Natural selection and Gene frequencies • Evolution is ‘ a change in the gene pool of a population from generation to generation over time’. • Although individuals are selected for, it is populations that evolve. • For natural selection to work there has to be variation in the population for selection to act on. Sources of heritable variation • Sexual recombination: meiosis mixes genes into new combinations, there is random joining of gametes in fertilisation. • Crossing over: as well as mixing in meiosis the homologous pairs can swap pieces. • Mutations: new gene variations for evolution to work on. Only source of new allelles, often harmful and carried as reccessive allelles. Agents that change gene frequencies • Natural Selection This is responsible for most evolutionary change by reducing and changing genetic variation. • the range of phenotypes associated with a characteristic in a population tend to be normally distributed. Selection pressures from the environment make some phenotypes more favourable which changes the alleles in the population (decreases genetic variability). • 3 types; Stabilising natural selection Disruptive natural selection Directional natural selection • Genetic drift: change in allele frequencies due to chance. Happens in small populations where chance will influence more. Often happens when arm of sea gets blocked off. • Founder effect: limited number of ancestors colonise an area = reduced number of alleles in the gene pool. • The bottleneck effect: disasters reduce population to low level but the survivors are random and not a representative sample. • Mutation: ultimate source of variation within a population (increases genetic variability) • Gene flow/migration: this is movement of individuals between populations. Immigration (increase genetic variability), migration (decreases).