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Chapter 23 Hardy-Weinberg & Evolutionary Forces What changes populations? AP Biology 2007-2008 Review… So far we have learned that evolution can occur through natural selection. Evolution - changes in allele frequencies (the % of a specific allele of a gene) that occur in a gene pool over time / change in genetic makeup of populations over time We have explored evidence that supports evolution…but how do we know if evolution is occurring or not? If allele frequencies remain constant from generation to generation, population is not undergoing any evolutionary change (genetic equilibrium). AP Biology If allele frequencies are changing, then…maybe! Hardy-Weinberg Principle/Equilibrium/Law Mathematical model that shows that under certain conditions, allele (& genotype) frequencies in a population will remain constant no matter how many generations have passed (genetic equilibrium) = evolution not occurring Represents an ideal situation that never really occurs in the real world AP Biology Hardy-Weinberg Principle/Equilibrium/Law 5 conditions must be met for genetic equilibrium (evolution not occurring): 1) 2) 3) 4) 5) Large population size Random mating No mutations No gene flow No natural selection These conditions remove all agents of evolutionary change (all mechanisms of evolution). Under these conditions, allele frequencies will remain the same indefinitely. AP Biology Hardy-Weinberg Principle/Equilibrium/Law p2 + 2pq + q2 = 1 p+q=1 p = frequency of dominant allele (A) q = frequency of recessive allele (a) p2 = % of homozygous dominant (AA) 2pq = % of heterozygous (Aa) q2 = % of homozygous recessive (aa) AP Biology If H-W has conditions for a non-evolving population, then a population can evolve by… 1. Small Population 2. Non-Random Mating 5 Agents of Evolutionary Change Mechanisms of Evolution Genetic Drift AP Biology Sexual Selection 3. Mutation 4. Gene Flow 5. Natural Selection Changes to DNA Migration Differential Survival 1. Small Population - Genetic Drift Small populations are more subject to the effects of genetic drift Genetic Drift - Random chance events can change frequency of traits in a population not adaptation to environmental conditions (not selection) Can result in a reduction or elimination of an allele (whether or not it was beneficial or harmful) Decreases genetic variation within the population Increases genetic variation between different populations 2 types: QuickTime™ and a founder effect bottleneck AP Biology decompressor are needed to see this picture. Founder Effect When a new population (colony) is started by only a small group of individuals. Who joins the group is random. just by chance some rare alleles may be at high frequency; others may be missing gene pool of new population is skewed human populations that started from small group of colonists examples: - birds get lost or blown off course by storm - the Amish AP Biology QuickTime™ and a decompressor are needed to see this picture. Distribution of blood types Distribution of the O type blood allele in native populations of the world reflects original settlement AP Biology Distribution of blood types Distribution of the B type blood allele in native populations of the world reflects original migration AP Biology Out of Africa Likely migration paths of humans out of Africa AP Biology Many patterns of human traits reflect this migration Bottleneck Effect A disaster reduces population to small number & population recovers and expands again but from a limited gene pool. Who survives the disaster is random. famine, natural disaster, human impact (loss of habitat, hunting)… loss of variation by chance event alleles lost from gene pool not due to fitness narrows the gene pool new population very different from original population AP Biology Bottleneck Effect - Cheetahs All cheetahs share a small number of alleles less than 1% diversity as if all cheetahs are identical twins 2 bottlenecks 10,000 years ago Ice Age last 100 years poaching & loss of habitat AP Biology Bottleneck Effect - Conservation Issues Peregrine Falcon Bottlenecking is an important concept in conservation biology of endangered species loss of alleles from gene pool reduces variation reduces adaptability Breeding programs must consciously outcross AP Biology Golden Lion Tamarin 2. Non-Random Mating - Sexual Selection Random mating - pairing by chance Non-random mating / sexual selection - individuals select mates based on phenotype. Can lead to changes in allele frequencies in gene pool attractiveness to potential mate fertility of gametes successful rearing of offspring Sexual selection may act in opposition to natural selection! competition or female choice AP Biology 3. Mutations Mutations are a source of new alleles on which natural selection can work Provide potential for evolution QuickTime™ and a decompressor are needed to see this picture. Sickle Cell Anemia Can be lethal if homozygous recessive, but beneficial if heterozygous! AP Biology HETEROZYGOTE ADVANTAGE 4. Gene Flow Movement of alleles between populations changes allele frequencies, introduces new variation (Ex:migration,immigration/emigration) Increases genetic variation within the population Decreases genetic variation between different populations Tends to counteract the effects of natural selection & genetic drift (these tend to cause populations to become more distinct). Helps maintain all the organisms over a large area as one species. AP Biology 5. Natural Selection Traits that improve survival or reproduction will accumulate in the population - adaptive change Over time the proportion of favorable alleles increases in the population AP Biology Ex: predation selection acts on both predator and prey (speed, camouflage, defense) Ex: physiological selection acts on body functions (disease resistance, efficient use of nutrients) Effects of Selection Changes in the average trait of a population DIRECTIONAL SELECTION STABILIZING SELECTION DISRUPTIVE SELECTION giraffe neck horse size human birth weight rock pocket mice AP Biology QuickTime™ and a decompressor are needed to see this picture. AP Biology