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Role of Variation in Evolution 4/28 Meiosis and Variation •Evolution can only take place if there is variability in a population. •There are two ways this can happen. 1 – Mutations: • Are a direct source of variation. • Are rare: • Relatively small amount of mutations • Cause physical change to the gene • Change in genotype • Possible change in phenotype • Excessive mutations have a negative impact on continuity. • Otherwise, it is a critical process for generating diversity in organisms. • Environmental factors can affect mutations. • Only mutations in gametes are passed on to offspring. 2) Recombination • This is a new combination of alleles during meiosis where most genetic variation occurs • Three processes: 1) Segregation of alleles and Independent Assortment • Recombination can happen during Prophase I when alleles separate from each other. • Members of a chromosome pair move to different of the cells independently of each other. Independent Assortment 3) Crossing Over • In Prophase I, homologous pairs of chromatids (one from mother and one from father) line up. • The arms of the pairs intertwine (cross-over) . • Pieces of chromosome partners exchange places and each has a new combination of genetic material. Meiosis is not always perfect • Can lead to fatal mistakes • Non-disjunction • Chromosome pairs do not separate • Half the cells ends up with more chromosomes and half have nothing. • Happens with first and second divisions