Abstract
BackgroundEcological speciation is a process in which a transiently resource-polymorphic species divides into two specialized sister lineages as a result of divergent selection pressures caused by the use of multiple niches or environments. Ecology-based speciation has been studied intensively in plant-feeding insects, in which both sympatric and allopatric shifts onto novel host plants could speed up diversification. However, while numerous examples of species pairs likely to have originated by resource shifts have been found, the overall importance of ecological speciation in relation to other, non-ecological speciation modes remains unknown. Here, we apply phylogenetic information on sawflies belonging to the 'Higher' Nematinae (Hymenoptera: Tenthredinidae) to infer the frequency of niche shifts in relation to speciation events.ResultsPhylogenetic trees reconstructed on the basis of DNA sequence data show that the diversification of higher nematines has involved frequent shifts in larval feeding habits and in the use of plant taxa. However, the inferred number of resource shifts is considerably lower than the number of past speciation events, indicating that the majority of divergences have occurred by non-ecological allopatric speciation; based on a time-corrected analysis of sister species, we estimate that a maximum of c. 20% of lineage splits have been triggered by a change in resource use. In addition, we find that postspeciational changes in geographic distributions have led to broad sympatry in many species having identical host-plant ranges.ConclusionOur analysis indicates that the importance of niche shifts for the diversification of herbivorous insects is at present implicitly and explicitly overestimated. In the case of the Higher Nematinae, employing a time correction for sister-species comparisons lowered the proportion of apparent ecology-based speciation events from c. 50-60% to around 20%, but such corrections are still lacking in other herbivore groups. The observed convergent but asynchronous shifting among dominant northern plant taxa in many higher-nematine clades, in combination with the broad overlaps in the geographic distributions of numerous nematine species occupying near-identical niches, indicates that host-plant shifts and herbivore community assembly are largely unconstrained by direct or indirect competition among species. More phylogeny-based studies on connections between niche diversification and speciation are needed across many insect taxa, especially in groups that exhibit few host shifts in relation to speciation.
Highlights
Ecological speciation is a process in which a transiently resource-polymorphic species divides into two specialized sister lineages as a result of divergent selection pressures caused by the use of multiple niches or environments
Ecological speciation is a process in which a shift in resource or habitat use within an ancestral species triggers the formation of two new sister species, each adapted to exploit different niches [1,2]
Phylogenetic trees The Bayesian and ML analyses of the sequence data produced relatively well-supported trees showing that the Higher Nematinae constitutes a monophyletic clade within the subfamily Nematinae (Figs. 3 and 4; the ML tree is included in Additional file 2)
Summary
Ecological speciation is a process in which a transiently resource-polymorphic species divides into two specialized sister lineages as a result of divergent selection pressures caused by the use of multiple niches or environments. Only a small fraction of available plants constitute a suitable food source for most insect species, contrasts of herbivore versus plant phylogenies have in many cases revealed drastic discrepancies between the phylogenetic trees [18,19,20]. In essence, this means that host-plant associations are evolvable and change occasionally during the evolutionary history of insect lineages [21,22,23]. Occasional colonizations of novel hosts can theoretically cause ecological speciation, which could provide an explanation for the enormous species diversity of plantfeeding insects on the Earth [24,25,26]
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