Figure 2

Theoretical models of the evolution of reproductive isolation. Three models were derived according to different assumptions about the rate of substitution and the way substitutions interact to produce incompatibilities between taxa. The ‘snowball’ model (constant substitution rate and epistatic interaction), the ‘linear’ model (constant substitution rate and non-epistatic interaction) and the ‘slowdown’ model (decelerating rate of substitution and non-epistatic interaction). (a, b) show the accumulation of incompatibilities and the decline in the logarithm of reproductive compatibility between taxa obtained from these models. Continuous, broken and dotted lines stand for the ‘slowdown’, ‘linear’ and ‘snowball’ model, respectively.