856 A. Colles, L. H. Liow and A. Prinzing hypotheses on the directionality of specialist-generalist evolution. This approach is relevant because ecological specialization is to some degree heritable i.e. phylogeneti- cally conservative (Bra¨ndle et al. 2002; but see Bo¨ hning- Gaese & Oberrath 1999). The impact of environmental changes on specialists is testable when for instance specialists of a given lineage colonize a new continent and thus become exposed to a new environment (Armbruster & Baldwin 1998). However, even when no direct information on environmental changes is available, it is obvious that both biotic and abiotic environments have changed drastically throughout the lifetime of entire phylogenetic lineages, i.e. millions of years (e.g. Thompson 1994). Results Examples of specialization as an evolutionary constraint Cases of excessive specialization on individual host plants constraining further evolutionary diversiﬁcation have been described for Dendroctonus wood beetles: those feeding only on a single tree genus appear only at or close to the tips of the phylogeny, in agreement with the generalist-to-specialist hypothesis (Kelley & Farrell 1998). The restriction of aphid nymphs to a single host–plant species also constrains diversiﬁcation (Moran 1988). A tendency of generalists to give rise to specialists, but not the reverse, was found in walking sticks (Crespi & Sandoval 2000). Specialization is thus a handicap. Once established, specialization is rarely reversible and many ancient specialists did not survive until present. Counterexamples However, numerous studies fail to conﬁrm the generalist- to-specialist and the dead-end hypotheses. For instance, in the seed beetle genus Stator, species specialized on Acacia may evolve into generalists, but also into specialists using a novel host plant (Morse & Farrell 2005). Similarly, butterﬂies of the tribe Nymphalini were originally special- ists, but later evolved into generalists or specialized on novel hosts (Janz et al. 2001). Hence, it was concluded that specialization is not a path of no return (Janz et al. 2001). Plant-pollinator relationships may be exempliﬁed by the continental-African euphorbia Dalechampia, pollinated by specialized pollinators collecting the plant s resin. After migration to Madagascar, Dalechampia interacted with a more diverse and generalist pollinator fauna as specialist pollinators were not available (Armbruster & Baldwin 1998). Here, a specialist plant responded to its novel environment by becoming more generalist and perhaps simultaneously escaping its previous natural enemies. Nevertheless, Rezende et al. (2007) found within a local pollinator community that specialized mutualistic interac- 2009 Blackwell Publishing Ltd/CNRS Review and Synthesis tions were phylogenetically conserved, i.e. specialist ances- tors neither evolved into generalist descendants nor did they go extinct. Such phylogenetic patterns within com- munities may reﬂect larger-scale evolutionary processes of entire lineages (Prinzing et al. 2008). Overall, some 80% of the studies reviewed indicate that specialists are well capable of changing back to generalists, and where they did not, they nevertheless survived, contradicting the generalist-to-specialist hypothesis and the dead-end hypothesis respectively (Table 2; column phylog- eny ). Earlier reviews (Futuyma & Moreno 1988; Thompson 1994; Schluter 2000) have already pointed this direction. Based on our more extensive review we are able to explore whether this lack of evidence for specialization as an extinction risk may result from particular limitations or biases (Why the Three Approaches Come to Different Conclusions: From Biases to Mechanisms). Limitations Limitations of the phylogenetic approach stem mainly from uncertainty and biases in reconstructing ancestral states. Stireman (2005) showed that standard methods like parsi- mony or symmetric maximum likelihood tend to assign specialist states to ancestors and generalist states to descendants. This bias may be caused by unequal numbers of specialist and generalist species in data, where most reconstructed trait transitions go from the rarer to the more common trait state (Nosil & Mooers 2005). We found evidence for a bias due to the relative frequency of specialists and non-specialists, but in the opposite direction: all studies in which specialists are more numerous than generalists concluded that specialization is not a dead-end and specialists can already be found among ancestors (Appendix S1) while 33% of the remaining studies con- cluded specialization to be a dead-end (n = 15, v2 = 3.46, d.f. = 1, P = 0.063). The accuracy of character-state recon- structions have been studied by comparing true character states observed in the fossil record or in vitro evolution of microbe lineages to reconstructed states: One study con- firmed the correctness of the reconstructions (Polly 2001) but two others did not, in particular where there was a strong evolutionary trend of traits across clades (Oakley & Cunningham 2000; Webster & Purvis 2002). Character state reconstructions as weighted averages within-lineage across descendants of a given ancestor appear to be more robust (Oakley & Cunningham 2000 and references therein) but do not permit testing of the polarity of trait changes. They do however permit testing for specialization as a dead-end (i.e. specialist ancestors having fewer descendants than their generalist sister-taxa). Where this has been performed (n = 5), the results were not drastically different from those in studies applying character-state reconstructions across