locked in an ancient,
ongoing evolutionary struggle, each adapting and diversifying in response to the other.
When such ancient plant/insect associations persist, coevolution may result in a pattern in
which insects which are “primitive” within a certain lineage may also be associated with
“primitive” plants (Farrell 1998, Ward et al. 2003). However, other kinds of historical
signatures may also be important in phytophagous insect evolution. For example, one
type of pattern has been noted (e.g. Farrell et al. 1992, Wiens and Donoghue 2004) which
could be called “biome tracking”. Because certain biomes (especially tropical forests)
have historically occupied much greater area in past epochs, many insect groups may
have originated in such biomes and later adapted to other climates and habitats (e.g.
temperate forests or grasslands). This kind of evolutionary trend may also result in
patterns detectable in relationships between modern species (i.e. phylogeny) and their
ecological characteristics.
2