The influence of multiple functional demands on morphological diversification: A test on turtle shells

Area of Science:

• Evolutionary Biology
• Comparative Anatomy
• Biomechanics

Background:

• Organismal parts often serve multiple functions, leading to evolutionary trade-offs.
• The impact of multiple functions on phenotypic diversity is less understood than its effect on evolution.
• Increased functional demands may either inhibit diversification or promote it by increasing adaptive landscape complexity.

Purpose of the Study:

• To investigate whether increased functional complexity in organismal parts influences phenotypic diversity.
• To compare the shell shape diversity of aquatic turtles (multiple functions) with terrestrial turtles (fewer functions).

Main Methods:

• Utilized 3D landmark data from 2722 specimens across 274 hard-shelled turtle species.
• Quantified shell shape variation using geometric morphometrics.
• Employed phylogenetic analyses to examine evolutionary patterns.

Main Results:

• Terrestrial turtles exhibited significantly higher phenotypic diversity in shell shape compared to aquatic turtles.
• Differences in diversity were attributed to distinct evolutionary modes, not differing rates of evolution.
• Aquatic turtle shells perform load resistance, hydrodynamics, self-righting, and thermoregulation; terrestrial shells lack hydrodynamic function.

Conclusions:

• The findings support the traditional view that multiple functions can constrain phenotypic diversity.
• Reduced functional complexity, as seen in terrestrial turtles, may facilitate greater morphological diversification.
• Evolutionary mode, influenced by functional demands, is a key driver of phenotypic diversity in turtle shells.