Rate of accumulation of reproductive isolation by chromosome rearrangements.

In populations with small or moderate effective population size, in the absence of strong homozygote advantage or drive, chromosomally induced isolation can occur only by the fixation of many weakly underdominant rearrangements. For these populations the minimal speciation time is an increasing function of Ne/u, the ratio of the effective population size and the spontaneous rearrangement rate. With moderate Ne and either strong rearrangement homozygote selective advantage or drive, isolation can occur by fixation of a few strongly underdominant rearrangements. Under these circumstances, speciation occurs at a more rapid rate, provided that such rearrangements occur with sufficient frequency. For large Ne, only rearrangements which are meiotically driven can become fixed, so that isolation times reflect the average strength of drive. A somewhat counterintuitive result for meiotic drive is that in certain cases the minimal speciation times decrease as the effective population size increases. The implications of these results for stasipatric speciation are that mildly to strongly underdominant rearrangements (those with h > .01) can contribute to hybrid sterility only in very small populations (Ne < 50) which are likely to have a high extinction probability and suffer inbreeding depression, or in moderate to large populations which have a high spontaneous occurrence rate of driven rearrangements. Even in populations with a high occurrence of driven rearrangements, only those rearrangements with D > h (the drive coefficient) can contribute to increasing hybrid sterility. Thus, the conditions under which the stasipatric model of speciation will operate in a reasonable time are fairly strict and speciation by fixation of a large number of weakly underdominant (cryptic) rearrangements is often more likely. -Author