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).