Is specialization an evolutionary dead end? Testing for differences in speciation, extinction and trait transition rates across diverse phylogenies of specialists and generalists

Abstract

Specialization has often been claimed to be an evolutionary dead end, with specialist lineages having a reduced capacity to persist or diversify. In a phylogenetic comparative framework, an evolutionary dead end may be detectable from the phylogenetic distribution of specialists, if specialists rarely give rise to large, diverse clades. Previous phylogenetic studies of the influence of specialization on macroevolutionary processes have demonstrated a range of patterns, including examples where specialists have both higher and lower diversification rates than generalists, as well as examples where the rates of evolutionary transitions from generalists to specialists are higher, lower or equal to transitions from specialists to generalists. Here, we wish to ask whether these varied answers are due to the differences in macroevolutionary processes in different clades, or partly due to differences in methodology. We analysed ten phylogenies containing multiple independent origins of specialization and quantified the phylogenetic distribution of specialists by applying a common set of metrics to all datasets. We compared the tip branch lengths of specialists to generalists, the size of specialist clades arising from each evolutionary origin of a specialized trait and whether specialists tend to be clustered or scattered on phylogenies. For each of these measures, we compared the observed values to expectations under null models of trait evolution and expected outcomes under alternative macroevolutionary scenarios. We found that specialization is sometimes an evolutionary dead end: in two of the ten case studies (pollinator-specific plants and host-specific flies), specialization is associated with a reduced rate of diversification or trait persistence. However, in the majority of studies, we could not distinguish the observed phylogenetic distribution of specialists from null models in which specialization has no effect on diversification or trait persistence.

Venom gland size and venom complexity—essential trophic adaptations of venomous predators: A case study using spiders

“…We found that the complexity of venom components was lower for specialists than for generalists. Specialization is more derived compared to an assumed ancestral generalist state (e.g., Day, Hua, & Bromham, ; Pekár, Coddington, & Blackledge, ). This seems to contradict the classical theory of venom evolution, which is based on frequent gene duplication, positive selection and protein neofunctionalization (Kordiš & Gubenšek, ), and should lead to more complex arsenal.…”

Phylogenetic evidence from freshwater crayfishes that cave adaptation is not an evolutionary dead‐end

“…1993; Day et al. 2016). However, failure to diversify or transition back to an ancestral state does not signify a lack of success per se.…”

“…Day et al. (2016) found mixed support for the evolutionary dead‐end hypothesis for specialists across a set of 10 phylogenies. In general, however, these studies focus on groups with either a small number of specialized taxa or with very few replicated events of specialist evolution.…”

Historical demography of four gecko species specializing in boulder cave habitat: Implications in the evolutionary dead end hypothesis and conservation

“…The decrease of specialist species in an ecosystem can be a major threat to global biodiversity (Clavel, Julliard, & Devictor, ). However, comparative studies often fail to find higher extinction rates in specialists than in generalists (Day, Hua, & Bromham, ; Stern et al, ; Tripp & Manos, ). Therefore, whether ecological specialization determines the fate of a species remains controversial (Day et al, ; Stern et al, ).…”