I am occasionally asked if various mechanisms might not help solve the accounting problem faced by advocates of evolution when they are forced to confront the massive gap left between the number of fixations that their primary mechanisms can account for and the number that are observed in the various inter-species divergences.
One such mechanism is gene flow. However, the answer is that gene flow not only doesn’t help, it is a structural hindrance to the pace of fixation by both natural selection and neutral substitution. But the similarity of the terms can be confusing, so here is a brief explanation.
Genetic drift is a process. It’s random change in allele frequencies caused by the luck of which individuals happen to reproduce and which alleles get passed on. It operates in every finite population, and it’s stronger in smaller ones. It has no direction and isn’t related to fitness.
Neutral substitution is an outcome. It’s a neutral mutation that has gone all the way from one copy to fixation, permanently replacing the old allele. Drift is the process that usually produces it. That’s why the Hard Limit treats the two together: the transit time is drift’s transit time, and a substitution is a mutation that completed the journey. Relictation and hitchhiking are two other ways to accomplish it.
Gene flow is a separate process. It’s the movement of alleles between populations through migration and interbreeding. It doesn’t create new alleles and doesn’t fix them. It redistributes alleles that already exist.
In population genetics, gene flow sits alongside mutation, drift, and selection as one of the four forces that change allele frequencies.Within a structured species, gene flow is what lets drift produce a species-wide substitution at all. An allele drifting upward in one deme needs migrants to carry it into the others. That’s why limited gene flow adds an extra stage to the journey and makes fixation slower, as discussed above. Between diverging lineages, gene flow works directly against substitution-driven divergence. It mixes the two gene pools back together and erases differences.