Chapter 13 Evolution of Populations PDF

“The world is before you, and you need not take it or leave it as it was when you came in.” James Baldwin Copyright Pearson Prentice Hall 13.10 – 13.18 Evolution of Populations & Mechanisms of Microevolution 16-2Evolution as Genetic Change...

“The world is before you, and you need not take it or leave it as it was when you came in.” James Baldwin Copyright Pearson Prentice Hall 13.10 – 13.18 Evolution of Populations & Mechanisms of Microevolution 16-2Evolution as Genetic Change Copyright Pearson Prentice Hall Learning Objectives Today you will learn to: ◦ Explain how natural selection leads to changes in phenotypes in a population over time. ◦ Predict the impact of environmental pressures on single-gene and polygenic traits in a population. Copyright Pearson Prentice Hall Unit Organizer The Theory of Evolution is about How Species & Populations as supported Change Evidence by the for using the to create Evolution mechanism of Natural Genetic Selection Change on In various forms Individual genetic Directional Disruptive variations Stabilizing Evolution as Genetic Change Natural selection affects which individuals survive and reproduce and which do not. Evolution is any change over time in the relative frequencies of alleles in a population. Populations, not individual organisms, can evolve over time. Copyright Pearson Prentice Hall The Current Theory of Evolution: Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Over time if a trait improves an organisms fitness, the organism could be more successful and produce more offspring. Over 1000’s of generations these small changes can slowly lead to the evolution of a species. Copyright Pearson Prentice Hall Natural Selection on Single-Gene Traits Natural Selection on Polygenic Traits How does natural selection affect polygenic traits? Copyright Pearson Prentice Hall Natural Selection on Polygenic Traits Natural selection can affect the distributions of phenotypes in any of three ways: ◦ directional selection ◦ stabilizing selection ◦ disruptive selection Copyright Pearson Prentice Hall Natural Selection on Polygenic Traits Directional Selection ◦ When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end, directional selection takes place. Copyright Pearson Prentice Hall Natural Selection on Polygenic Traits Stabilizing Selection ◦ When individuals near the center of the curve have higher fitness than individuals at either end of the curve, stabilizing selection takes place. Copyright Pearson Prentice Hall Natural Selection on Polygenic Traits Disruptive Selection ◦ When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle, disruptive selection takes place. Copyright Pearson Prentice Hall Figure 13.14 Original population Frequency of individuals Original Evolved Phenotypes (fur color) population population Stabilizing selection Directional selection Disruptive selection Genetic Drift Genetic drift is random change in allele frequency. ◦ It may occur when a small group of individuals colonizes a new habitat. Individuals may carry alleles in different relative frequencies than did the larger population from which they came. Copyright Pearson Prentice Hall Genetic Drift Copyright Pearson Prentice Hall Genetic Drift Copyright Pearson Prentice Hall  Genetic Drift Descendants Population A Population B When allele frequencies change due to migration of a small subgroup of a population it is known as the founder effect. Copyright Pearson Prentice Hall Humans Influence Evolution ◦ Human activities such as overhunting and habitat destruction may create severe bottlenecks for other species. ◦ Examples of species affected by bottlenecks include the endangered Florida panther, the African cheetah, and the greater prairie chicken. Figure 13.12a-1 Original population Figure 13.12a-2 Original Bottlenecking population event Figure 13.12a-3 Original Bottlenecking Surviving population event population Evolution Versus Genetic Equilibrium The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. When allele frequencies remain constant it is called genetic equilibrium. Copyright Pearson Prentice Hall Evolution Versus Genetic Equilibrium Fiveconditions are required to maintain genetic equilibrium from generation to generation: 1. there must be random mating, 2. the population must be very large, 3. there can be no movement into or out of the population, 4. there can be no mutations, and 5. there can be no natural selection. Copyright Pearson Prentice Hall Evolution Versus Genetic Equilibrium 1. Random Mating ◦ Ensures that each individual has an equal chance of passing on its alleles to offspring. ◦ Mating is rarely completely random. Many species select mates based on particular heritable traits. Copyright Pearson Prentice Hall Copyright Pearson Prentice Hall Evolution Versus Genetic Equilibrium 2. Large Population ◦ Genetic drift has less effect on large populations than on small ones. ◦ Allele frequencies of large populations are less likely to be changed through the process of genetic drift. Copyright Pearson Prentice Hall Evolution Versus Genetic Equilibrium 3. No Movement Into or Out of the Population ◦ Because individuals may bring new alleles into a population, there must be no movement of individuals into or out of a population. ◦ The population's gene pool must be kept together and kept separate from the gene pools of other populations. Copyright Pearson Prentice Hall Evolution Versus Genetic Equilibrium 4. No Mutations o If genes mutate, new alleles may be introduced into the population, and allele frequencies will change. Copyright Pearson Prentice Hall Evolution Versus Genetic Equilibrium 5. No Natural Selection ◦ All genotypes in the population must have equal probabilities of survival and reproduction. ◦ No phenotype can have a selective advantage over another. ◦ There can be no natural selection operating on the population. Copyright Pearson Prentice Hall  Random No Mutations Mating Genetic Equilibrium No Natural Large No Movement Selection Population In or Out of the Population Copyright Pearson Prentice Hall The equation for Hardy-Weinberg equilibrium: p2 + 2pq + q2 = 1 p = the frequency of the dominant allele (A) q = the frequency of the recessive allele Allele (a) frequencies p + q = 1 Genotype frequencies p2 + 2pq + q2 = 1 Dominant Heterozygotes Recessive homozygotes homozygotes Copyright Pearson Prentice Hall Video on Gene Flow Section Review 1. Explain how natural selection leads to changes in phenotypes in a population over time. 2. Predict the impact of environmental pressures on single-gene and polygenic traits in a population. Copyright Pearson Prentice Hall