Try these parameter values:

Allopatric speciation occurs when a population is separated geographically, restricting the gene flow between the subpopulations. Genetic differences between the two populations accumulate as genetic drift and differential selection pressures affect the subpopulations.

Given enough time, the populations may be different enough that if they were to come into secondary contact with each other, they would remain two genetically distinct populations. Two forces can keep these populations from rejoining upon secondary contact: mating preferences, and hybrid inferiority.

Genetic drift occurs more rapidly in small populations. Therefore, subpopulations of small size accumulate genetic differences faster accelerating the speciation process. However, small subpopulations are also more susceptable to extinction since deleterious mutations may also be amplified by genetic drift.

The population to the right undergoes three phases:

1) Initial, Single Population. In this phase, individuals readily interbreed as the preference character becomes correlated with the trait character. There is no differential survival at this point.

2) Geographic Separation. The population is subdivided into two in space (gene flow is completely restricted). In this phase, genetic drift influences the populations independently and produces some genetic differences between the two populations. If the trait value in each population reaches a certain limit, a new niche becomes available and the trait value is selected for in the population. In one population, the trait is then selected for, in the other it is selected against(due to different environments).

3) Secondary Contact. In this phase the two populations are no longer separated geographically and the individuals are free to mate as they wish. If the populations are sufficiently diverged, they will remain distinct; gene flow will remain restricted due to the preference trat that is correlated with the trait.

Adjust the population size and the time of geographic separation to the right: