Final answer:
Patrilocal societies can affect gene flow by directing genes towards the husband's community as wives move away from their birth community, potentially increasing certain genetic traits in one area while decreasing genetic diversity in another.
Step-by-step explanation:
The effect of patrilocal societies on gene flow can be substantial due to residential patterns after marriage. In these societies, it's customary for a wife to live with her husband's family, which leads to women moving away from their own lineage. As a result, genetic variation can decrease in the woman's birth community while increasing in the husband's community as new genes are introduced through the wives moving in. Over time, this can result in particular genes becoming more common in certain areas, resembling a sort of genetic drift, where the frequency of certain alleles in the gene pool can increase due to the consolidation of genes within a specific community. Conversely, gene flow is restricted in the women's birth communities since fewer new genetic variations are introduced when women leave to join their husbands' families.