Evolution of Foraging Strategy
Comparison of traits among species is a critical part of investigating
adaptations, but it is important to take into account the evolutionary
relationships among the species being compared (Harvey and Pagel,
1991). Two species may share a trait due to common descent or because
both species evolved the same response to similar selection pressure. The
considerable variation in foraging strategy within the genusSteinernema
leads us to questions about how these strategies may have evolved and if
the suites of traits associated with foraging strategy represent adaptations.
The mapping of behavioural traits on to a recent molecular and morpho-
logical phylogeny ofSteinernema(Stocket al., 2001) provides a means of
developing hypotheses about the evolution of foraging strategies within
this group. Mapping the ability to find mobile versus sedentary hosts on to
the phylogeny ofSteinernemasuggests that the ancestral species was an
intermediate forager and that ambush and cruise foraging both evolved at
least once in the genus (Fig. 2.3). We can also hypothesize that the30 J.F. Campbell and E.E. Lewis
Fig. 2.3. Foraging strategy mapped on to the phylogeny ofSteinernemabased on Stock
et al. (2001). Foraging-strategy categories were based on ratio between ability to find mobile
versus non-mobile hosts as shown in Fig. 2.1. Behavioural data are lacking for two species;
in one case this results in an equivocal mapping.