● Francis Galton helped invent a model that describes how family names can disappear from a population purely by chance; it provides a good entrée to the subject of neutral evolution. ● Strict neutrality needs to be distinguished from near neutrality. With strict neutrality, each token allele at a locus has a probability of 1/2N of eventually going to fixation if the population is diploid and contains N individuals; there is strict neutrality here because there is zero variation in fitness. ● The relationship of strict neutrality to the hypothesis of a molecular clock is clarified. ● Strict neutrality is a null hypothesis; the epistemic status of null hypotheses is discussed – are they default assumptions, to be viewed as innocent until proven guilty? ● Alternatives to the hypothesis of strict neutrality are discussed, including Tomoko Ohta’s theory of near neutrality, which says that nearly all genes are strictly neutral or slightly deleterious. ● Whether drift is a cause of evolution, and whether it is a process distinct from natural selection, are discussed, and so is the question of what it means for drift to be a stronger cause than selection in the evolution of an allele.

● Darwin believed that mutations have their causes, but they do not arise because they would be good for the organisms in which they occur; they are “random,” not “guided.” ● Darwin also held that adaptive evolution is a gradual process, meaning that it involves the accumulation of small phenotypic changes rather than large ones. ● An experiment by the Lederbergs is analyzed that is widely taken to refute the idea that mutations are guided. ● A thought experiment is provided to clarify what the theses of random and guided mutation mean. ● It is argued that the fact that microbial populations increase their mutation rates when they are starved does not show that mutations are guided. ● R.A. Fisher’s geometric argument for gradualism is analyzed. Motoo Kimura’s formula for the fixation probability of an allele shows why Fisher’s argument fails; Allen Orr’s follow-up does better. ● Fisher and Orr are talking about the likelihood of gradualism, not its probability.