Convergence and divergence in the subterranean realm: a reassessment

The dominant neo-Darwinian paradigm of the evolution of cave animals is that the severe aphotic, low food environment with little environmental cyclicity imposes strong selective pressures leading to a convergent (troglomorphic) morphology of reduced pigment and eyes, and elaborated extra-optic sensory structures. Challenges to the paradigm come from two fronts. First, troglomorphic animals occur in many aphotic habitats with relatively abundant food and environmental cyclicity. Second, many permanent reproducing populations in caves are not troglomorphic. A review of data on patterns of troglomorphy confirms both of these points. This suggests that the absence of light, rather than resource level and environmental cyclicity, is the important selective factor, and that other forces are at work, including competition and differences in the age of lineages in subterranean environments.