PATTERNS OF DIVERSIFICATION IN PHYTOPHAGOUS INSECTS

An alternative approach focuses on the genetic and ecological mechanisms by
which host range changes. Thus, Crespi and Sandoval (2000; see also Nosil et al. 2003)
conclude that host specialization in Timema walking sticks comes about when host-
associated color polymorphism in polyphagous ancestors is converted into species
differences under disruptive selection by predators. Phylogenetic evidence by itself is
consistent with but does not strongly establish ancestral polyphagy. However, that
interpretation is supported by abundant experimental and other evidence. Similar logic is
reflected in the elaboration of a novel hypothesis about butterfly host range (e.g., Janz et
al. 2001, Weingartner et al. 2006, Janz and Nylin 2008). A phylogeny for the nymphalid
tribe Nymphalini suggests ancestral restriction to Urticales followed by repeated host
range expansions as well as contractions, with multiple ostensibly independent
colonizations of a set of disparate plant families. Complementary experiments show that
larvae of many species are able to feed on hosts not presently used by that species, but
characteristic of their inferred ancestors and/or extant relatives. Retained latent feeding
abilities may help to explain rapid expansions (and hence observed lability) of host range.
Polyphagy may also facilitate radical host shifts (and/or further broadening of host
range), given that less specialized species seem to generally make more oviposition
mistakes (Janz et al. 2001), and has been suggested to thereby promote diversification
(Weingartner et al. 2006, Janz et al. 2006, Janz and Nylin 2008). This postulate stands in
direct contrast to the prediction that specialization promotes faster speciation, for which
evidence is currently lacking (see below).
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