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Transcript
The Evolution of Populations CHAPTER 23 Overview: The Smallest Unit of Evolution One common misconception about evolution is that individual organisms evolve, in the Darwinian sense, during their lifetimes Natural selection acts on individuals, but populations evolve!! Genes, Populations, and Evolution A population is all the members of a single species occupying a particular area at the same time. Diversity exists among members of a population. Population genetics is the study of this diversity in terms of allele differences. Evaluates the diversity of a population by studying genotype and phenotype frequencies over time Microevolution In the 1930s, population geneticists began to describe variations in a population in terms of alleles Microevolution pertains to evolutionary changes within a population. Various alleles at all the gene loci in all individuals make up the gene pool of the population. The gene pool of a population can be described in terms of: Genotype frequencies Allele frequencies Genes, Populations, and Evolution Allele Frequencies The proportion of each allele within a population’s gene pool. Frequencies of the dominant and recessive allele must add up to 1. This relationship is described by the expression p + q = 1 p is the frequency of one allele and q is the frequency of the other Microevolution involves a change in these allele frequencies within populations over time. If the gene frequencies do not change over time, microevolution has not occurred. Genes, Populations, and Evolution The Hardy-Weinberg Equilibrium states that: Allele frequencies in a population will remain constant assuming: No Mutations No Gene Flow Random Mating No Genetic Drift No Natural Selection Genes, Populations, and Evolution Hardy-Weinberg Equilibrium: Required conditions are rarely (if ever) met Deviations from a Hardy-Weinberg equilibrium indicate that evolution has taken place Analysis of allele changes in populations over time determines the extent to which evolution has occurred. Genes, Populations, and Evolution Frequencies of the phenotypes will equal one and follow the equation: p2+2pq+q2=1 The genotypes in a Hardy-Weinberg population are indicated by each term on the left side of the equation above Hardy-Weinberg Equilibrium Learning to solve the problem: If the question states “frequency of alleles in the a population”, then you should start with p+q=1 and solve for p or q If the question says frequencies of organisms that express the trait (dominant or recessive) then you start with p2+2pq+q2=1 Your turn.... If 9% of the population has blue eyes, what percent of the population is hybrid for brown eyes? Homozygous for brown eyes? Practice Determine the percent of the population that is homozygous dominant if the percent of the population that is homozygous recessive is 16%. Practice Determine the percent of the population that is hybrid if the allele frequency of the recessive trait is 0.5. Causes of Evolution Mutations Gene Flow Nonrandom mating Natural selection Genetic Drift Mutations Genetic The mutations raw material for evolutionary change Provide new alleles Increase Some diversity mutations might be more adaptive than others Ex: Genetic mutations affecting pigment color in peppered moths have provided the variation needed for natural selection to occur Average about 1 mutation every 100,000 genes per generation Gene Flow Gene Flow (gene migration) Increases diversity Movement of alleles between populations when: Gametes or seeds (in plants) are carried into another population Breeding individuals migrate into or out of population Continual gene flow reduces genetic divergence between populations Nonrandom Mating Nonrandom Mating When individuals do not choose mates randomly Assortative mating: Individuals select mates with the same phenotype with respect to a certain characteristic Individuals Increases loci reject mates with differing phenotype the frequency of homozygotes for certain Genetic Drift Genetic Drift Changes in the allele frequencies of a population due to change rather than selection by the environment Tends to limit diversity Does not necessarily lead to adaptation to the environment Occurs by disproportionate random sampling from population Can cause the gene pools of two isolated populations to become dissimilar Some alleles are lost and others become fixed (unopposed) Likely to occur: After a bottleneck When severe inbreeding occurs, or When founders start a new population Genetic Drift Bottleneck effect A random event prevents a majority of individuals from entering the next generation Fire, earthquake, floods and human hunting The next generation is composed of alleles that just happened to make it Example: Tay-Sachs disease among Eastern European Jews Genetic Drift Founder effect When a new population is started from just a few individuals The alleles carried by population founders are dictated by chance Formerly rare alleles will either: Occur at a higher frequency in the new population, or Be absent in new population Example: polydactyly in Old Order of Amish in Lancaster, PA Original population gene pool = 3,800 alleles* Remnant population gene pool = 90 alleles* 11% 13% 26% 8% 44% 53% 45% Natural Selection Results in: A change in allele frequencies of the gene pool Improved fitness of the population Major cause of microevolution Natural Selection Natural selection can alter the frequency of inherited traits in a population in five different ways, depending on which phenotype in a population are favored Stabilizing Disruptive (diversifying) Directional Sexual Artificial Natural Selection Stabilizing Selection: Eliminates the extremes and favors the more common intermediate forms Example: Human birth weights (6-8 lbs) Natural Selection Disruptive (diversifying) selection: Increases the extreme types in a population at the expense of intermediate forms. May result in balanced polymorphism One population divided into two distinct types Example: British land snails in different habitat ranges Disruptive Selection Initial Distribution Number of Individuals Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. After Time Number of Individuals Banding Pattern After More Time Number of Individuals Banding Pattern Banding Pattern a. b. b: © Bob Evans/Peter Arnold/Photolibrary Natural Selection Directional selection: Changing environmental conditions causes one phenotype to replace another in the gene pool Example: Peppered moths and antibiotic resistance bacteria Directional Selection Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. After More Time Number of Individuals After Time Number of Individuals Number of Individuals Initial Distribution Body Size Body Size Body Size a. Hyracotherium Merychippus Equus b. Natural Selection Sexual selection: adaptive changes in males and females lead to an increased ability to secure a mate. Select for traits that enhance an individual’s chance of mating Males - increased ability to compete with other males for a mate Females choose to select a male with the best fitness (ability to produce surviving offspring). Natural Selection Artificial selection: Humans breed plants and animals by seeking individuals with desired traits as breeding-stock. Racehorses are bred for speed Dogs for desired traits Maintaining Variations Variation in a population is necessary in order for a population to evolve as the environment changes. The more variation, the more capacity for evolution Balanced polymorphism Geographic variation Sexual reproduction Outbreeding Diploidy Heterozygote advantage Frequency-dependent selection Maintaining Variations Balanced polymorphism: The presence of two or more phenotypes in a single population Freckles and no freckles in humans Geographic variation: Results from differences in phenotypes or genotypes between populations that inhabit different areas Rabbits in cold vs. warm climates Graded variation in the phenotype is known as a cline Maintaining Variations Diploidy (2n): Prevents the elimination of recessive alleles At certain times the recessive allele may not be favored but at other times it might be the advantage Heterozygote advantage Preserves multiple alleles in a population The hybrid individual is selected for because of greater reproductive success Sickle cell anemia in West Africa Sickle Cell Disease malaria sickle-cell overlap of both Maintaining Variations Frequency-Dependent selection: Decreases the frequency of the more common phenotypes and increase the frequency of the less common ones A predator develops a vision for a specific characteristic and therefore the less desired characteristic ends up becoming more frequent