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CHAPTER 16 POPULATION GENETICS AND SPECIATION

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SECTION 1 GENETIC EQUILIBRIUM Population genetics-is the study of evolution from a genetic point of view. Evolution at the genetic level is called microevolution a change in the genetic material of a population Genetic material of organisms consists of many alleles-or variations-of many genes that code for various traits Do you remember that a population consists of a group of individuals of the same species that routinely interbreed? Populations are the smallest unit in which evolution occurs

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CAUSES OF VARIATION What causes variation in traits? Environment Heredity Some occur as a range of phenotypes Range of body sizes Two colors of flowers What causes genes to vary? Mutation- random change in genes that is passed on Recombination-reshuffling of genes in a diploid offspring Random pairing-organisms produce large quantities of gametes. This ensures the union of gametes is partly a matter of chance.

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THE GENE POOL Population genetics uses the term gene pool to describe the total genetic information available in a population

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HARDY-WEINBERG GENETIC EQUILIBRIUM Unless acted on by an outside influence, genotype frequencies will stay the same. If the next 5 items occur, there will be no genetic variation 1.No net mutations occur; alleles remain the same 2.Individuals neither enter or leave the population 3.The population is large; infinitely large 4.Individuals mater randomly 5.Selection does not occur

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SECTION 2 DISRUPTION OF GENETIC EQUILIBRIUM RememberEvolution is a change in a populations genetic material over generations!! MUTATIONS The first requirement for genetic equilibrium is that allele frequencies dont change due to mutations Spontaneous mutations occur constantly, but at very low rates This is at normal conditions When mutagens are introduced, this will affect genetic equilibrium, by producing totally new alleles for a new trait Mutagens increase the rates of mutations Slow changes in alleles take long periods of time to remove harmful mutations In the long run, beneficial mutations with arise and be beneficial to evolution

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NOT THESE MUTANTS

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GENE FLOW Second requirement for genetic equilibrium, is that the size of the population remains constant. Immigration- the movement of individuals into a population Emigration- the movement of individuals out of a population The constant movement of genes rom one population to another Gene flow Gene flow can occur through various mechanisms, such as migration of individuals, or dispersal of seeds or spores

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GENETIC DRIFT Third requirement of genetic equilibrium is the presence of a large population The Hardy-Weinberg principle is based on probability Genetic drift- in the phenomenon by which allele frequencies in a population change as a result of random events. this requires a small to medium sized population

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NON RANDOM MATING The fourth requirement of genetic equilibrium is random matings, without care of genetic makeup. Many species do not mate randomly Mating influenced by geographic Many species chose mates with similar traits The selection of a mate based on similar traits is called assortative mating Nonrandom mating affects what alleles will be combined within individuals, but does not affect the overall frequencies within a population

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SEXUAL SELECTION The tendency to choose a mate based on certain traits

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NATURAL SELECTION The fifth and final requirement of genetic equilibrium is the absence of natural selection. Natural selction is ongoing, and therefore disrupts genetic equilibrium. Remember!!! Natural selection means some members of a population are more likely than others to survive and reproduce, passing their genes to the next generation. Remember that natural selection works off 4 different variables When natural selection is working, the distribution of traits shift between 3 types of bell curves Stabilizing selection Disruptive selection Directional selection

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STABILIZING SELECTION Individuals with the average form of a trait have the highest fitness

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DISRUPTIVE SELECTION Individuals with either extreme variation of a trait have greater fitness that individuals with the average form of a trait

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DIRECTIONAL SELECTION Individuals that display a more extreme form of a trait have greater fitness than individuals with an average form of the trait.