Review and Synthesis Finally, rather than studying ecological specialization directly, multivariate morphology has been used to indirectly infer an average-special continuum. Results suggest that morphologically average lineages have longer evolutionary lifespans than special or outlier ones within clades (Liow 2007b). Limitations The true taxonomic duration or survivorship of the taxa under study is difﬁcult to estimate with precision due to the patchy nature of the fossil record. There may also be worries that certain types of specialists (e.g. living in uncommon habitats that are rarely preserved) may be seldom sampled such they appear to have shorter durations than they actually do. However, various measures are routinely used in paleobiolo- gical studies to alleviate the sampling issue, including bootstrapping (e.g. Baumiller 1993; Labandeira et al. 2002), conﬁdence intervals (e.g. Labandeira et al. 2002), rarefaction and weighing (e.g. Liow 2007a). Also, paleontological studies estimating taxon durations (rather than comparing survivor- ship across extinction events) often remove extant taxa to avoid duration truncation albeit this may cause new problems: if extant species are skewed in their representation of specialists, we may be left with biased datasets. Direct observations of resource and habitat-use are often not accessible in paleontological studies (Table 2: 1). Studies using proxies to infer specialization tend to conclude that specialist species have shorter durations more often than studies that directly observe the use of environments (Table 2: 1). Moreover, decline within a given resource type has been estimated in only one of the studies in our review (Labandeira et al. 2002). The authors of this study concluded that specialist herbivorous insects were at greater extinction risk. Where habitat ⁄ resource use has actually been observed, we often do not know whether individuals in generalist species can live on only a single one of the resource ⁄ habitat types in question or whether they require a combination of multiple types. Where we do know that they can live on a single resource, these generalist species also tend to have longer durations than specialist species (Table 2: 6). Most studies do infer specialization independently from decline, but those that do not tend to report overly weak tendency of specialists to decline [Table 2: 8())]. Finally, paleoecological studies largely focus on marine invertebrates and on dietary specialization, although this does not seem to bias the conclusions (Table 2). Summary The majority of the paleoecological studies reviewed indicate that specialization may shorten species durations (10 of 16 studies) or reduce survivorship during major Are specialists at risk? 855 extinction events (four of six). Exceptions are more common among cases of habitat specialization. Moreover, the number of quantitative studies is relatively limited, and studies using directly observed specialization often lead to different conclusions compared with those using inferred specialization. From an evolutionary perspective, a possible response of a specialist to environmental change, other than extinction, is speciation. Alternatively, most specialists may become extinct rather than undergo speciation and most species would thus have non-specialist ancestors. These questions can be addressed by a phylogenetic approach where traits, including specialization, can be mapped onto phylogenies, and their evolution traced. T H E P H Y L O G E N E T I C A P P R O A C H Why phylogenetic biologists study specialization Phylogenetic trees can be used to address many macroevo- lutionary questions, including the direction and reversibility of evolutionary trait changes (Brooks & McLennan 2002), and for our purposes, of the trait state specialist . Specialization has long been thought to be a dead-end leading to diminished speciation and increased extinction (Cope 1896; Simpson 1944; Moran 1988). If specialization is a dead-end, this trait state should always be phylogenetically young (Futuyma & Moreno 1988). Most earlier origins of specialists should have disappeared too rapidly to leave a phylogenetic trace (Brooks & McLennan 2002). Specialists are thus expected to be relatively recent in origin and short in persistence. Specialists should unlikely be ancestors of generalists, whereas many generalists are expected to have evolved into specialists (generalists-to-specialist-hypothesis; Futuyma & Moreno 1988; Thompson 1994; Schluter 2000; Stephens & Wiens 2003). The hypothesis of specialization as a dead-end and the generalist-to-specialist hypothesis both imply that specialists lose the capacity to adapt to novel environments due to either of two microevolutionary mechanisms (Futuyma 2001): (1) Trade-off: individuals may sacriﬁce their capacity to use different resources to gain competitive superiority on one of the resources ⁄ habitats. Trade-offs will drive generalists to become increasingly specialized; (2) Neutral processes: a specialist accumulates mutations that would be disadvanta- geous in other environments but are not selected against in the environment that the specialist actually uses. This may render future expansions of the environmental niche impossible and specialization thus becomes ﬁxed. Methods and data The reconstruction of ancestral states of specialist ⁄ generalist traits using data from extant species is a key tool used to test 2009 Blackwell Publishing Ltd/CNRS