Question

how does reduced gene flow contribute to variation...???

Answer 1

In population genetics, gene flow (also known as gene migration) is the transfer of genetic variation from one population to another. If the rate of gene flow is high enough, then two populations are considered to have equivalent genetic diversity and therefore effectively a single population. It has been shown that it takes only "One migrant per generation" to prevent population diverging due to drift.Gene flow is an important mechanism for transferring genetic diversity among populations. Migrants into or out of a population may result in a change in allele frequencies (the proportion of members carrying a particular variant of a gene), changing the distribution of genetic diversity within the populations. Immigration may also result in the addition of new genetic variants to the established gene pool of a particular species or population. High rates of gene flow can reduce the genetic differentiation between the two groups, increasing homogeneity. For this reason,gene flow has been thought to constrain speciation by combining the gene pools of the groups, and thus, preventing the development of differences in genetic variation that would have led to full speciation

Answer 2

Gene flow or migration between two population will help maintain similarities between the species thus preventing or slowing down the process of speciation.In other words Gene flow migration can help maintain genetic diversity in a population but it will decrease variation between two population.