Trait independence primes convergent trait loss.

Area of Science:

• Evolutionary biology
• Developmental biology
• Comparative anatomy

Background:

• Convergent evolution is typically linked to environmental pressures.
• Developmental constraints can influence evolutionary trajectories by limiting phenotypic variation.
• The loss of the tympanic middle ear in toads is a labile trait lacking a clear adaptive explanation.

Purpose of the Study:

• To investigate the role of developmental dependencies in the convergent loss of the tympanic middle ear in toads.
• To determine if evolutionary trade-offs with other skull features influence middle ear evolution.
• To test whether developmental independence contributes to the evolutionary lability of the toad middle ear.

Main Methods:

• Comparative analysis of skull morphology in 55 Bufonidae species (39 eared, 16 earless).
• Examination of six hypothesized independent transitions of middle ear loss.
• Testing for shared versus lineage-specific changes in skull shape and the presence of late-forming skull bones in earless species.

Main Results:

• No evidence of pleiotropic trade-offs between middle ear loss and other skull structures was found.
• Earless toads did not consistently lack other late-forming skull bones.
• Skull shape changes did not uniformly distinguish earless from eared species.

Conclusions:

• Developmental dependencies do not appear to constrain middle ear evolution in Bufonidae.
• The loss of the tympanic middle ear in anurans may be driven by developmental independence rather than selection-related trade-offs.
• This study provides a rare example where developmental independence facilitates the evolutionary lability of a sensory system.