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Microevolutionary Processes Chapter 21 Microevolutionary Processes Microevolution is a change in allele frequencies in a population over generations Drive a population away from genetic equilibrium Small-scale changes in allele frequencies brought about by: Natural selection Gene flow Genetic drift Populations Evolve Biological evolution does not change individuals It changes a population Traits in a population vary among individuals Evolution is change in frequency of traits Gene Pools and Allele Frequencies Population localized group of individuals capable of interbreeding and producing fertile offspring Gene Pool consists of all the alleles in a population Gene Mutations Infrequent but inevitable Each gene has own mutation rate The average is about one mutation in every 100,000 genes per generation Mutation rates are often lower in prokaryotes and higher in viruses Short generation times allow mutations to accumulate rapidly in prokaryotes and viruses Lethal mutations Neutral mutations Advantageous mutations Variation in Phenotype Each kind of gene in gene pool may have two or more alleles Individuals inherit different allele combinations This leads to variation in phenotype Offspring inherit genes, not phenotypes Variation in Phenotype Variation in Phenotype Variation in Phenotype What Determines Alleles in New Individuals? Mutation Crossing over at meiosis I Independent assortment Fertilization Change in chromosome number or structure The Hardy-Weinberg Principle The Hardy-Weinberg principle describes a population that is not evolving If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving Hardy-Weinberg Equilibrium The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation In a given population where gametes contribute to the next generation randomly, allele frequencies will not change Mendelian inheritance preserves genetic variation in a population Hardy-Weinberg Rule At genetic equilibrium, proportions of genotypes at a locus with two alleles are given by the equation: p2 + 2pq + q2 = 1 Frequency of allele A = p Frequency of allele a = q Conditions for Hardy-Weinberg Equilibrium The Hardy-Weinberg theorem describes a hypothetical population that is not evolving In real populations, allele and genotype frequencies do change over time Conditions for Hardy-Weinberg Equilibrium The five conditions for non-evolving populations are rarely met in nature 1. No mutations 2. Random mating 3. No natural selection 4. Extremely large population size 5. No gene flow Applying the Hardy-Weinberg Principle We can assume the locus that causes phenylketonuria (PKU) is in Hardy- Weinberg equilibrium given that 1. The PKU gene mutation rate is low 2. Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele 3. Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions 4. The population is large 5. Migration has no effect, as many other populations have similar allele frequencies Applying the Hardy-Weinberg Principle The occurrence of PKU is 1 per 10,000 births q2 0.0001 q 0.01 The frequency of normal alleles is p 1 – q 1 – 0.01 0.99 The frequency of carriers is 2pq 2 0.99 0.01 0.0198 or approximately 2% of the U.S. population Altering Allele Frequencies in a Population Three major factors alter allele frequencies and bring about most evolutionary change Natural selection Genetic drift Gene flow 1.) Natural Selection Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions Evolution by natural selection involves both chance and “sorting” New genetic variations arise by chance Beneficial alleles are “sorted” and favored by natural selection Only natural selection consistently results in adaptive evolution, an increase in the frequency of alleles that improve fitness Results of Natural Selection Three possible outcomes: A shift in the range of values for a given trait in some direction Stabilization of an existing range of values Disruption of an existing range of values in the population Number of individuals phenotypic range Allele frequencies shift in one direction Range of values for the trait at time 1 Number of individuals in the population Favors individuals at one end of the Range of values for the trait at time 2 Number of individuals in the population Directional Selection Range of values for the trait at time 3 Peppered Moths Prior to industrial revolution, most common phenotype was light colored After industrial revolution, dark phenotype became more common PEPPERED MOTHS - KETTLEWELL Adaptation Darwin noticed that every species seemed well adapted to the life it leads Adaptations physical and behavioral characteristics combine to help the organism catch food, handle harsh conditions and reproduce Examples of Adaptations Camouflage: a structural adaptation enabling an organism to blend in with its environment Octopus Cuttlefish Mimicry: a structural adaptation that provides protection for an organism by copying the appearance of another species black on yellow kill fellow red on black friend of jack Pesticide Resistance Pesticides kill susceptible insects Resistant insects survive and reproduce If resistance has heritable basis, it becomes more common with each generation Antibiotic Resistance First came into use in the 1940s Overuse has led to increase in resistant forms Most susceptible cells died out and were replaced by resistant forms Intermediate forms are favored and extremes are eliminated Number of individuals in the population Stabilizing Selection Range of values for the trait at time 1 Range of values for the trait at time 2 Range of values for the trait at time 3 Stabilizing Selection Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes Natural selection will tend to maintain two or more alleles at that locus For example, the sickle-cell allele causes deleterious mutations in hemoglobin but also confers malaria resistance Figure 21.17 Key Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% Distribution of malaria caused by Plasmodium falciparum (a parasitic unicellular eukaryote) 5.0–7.5% 7.5–10.0% 10.0–12.5% 12.5% variation are favored Intermediate forms are selected against Number of individuals Number of individuals in the population in the population Forms at both ends of the range of Range of values for the trait at time 1 Range of values for the trait at time 2 Number of individuals in the population Disruptive Selection Range of values for the trait at time 3 Sexual Selection Sexual selection natural selection for mating success can result in sexual dimorphism marked differences between the sexes in secondary sexual characteristics 2.) Genetic Drift Genetic Drift how allele frequencies fluctuate unpredictably from one generation to the next tends to reduce genetic variation through losses of alleles, especially in small populations Effects of Genetic Drift 1. Genetic drift is significant in small populations 2. Genetic drift can cause allele frequencies to change at random 3. Genetic drift can lead to a loss of genetic variation within populations 4. Genetic drift can cause harmful alleles to become fixed Founder Effect Founder effect occurs when a few individuals become isolated from a larger population allele frequencies in the small founder population can be different from those in the larger parent population due to chance Bottleneck Bottleneck effect results from a drastic reduction in population size due to a sudden environmental change if the population remains small, it may be further affected by genetic drift understanding the bottleneck effect can increase understanding of how human activity affects other species 3.) Gene Flow Gene flow movement of alleles among populations alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen) tends to reduce genetic variation among populations over time Barriers to Gene Flow Whether or not a physical barrier deters gene flow depends upon: Organism’s mode of dispersal or locomotion Duration of time organism can move Inbreeding Nonrandom mating between related individuals Leads to increased homozygosity Can lower fitness when deleterious recessive alleles are expressed Amish, cheetahs