The end of regressive evolution: examining and interpreting the evidence from cave fishes

The evolution of hypogean fauna in general and hypogean fishes in particular has been controversial. Explanations regarding the reduction or loss of phenotypic characters such as eyes and pigmentation range from neo‐Lamarckism to neutral mutations, with ‘regressive evolution’ being a catch‐all characterization for such processes. The assumptions required for special evolutionary mechanisms underlying the evolution of cave dwellers have been based on generalizations about the animals and their environments drawn from relatively few observations. The evidence offered for notions such as pre‐adaptation of colonizing fauna and the purported impoverished nutrients in all caves is examined and it appears that the generalizations cannot be supported. Some major accomplishments in field and laboratory studies of hypogean fishes are summarized, including work highlighting developmental phenotypic plasticity. At the end, it is argued that evolution of hypogean fauna can be explained by well‐known mechanisms within the current context of evolutionary biology.