Stasis of functionally versatile specialists.

Area of Science:

• Evolutionary biology
• Paleontology
• Morphological evolution

Background:

• Long-term stasis is often attributed to broadly adapted generalists.
• However, generalists may lack stabilizing constraints, leading to phenotypic drift.
• An alternative hypothesis suggests morphological specialists acting as ecological generalists might experience stasis due to stabilizing functional constraints.

Purpose of the Study:

• To investigate the evolutionary dynamics of trophic morphology in tree squirrels (subfamily Callosciurinae and tribe Sciurini).
• To determine if these ecologically versatile specialists exhibit evolutionary stasis in mandible size and shape.
• To test the hypothesis that morphological specialization can stabilize adaptive optima without limiting niche breadth.

Main Methods:

• Analysis of morphological disparity and evolutionary dynamics of mandible size and shape in tree squirrels.
• Comparative analysis of trophic morphology across different dietary specialists and generalists within the studied lineages.
• Phylogenetic comparative methods to infer evolutionary trends and identify periods of stasis.

Main Results:

• A few dietary specialists accounted for a disproportionate amount of morphological disparity.
• Excluding these specialists, compelling evidence for stasis in jaw shape was found, but not in jaw size.
• The primary adaptive optima for mandible shape showed minimal divergence over approximately 30 million years.

Conclusions:

• Morphological specialization in tree squirrels steepened their adaptive peak, stabilizing optima.
• This stabilization occurred without limiting their ecological niche breadth, supporting the hypothesis for specialists acting as generalists.
• Evolutionary stasis in trophic morphology is driven by functional constraints that stabilize adaptive optima, even in ecologically versatile species.