Dwarfism in insular sloths: biogeography, selection, and evolutionary rate

Area of Science:

• Evolutionary Biology
• Island Biogeography
• Population Genetics

Background:

• Rising sea levels over 10,000 years created islands in Bocas del Toro, Panama.
• Insular populations of three-toed sloths (Bradypus) exhibit smaller body sizes than mainland counterparts.
• Intra-island variation in sloth body size suggests potential evolutionary drivers.

Purpose of the Study:

• To investigate the relationship between island characteristics (area, distance, age) and sloth body size.
• To test hypotheses regarding optimal body size, evolutionary equilibria, and dispersal.
• To quantify evolutionary rates and selection intensity in insular sloth populations.

Main Methods:

• Linear regressions of sloth body size against island area, distance from mainland, and island age.
• Retroactive calculation of evolutionary rates (haldanes, darwins) and selection intensity (i).
• Testing observed morphological changes against models of evolution by genetic drift.

Main Results:

• Mean sloth body size decreased linearly with island age, explaining up to 97% of variation.
• Island area and distance from the mainland were not significant predictors of body size when island age was included.
• Consistent directional selection for smaller body size (dwarfism) was observed across insular populations.

Conclusions:

• Island age is the primary driver of three-toed sloth dwarfism in Bocas del Toro.
• Evidence suggests uniform directional selection for smaller body size, rather than genetic drift or dispersal.
• Observed evolutionary rates align with theoretical predictions, uniting micro- and macroevolutionary scales, but linearity remains unexplained.