Cascading Chromosomal Speciation and the Paradoxical Role of Contact Hybridization as a Sink for Gene Flow

±120 species in 9 sceloporine iguanid lizard genera are surveyed for modes of correlation between speciation and distribution, ecology, phylogeny, and chromosomal variability. The most common mode correlates speciation with allopatric isolation, no chromosomal, and little ecological change. 8 genera with less than 15 species each, show only this mode. Some Sceloporus show another mode: speciation not clearly involving allopatry correlates with ecological shifts and the fixation of negatively heterotic chromosomal mutations. 5 chromosomally derived lines include rapidly formed chains or cascades of chromosomal speciation. Species ending 3 chains are either ecologically super-specialist or super-dominant and have used up substrates for particular types of chromosomal changes. Ancestral and derived species in the most recent chain contact geographically in hybrid zones less than 1000 meters wide, where they hybridize and backcross freely, but paradoxically, they exchange no chromosomes or genes. Even in Sceloporus, less than 25% of the speciation events involve chromosomal-change, yet nearly 60 of its ±75 species have such changes in their phylogenies. Allopatric speciation models explain all features of the first mode, but not the second. A sequence of 3 models accounts for the conditions associated with chromosomal speciation. Each is falsifiable, at least statistically, and tests are suggested.