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BIOL 103 Lecture Notes Mar 4

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BIOL 103
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Week of March 4 th Variation and Natural Selection in Populations Key Terms:  Phenotype: physical expression of a genotype; observable characteristics on an individual resulting from the interaction of its genotype with the environment  Genotype: genetic constitution of an individual organism  Locus (plural – loci): the specific location of a gene or DNA on a chromosome  Allele: one or two more alternate forms of a gene that arise by mutation and are found at the same place on the chromosome  Dominant Allele: allele that produces same phenotype whether its paired allele is identical or different  Recessive Allele: allele that produces it’s phenotype only when it is paired with an identical allele  Homozygous: having identical alleles at corresponding loci  Heterozygous: having dissimilar alleles at corresponding loci A population’s gene pool:  Includes all alleles for all loci in population o Each diploid has maximum two alleles at each locus; each additional allele may exist at the same locus in other individuals o Of thousands of loci, one individual has small fraction of alleles present in entire population Evolution of populations is best understood in terms of:  Frequency of phenotypes  Frequency of genotypes  Frequency of alleles How is genetic variation caused in populations?  Mutations (eg. UV light on skin cells) o Changes in DNA sequences lead to new alleles o Cannot enter population unless in gametes  Sexual reproduction – variable gametes o Meiosis shuffles maternal/paternal chromosomes to new combinations  This shuffling of homologous chromosomes and crossing over leades to new gene combos and variable gametes Sex promotes variation:  In an organism with three haploid sets (n=3), 8 gamete types result from randomly grouped maternal/paternal chromosomes n  A diploid organism produces 2 maternal/paternal chromosome combinations (n=haploid chromosome number)  With n=23, a human can produce 2 =8.4 million different gametes (not including recombination/mutation) Persistence of Sexual Reproduction: Major unsolved problem in evolution –Why is there sex?  In theory, male-producing sexual populations are subject to invasion and replacement by clonal females  A population of one million individuals would be replaced in less than 50 generations by a clone beginning with a single asexual female Why is sex beneficial for snails? (Note: study done in New Zealand)  Sexually produced offspring have higher fitness than asexually produced in
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