Darwin is rightly honored for his understanding of natural selection as a main cause of evolution, but he failed to include genetic drift in his picture because he knew nothing about genes.
Genetic drift is the change in the average composition of a population due to random mutations of individual genes. Genetic drift causes species to evolve even in the absence of selection. Genetic drift and natural selection work together to drive evolution, selection being dominant when populations are large, genetic drift being dominant when populations are small.
Genetic drift is particularly important for the formation of new species, when populations may remain small for a long time. The predominance of genetic drift for small populations is due to a simple scaling law. Genetic drift scales with the inverse square root of population. This means that genetic drift is ten times faster for a population of ten thousand than for a population of a million. The scaling is the same for any kind of random mutations. If we observe any measurable quantity such as height, running speed, age at puberty, or intelligence test score, the average drift will vary with the inverse square root of population. The square root results from the statistical averaging of random events.
If a small population is inbreeding, the rate of drift of the average measure of any human capability scales with the inverse square root of the population. Big fluctuations of the average happen in isolated villages far more often than in cities. On the average, people in villages are not more capable than people in cities. But if ten million people are divided into a thousand genetically isolated villages, there is a good chance that one lucky village will have a population with outstandingly high average capability, and there is a good chance that an inbreeding population with high average capability produces an occasional bunch of geniuses in a short time. The effect of genetic isolation is even stronger if the population of the village is divided by barriers of rank or caste or religion.
This is from Dyson's review of Scale in the NYRB.
Generally, thinking about genetics and intelligence is distasteful. But I find that the demystification that science offers is a kind of protection from the more twisted ideas that we can conjure up in the evenings of our minds. What does genetic drift mean for India where endogamy has been practised for thousands of years? It means many things - some more repulsive than others. But one takeaway that I offer is that inequality breeds inequality. This was always true of cultural capital (what many call "merit) and now seems true of genetic capital as well.