Migration gene flow Transfer of alleles from the gene pool of 1 population to the gene pool of another population Potent mechanism of evolution Tends to homogenize allele frequencies across populations Migration prevents populations from diverging due to constant gene flow from one place to another Genetic drift Chance events can alter allele and genotype frequencies In populations of finite size, chance events (in form of sampling error) can cause evolution Sampling error random discrepancy between theoretical expectations and actual results Random genetic driftsampling error in the production of zygotes from gene pool Genetic drift cannot produce adaptation because it is only cumulative effect of random events but causes changes in allele frequencies so mechanism of evolution Selection differential reproductive success that happens for a reason Genetic drift differential reproductive success that just happens Genetic drift is most important in small populations Founder effect allele frequencies in the new population are likely, simply by change, to be different from what they were in the source population Small populations that have been founded by group of individuals that have moved When a new population is founded by a small number of individuals, it is likely that chance alone will cause the allele frequencies in the new population to be different from those in source population Patterns 1. Fluctuations in allele frequency in each generation caused by random sampling error, so population follows unique evolutionary path 2. Genetic drift has more rapid and dramatic effect on allele frequencies in small population 3. In time, genetic drift can produce substantial changes in allele frequencies even in large populations *Wandering of allele frequencies produces 2 important effects Eventually alleles drift to fixation or loss Frequency of heterozygotes declines Under genetic drift, every population follows a unique evolutionary path. Genetic drift is rapid in small populations and slow in large populations. If genetic drift is the only evolutionary process at work, eventually one allele will drift to a frequency of 1 (that is, to fixation) and all other alleles will be lost. Heterozygosity frequency of heterozygotes in a population As alleles drift to fixation or loss, the frequency of heterozygotes in the population declines Under genetic drift alleles become fixed or lost, and the frequency of heterozygotes declines Due to drift, small isolated populations lose their genetic diversity Rate of evolution rate at which new alleles created by mutation are substituted for other alleles already present Mutation creation of new allele Substitutionfixation of the new allele, with or without additional mutational change Interaction of mutation, genetic drift and selection When mutation, genetic drift, and selection interact, three processes occur (1) Deleterious alleles appear and are eliminated by selection (2) Neutral mutations appear and are fixed or lost by chance (3) Advantageous alleles appear and are swept to fixation by se- lection When populations are subject to both selection and genetic drift, smaller populations follow more diverse evolutionary paths