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Variation and Natural Selection Continued.docx

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BIOL 103
Peter T Boag

Variation and Natural Selection - Genetic Variation in populations is caused by: o Mutation (e.g. UV light hitting skin cells)  Changes in DNA sequences  new alleles  Cannot enter population unless in gametes o Sexual reproduction  variable gametes  Meiosis shuffles maternal and paternal chromosomes into new combinations  Crossing over shuffles alleles into new combinations on each chromosome  Result is genetic and phenotypic variation even in absences of new mutations - Shuffling of homologous chromosomes and crossing over in meiosis leads to new gene combinations and variable gametes - Sex promotes variation: o In an organism with three chromosomes per haploid set (n =3), 8 gamete types result from randomly grouped maternal and paternal chromosomes o A diploid organism produces 2 maternal and paternal chromosome combinations (n = haploid chromosome number) o With n=23, a human can produce 2 = 8.4 million different gametes (not including recombination or mutation) - The persistence of sexual reproduction in natural populations  unsolved problems for evolutionary biology - In theory: male-producing, sexual populations are subject to invasion and rapid replacement by clonal females - Population of one million sexual individuals would be replaced in less than 50 generation by a clone beginning with a single asexual female - New molecular techniques in 1960’s showed large amounts of genetic variation o New techniques such as protein electrophoresis revealed more variation than expected; excess variation = genetic load o Genetic polymorphism maintained by:  Balanced polymorphisms, where heterozygotes are more fit than either homozygote  Frequency dependent selection, where fitness depends on the relative abundance of a morph  Neutrality, e.g. not being exposed to selection o Also spatial/temporal environmental variation; but resulting geographic variation in phenotype may have a genetic and/or environmental origin – need to test o Environment also affects developing phenotypes of quantitative
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