Activity-dependent scaling of quantal amplitude inneocortical neurons

of migration into a particular region can be eliminated, in part, if similar numbers of immigrants come from the north and the south. However, if more immigrants come from the south, the mean phenotype in a given region will tend to be pushed to a value that is optimal for a more southerly region. It can be shown that the fertility of a local population, in comparison with the fertility of the population immediately to its south, becomes smaller (in proportional terms) as one moves further north. Thus we have another reason why the loss of fitness resulting from migration tends to be largest in the far north. In the trial shown in Fig. 1a, Θ Ͻ 1, so asexuals have an intrinsic fertility disadvantage. Analysis shows that this disadvantage precludes a successful invasion by asexuals with a phenotype that is optimal for region number 5, or by asexuals with phenotypes optimal for any of the regions south of region 5. However, a successful invasion is possible by an asexual phenotype that is optimal for any of the regions north of region 5. This pattern of asexuals being able to invade the initial equilibrium successfully only if they are adapted to northern regions was often found among the polymorphic trials (about 25% of the time). However, we also found that, in about 75% of the polymorphic trials, asexuals optimal for any region could invade the initial equilibrium successfully. Nevertheless, even among these trials, the probability of establishment of a clone after its introduction was generally higher if the clone was adapted to the north and was introduced in the north than if the clone was adapted to the south and introduced in the south. What accounts for the difference in the likely fate of north-and south-adapted asexual clones? Roughly speaking, members of these clones do not mate, and so individuals with the optimal phenotype for a given region produce offspring with this same optimal phenotype. In contrast, a sexual adult with the locally optimal phenotype may have a non-optimal mate (possibly a migrant), and so many of the offspring may also be non-optimal. Immigrants and their recent descendants are most common in the north, so the initial invasion of asexuals is more likely in northern areas (where their mode of reproduction confers the largest advantage) than in the south. A similar explanation has been proposed to account for the presence …