The accumulation of reproductive incompatibilities in African cichlid fish.

The rate at which different components of reproductive isolation accumulate with divergence time between species has only been studied in a limited, but growing, number of species. We measured premating isolation and hybrid inviability at four different ontogenetic stages from zygotes to adults in interspecific hybrids of 26 pairs of African cichlid species, spanning the entire East African haplochromine radiation. We then used multiple relaxed molecular clock calibrations to translate genetic distances into absolute ages to compare evolutionary rates of different components of reproductive isolation. We find that premating isolation accumulates fast initially but then changes little with increasing genetic distance between species. In contrast, postmating isolation between closely related species is negligible but then accumulates rapidly, resulting in complete hybrid inviability after 4.4/8.5/18.4 million years (my). Thus, the rate at which complete intrinsic incompatibilities arise in this system is orders of magnitude lower than rates of speciation within individual lake radiations. Together these results suggest divergent ecological adaptations may prevent populations from interbreeding and help maintain cichlid species diversity, which may be vulnerable to environmental degradation. By quantifying the capacity to produce viable hybrids between allopatric, distantly related lineages our results also provide an upper divergence time limit for the "hybrid swarm origin" model of adaptive radiation.