Freshwater pearl mussels live in rapid, cold rivers and streams, occupying the ecological niche that few of other bivalves fit. Therefore, these mussels are abundant in undisturbed habitats, and there is practically no struggle for existence in their populations (with the exception of strong stabilizing selection for adaptation of larvae to parasitizing on specific host fish species). As a result, freshwater pearl mussels have almost lost their adaptive potential, which prevents their further evolution, including the evolutionary response to human pressure on their populations. Problems encompassed in the assessment of species adaptive potentials are discussed. The assumption on “non-Darwinian” evolutionary mechanisms is rejected.