Phenotypic constraints offer a general, evolutionary explanation for species diversity gradients

Differences in species richness with latitude and along environmental gradients in general are striking and have long been recognized, yet there is still no consensus on their causes. The consistent pattern of lower species richness in harsher environments suggests environmental stress as an obvious explanation, but why should environments with conditions that are adverse for growth and survival have lower species diversity? We used a trait optimization model to investigate how altering the severity of constraints on organism performance changes the diversification rate. Specifically, in the model, diversification occurred via allopatric speciation on an archipelago where island environments varied in environmental severity along a gradient. When species invaded new islands they persisted only if their trait values both allowed survival in the new environment and conferred a level of fitness that was competitive with species already present. A genetic algorithm, encompassing mutation, recombination, and selection, was used to simulate species adapting to these environmental differences via trait evolution over time. Species were more able to adapt to lower constraint than to higher constraint, resulting in an asymmetry in the diversification rate. This asymmetry in diversification rates not only provides a general explanation for environmental gradients in species diversity but also diversity differences among functional types or life forms, because constraint severity also differs among organism types.