Abstract
BackgroundMajor modifications to the pharyngeal jaw apparatus are widely regarded as a recurring evolutionary key innovation that has enabled adaptive radiation in many species-rich clades of percomorph fishes. However one of the central predictions of this hypothesis, that the acquisition of a modified pharyngeal jaw apparatus will be positively correlated with explosive lineage diversification, has never been tested. We applied comparative methods to a new time-calibrated phylogeny of labrid fishes to test whether diversification rates shifted at two scales where major pharyngeal jaw innovations have evolved: across all of Labridae and within the subclade of parrotfishes.ResultsDiversification patterns within early labrids did not reflect rapid initial radiation. Much of modern labrid diversity stems from two recent rapid diversification events; one within julidine fishes and the other with the origin of the most species-rich clade of reef-associated parrotfishes. A secondary pharyngeal jaw innovation was correlated with rapid diversification within the parrotfishes. However diversification rate shifts within parrotfishes are more strongly correlated with the evolution of extreme dichromatism than with pharyngeal jaw modifications.ConclusionThe temporal lag between pharyngeal jaw modifications and changes in diversification rates casts doubt on the key innovation hypothesis as a simple explanation for much of the richness seen in labrids and scarines. Although the possession of a secondarily modified PJA was correlated with increased diversification rates, this pattern is better explained by the evolution of extreme dichromatism (and other social and behavioral characters relating to sexual selection) within Scarus and Chlorurus. The PJA-innovation hypothesis also fails to explain the most dominant aspect of labrid lineage diversification, the radiation of the julidines. We suggest that pharyngeal jaws might have played a more important role in enabling morphological evolution of the feeding apparatus in labrids and scarines rather than in accelerating lineage diversification.
Highlights
Major modifications to the pharyngeal jaw apparatus are widely regarded as a recurring evolutionary key innovation that has enabled adaptive radiation in many species-rich clades of percomorph fishes
The rest of our estimates within parrotfishes including the age of the seagrass clade, reef clade, Scarus + Chlorurus and crown ages of those genera are slightly older than those Smith et al, [33] though in almost all cases their mean is captured in our 95% credible interval
We found evidence for two significant rate shifts within labrids, though neither of these corresponded to the predictions generated by the pharyngeal jaw apparatus (PJA) key-innovation hypothesis
Summary
Major modifications to the pharyngeal jaw apparatus are widely regarded as a recurring evolutionary key innovation that has enabled adaptive radiation in many species-rich clades of percomorph fishes. One of the most speciesrich groups of labrids, the parrotfishes, exhibit further modifications of the PJA that are associated with forceful grinding [13,16,17] These include a laterally expanded fourth epibranchial, laterally compressed upper pharyngeal jaws (pharyngobranchials), an anterior muscular sling through novel attachments of the transversalis ventralis muscle (complementing the existing posterior muscle sling), a well developed sliding joints between the pharyngobranchial, neurocranium and epibranchial that permit extensive anterior-posterior motion of the upper jaw, and a posterior to anterior progression of ordered tooth tows on the lower pharyngeal jaws. These modifications are thought to enable trophic diversification by allowing the pharyngeal jaws to take on enhanced functions in prey processing, freeing the oral jaws to become specialized for prey capture [15]
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