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Download Ch 23 Evolution of Populations
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The Evolution of Populations Ch 23 Modern Synthesis= Natural Selection + Genetics • Population- a group of the same species that will mate and make viable offspring • Gene pool- all the genes in a population • MICROEVOLUTION= change in allele frequencies in a population • Measured using Hardy- Weinberg Theorem Hardy Weinberg Equilibrium • How many alleles does an individual have per gene? • What are the possibilities? 1. 2. 3. • HW is a way to measure if the allele frequency is evolving (changing) or in equilibrium(stable). To observe equilibrium these must be met: • Large population • No gene flow (migration) • No mutations • Random mating • No natural selection Equation Mutations and Sexual Reproduction produce Variations • Mutations may be random or induced by the environment. The ONLY source of new genes and NEW alleles. • Deletions, duplications or rearrangements of many loci are usually harmful. • Point mutations may or may not change an amino acid/protein. • Duplications within ONE gene provide a large variation for selection to work. • Rates • Rare in plants/animals (about 1/100,000 genes per generation) • Highest in bacteria and viruses Sexual Reproduction • Variation arise from • meiosis (crossing over, independent assortment) • Random fertilization • Conjugation- sexual reproduction in bacteria (not common) ***Horizontal gene transfer can occur between DIFFERENT closely related species Genetic Drift • Small populations can see changes in a phenotype frequency by chance alone • Bottleneck Effect- a disaster may greatly reduce population size • Founder effect- when a small group establishes a new population Gene Flow • Results from movement into or out of a population. • Tends to reduce the differences between populations Genetic Variation • Not ALL variation is heritable. Some results from environmental influences. • Phenotypic Polymorphism- if there are two+ distinct morphs Types of Selection • Directional- a shift in a phenotype towards one of the extremes • Disruptive- when BOTH extremes are favored • Stabilizing- when the average individual is favored Heterozygous Advantage Heterozygote advantage- if the hetero. has a greater fitness, then selection favors it EX: Sickle cell heteros. are protected against malaria but if sickle cell(recessive) will probably not survive • Neutral variations have no selective advantage • Low fitness, genes decline… • Fixed alleles are when all members of a population are homozygous Sexual Selection • Selection for mating success can result in dimorphism (differences between the sexes) • Intrasexual- same sex competition • Intersexual- usually the female is picky when selecting a partner