Stasis of functionally versatile specialists

Abstract

A classic hypothesis posits that lineages exhibiting long‐term stasis are broadly adapted generalists that remain well‐adapted despite environmental change. However, lacking constraints that steepen adaptive peaks and stabilize the optimum, generalists’ phenotypes might drift around a broad adaptive plateau. We propose that stasis would be likely for morphological specialists that behave as ecological generalists much of the time because specialists’ functional constraints stabilize the optimum, but those with a broad niche, such as generalists, can persist despite environmental change. Tree squirrels (Callosciurinae and Sciurini) exemplify ecologically versatile specialists, being extreme in adaptations for forceful biting that expand rather than limit niche breadth. Here, we examine the structure of disparity and the evolutionary dynamics of their trophic morphology (mandible size and shape) to determine if they exhibit stasis. In both lineages, a few dietary specialists disproportionately account for disparity; excluding them, we find compelling evidence for stasis of jaw shape but not size. The primary optima of these lineages diverge little, if at all over approximately 30 million years. Once their trophic apparatus was assembled, their morphological specialization steepened the slopes of their adaptive peak and constrained the position of the optima without limiting niche breadth.

Insectivory leads to functional convergence in a group of Neotropical rodents

“…However, considering the observed coordinated shifts in diet and functional morphology, we suggest that the convergence is related to ecological pressures of the demands of processing prey in insectivorous species. The bite force is mostly limiting for the consumption of harder food items (Zelditch et al., 2020), and, considering the trade‐off between bite force and jaw‐closing speed, this relationship between form and function may depend on crossing a threshold where a species gives up its bite force in favour of a configuration that increases hunting efficiency, allowing a functional specialist species to base its diet almost entirely on invertebrates. Small insectivorous placentals, like shrews and moles, are absent in most of South America, and this niche is mainly occupied by marsupials and some rodents.…”

Incomplete convergence of gliding mammal skeletons*

“…To calculate multivariate phylogenetic half‐lives (ln(2)/α), we first summed the σ 2 values of individual PCs and summed stationary variances of PCs, and then used those results and the equation for stationary variance (σ 2 /2α) to generate a multivariate α (Zelditch et al. 2020). Support for OUM models would suggest the presence of selection for trait optima (possibly due to unique adaptive peaks or zones; Wright 1932; Simpson 1944), likely driving convergence among clades.…”

Snout shape and masticatory apparatus of the rodent‐like mesotheriid ungulates (Notoungulata, Typotheria): exploring evolutionary trends in dietary strategies through ancestral reconstructions

“…2012; Sharp & Trusler 2015; Zelditch et al . 2020). The ability for strong biting and scratch digging (Shockey et al .…”

“…At this point, ‘Intermediate’ Morphotype taxa can be inferred to be functional specialists but ecological generalists (Zelditch et al . 2020; and references therein).…”

“…These functional inferences are unexpected for species whose diets are composed exclusively of soft leaves, but do not preclude their consumption (Zelditch et al . 2020). Wombats and sciuromorph rodents also present expanded zygomatic plates related to the great development and anterior origin of the superficial masseter (e.g.…”