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Evolution of Populations Chapter 11 11-1: Genes and Variation Population: group of individuals in the same species that interbreed; share a common gene pool Gene Pool: all of the alleles of all the individuals in a population 11-1: Genes and Variation Allele Frequency: proportion of one allele, compared with all the alleles for that trait, in the gene pool; given as a decimal or a percentage ◦ Evolution occurs when there is a change in the allele frequency of a population 11-1: Genes and Variation Where does genetic variation come from? ◦ Mutations: change in DNA Occur during replication or due to chemicals, radiation, etc. Don’t always change the phenotype ◦ Gene Shuffling/Recombination Mostly occurs during meiosis (independent assortment, crossing-over) 11-2: Natural Selection in Populations Natural Selection acts on organisms that survive and reproduce, NOT the genes 11-2: Natural Selection in Populations Single Gene Traits: one gene controls the trait Polygenic Traits: more than one gene controls the trait; this produces a range of phenotypes 11-2: Natural Selection in Populations Normal Distribution: graph of numerical data that forms a bell-shaped curve and is symmetrical; typical for a polygenic trait ◦ Natural selection leads to changes in allele frequency, which can shift the curve Remember: changes in allele frequency = evolution! 11-2: Natural Selection in Populations Stabilizing Selection: favors the average phenotype; the curve narrows 11-2: Natural Selection in Populations Directional Selection: one extreme form is favored (one end OR the other) 11-2: Natural Selection in Populations Disruptive Selection: both extremes are favored and the intermediate forms are eliminated 11-3: Other Mechanisms of Evolution Gene Flow: movement of alleles from one population to another ◦ Causes allele frequency to increase or decrease by chance ◦ Occurs often in fungi or plants who have seeds, spores, etc. that are easy for dispersal 11-3: Other Mechanisms of Evolution Genetic Drift: the random change in allele frequency in a population ◦ Causes a loss of genetic diversity ◦ More prevalent in smaller populations (easier for change to have an affect) 11-3: Other Mechanisms of Evolution Types of Genetic Drift: ◦ Bottleneck Effect – results from an event that drastically reduces the size of a population Greatly reduces genetic variation 11-3: Other Mechanisms of Evolution Types of Genetic Drift: ◦ Founder Effect – occurs after a small number of individuals colonize a new area 11-3: Other Mechanisms of Evolution 11-3: Other Mechanisms of Evolution Sexual Selection: traits that increase the ability of individuals to attract or acquire mates appear with increasing frequency in a population ◦ Types: Competition between males for female attention; Physical traits attract females FORMATION OF A NEW SPECIES SPECIATION FOR NEW SPECIES TO FORM, ISOLATION MUST OCCUR CAUSING THEM TO FAIL TO INTERBREED AND PRODUCE FERTILE OFFSPRING. TYPES OF REPRODUCTIVE ISOLATION BEHAVIORAL ISOLATION - BEHAVIOR INTERFERES GEOGRAPHIC ISOLATIONBARRIER SEPARATES TEMPORAL ISOLATION – GROUPS REPRODUCE AT DIFFERENT TIMES OF THE DAY OR YEAR, SO THEY ARE NOT COMPATIBLE 11-4: Hardy-Weinberg Equilibrium If no change takes place, the population doesn’t evolve and genetic equilibrium has been reached. Hardy-Weinberg Equilibrium: condition in which a population’s allele frequencies for a given trait do not change 11-4: Hardy-Weinberg Equilibrium Requirements for Hardy-Weinberg ◦ Very large populations So genetic drift doesn’t have much effect ◦ No emigration or immigration Gene pool must be kept separate ◦ No mutations Would cause allele frequency to change ◦ Random mating Atypical because some traits are favored over others ◦ No natural selection Equal opportunity for survival and reproduction of all genotypes 11-4: Hardy-Weinberg Equilibrium Populations rarely meet all of the HardyWeinberg conditions, and the idea is more often used to compare real-life models to predicted data. Hardy-Weinberg Equation used to predict genotype frequencies in populations 11-4: Hardy-Weinberg Equilibrium p2 + 2pq + q2 = 1 and p + q = 1 p = frequency of the dominant allele q = frequency of the recessive allele p2 = % of homozygous dominant individuals q2 = % of homozygous recessive individuals 2pq = % of heterozygous individuals