Development and function explain the modular evolution of phalanges in gecko lizards

Area of Science:

• Evolutionary developmental biology
• Comparative morphology
• Biomechanics

Background:

• Functional specialization can alter covariation among phenotypic traits.
• Tetrapod phalanges typically develop from conserved modules, constraining proportions.
• Gecko adhesive toepads involve autopodium variation, potentially reorganizing modular structures.

Purpose of the Study:

• To test developmental versus functional hypotheses for gecko hand bone modularity.
• To compare modular architecture between gecko lineages with and without adhesive toepads.
• To investigate how selective regimes associated with adhesive toepads influence digit evolution.

Main Methods:

• Comparative analysis of hand bone modularity in padded and padless gecko species.
• Testing of two hypotheses: one based on developmental interactions, another on functional associations.
• Empirical evaluation of module support across different gecko lineages.

Main Results:

• Strong evidence for developmental modules in most gecko species, indicating embryological constraints.
• Padless geckos show high integration in a distal functional module, despite padded species having distal phalangeal specialization.
• Ancestrally padless geckos with weak phalangeal modules and strong distal modules suggest adaptation to incipient frictional adhesion.

Conclusions:

• Digit element modularity in geckos is dynamic and evolutionarily plastic.
• The presence or absence of adhesive toepads is strongly associated with shifts in digit modularity.
• Functional demands, particularly those related to adhesion, can override or modify developmental constraints on bone morphology.