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CH 13 – Microevolution - Modern Synthesis Outline 1. Recap 2. Definitions 3. Analyzing Gene Pools (Hardy Weinberg Equilibrium) 4. Mechanisms of Evolution a) Genetic Drift b) Gene Flow c) Mutation d) Natural Selection 5. Sexual Selection – a form of Natural Selection 6. General Outcomes of Natural Selection Directional, Disruptive, Stabilizing 7. Evolution Connection: Sickle Cell Anemia The Modern Synthesis: Darwinism Meets Genetics (Mendel) – The modern synthesis is the fusion of genetics with evolutionary biology. Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings Populations as the Units of Evolution – A population • Is a group of individuals of the same species living in the same place at the same time. • Is the smallest biological unit that can evolve. – Population genetics • Focuses on populations as the evolutionary units. • Tracks the genetic makeup of populations over time. Genetic Variation in Populations – Individual variation abounds in populations. • Not all of this variation is heritable. • Only the genetic component of variation is relevant to natural selection. Sources of Genetic Variation • Mutations – Are changes in the DNA of an organism. • Sexual recombination – Shuffles alleles during meiosis. (independent assortment, random fertilization, crossing over) • • • • • • • • Allele Gene pool Gene Genome Phenotype Genotype Homozygous Heterozygous Definitions Analyzing Gene Pools – The gene pool • Consists of all alleles of all individuals making up a population. – Alleles in a gene pool • Occur in certain frequencies. • Can be symbolized by p for the relative frequency of the dominant allele in the population and q for the frequency of the recessive allele in the population. • p+q=1 The Hardy-Weinberg formula criteria (non-evolving population): – Large population – must be large to minimize sampling error. – Random mating – no mating preference – No mutation- alleles must not change. – No migration – exchange of genes between the population and another population must not occur. – No natural selection- natural selection must not favor any individual. – Genotype frequencies – Can be calculated from allele frequencies. – Are symbolized by the expressions p2, 2pq, and q2. p2 + 2pq + q2 = 1 Figure 13.20 Sample Problems • 1.You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that 36%, calculate the following: • A. The frequency of the "aa" genotype. • B. The frequency of the "a" allele. • C. The frequency of the "A" allele. • D. The frequencies of the genotypes "AA" and "Aa." • E. The frequencies of the two possible phenotypes if "A" is completely dominant over "a." • #2. Sickle-cell anemia is an interesting genetic disease. Normal homozygous individials (SS) have normal blood cells that are easily infected with the malarial parasite. Thus, many of these individuals become very ill from the parasite and many die. Individuals homozygous for the sickle-cell trait (ss) have red blood cells that readily collapse when deoxygenated. Although malaria cannot grow in these red blood cells, individuals often die because of the genetic defect. However, individuals with the heterozygous condition (Ss) have some sickling of red blood cells, but generally not enough to cause mortality. In addition, malaria cannot survive well within these "partially defective" red blood cells. Thus, heterozygotes tend to survive better than either of the homozygous conditions. If 9% of an African population is born with a severe form of sickle-cell anemia (ss), what percentage of the population will be more resistant to malaria because they are heterozygous (Ss) for the sickle-cell gene? • #3. There are 100 students in a class. Ninety-six did well in the course whereas four blew it totally and received a grade of F. Sorry. In the highly unlikely event that these traits are genetic rather than environmental, if these traits involve dominant and recessive alleles, and if the four (4%) represent the frequency of the homozygous recessive condition, please calculate the following: • A. The frequency of the recessive allele. • B. The frequency of the dominant allele. • C. The frequency of heterozygous individuals. • #4. Within a population of butterflies, the color brown (B) is dominant over the color white (b). And, 40% of all butterflies are white. Given this simple information, which is something that is very likely to be on an exam, calculate the following: • A. The percentage of butterflies in the population that are heterozygous. • B. The frequency of homozygous dominant individuals. Microevolution as Change in a Gene Pool – Hardy-Weinberg equilibrium • Describes a non-evolving population that is in genetic equilibrium. – Microevolution is defined as • A generation-to-generation change in a population’s frequencies of alleles. Mechanisms of Microevolution – The main causes of microevolution are • • • • Genetic drift. Gene flow. Mutations. Natural selection. Genetic Drift – Genetic drift • Is a change in the gene pool of a small population due to chance. The Bottleneck Effect – The bottleneck effect • Is an example of genetic drift. • Results from a drastic reduction in population size. Figure 13.23 – Bottlenecking in a population usually reduces genetic variation. The Founder Effect – The founder effect • Is genetic drift in a new colony. The Founder Effect Explains the relatively high frequency of certain inherited disorders among some populations. Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings – Gene flow Gene Flow • Is genetic exchange with another population. • Tends to reduce genetic differences between populations. http://www.bedbugdatabase.com/about-usainformation.php Figure 13.26 – Mutations Mutation • Are changes in an organism’s DNA. • Alone do not have much effect on a large population. • Can have significant cumulative effects on a population. Natural Selection: A Closer Look – Of all causes of microevolution, only natural selection promotes adaptation. http://media.pearsoncmg.com/bc/bc_0media_bio/bioflix/bioflix.htm?eb3evolution Darwinian Fitness – Darwinian fitness • Is the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals. Sexual Selection – a form of Natural Selection http://www.youtube.com/watch?v=REP4S0uqEOc Figure 13.27 Three General Outcomes of Natural Selection – Directional selection • Shifts the phenotypic “curve” of a population. • Selects in favor of some extreme phenotype. – Disruptive selection • Can lead to a balance between two or more contrasting morphs in a population. – Stabilizing selection • Maintains variation for a particular trait within a narrow range. Figure 13.28 Evolution Connection: Population Genetics of the Sickle-Cell Allele – Sickle-cell disease • Affects about one out of every 400 African Americans. • Is more common among African Americans; but why? Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings – The sickle-cell allele • Confers resistance to the disease malaria. • Is adaptive in the African tropics where malaria is common.