INTROGRESSION IN FISHES - SIGNIFICANCE FOR PALEONTOLOGY, CLADISTICS, AND EVOLUTIONARY RATES

Hybridization is frequent in certain freshwater fishes, but evidence for introgression is usually inconclusive. Most putative cases of introgression are unsubstantiated because morphological data alone are not constrained enough to provide unambiguous evidence for it. Definitive evidence for some cases has been provided by studies of proteins and mitochondrial DNA. The ability to hybridize is a context-dependent retained plesiomorphic trait, at least among lineages separated for up to about 5 million years. Hybridization and possibly introgression occurred in the distant past; they are not limited to anthropogenically disturbed habitats. Local variation and homoplasy are usual consequences. Introgression may substitute blocks of homoplastic characters, which may then become widespread in a recipient lineage. The mitochondrial DNA molecule may become such a block. The consequence for evolutionary studies is the loss of hierarchical structure in phylogenetic data and the loss of evidence of evolutionary history. False cladograms and poor estimates of evolutionary rates may result. Estimates of evolutionary divergence times and rates of evolution in fishes are often based simply on phenograms of genetic distances scaled with mammalian rate estimates. Rates calculated in this way are about two times slower than those calculated from divergence times based on fossil evidence for apomorphies that diagnose branching of fish lineages.