* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Population Genetics
Survey
Document related concepts
The Selfish Gene wikipedia , lookup
Sociobiology wikipedia , lookup
State switching wikipedia , lookup
Sexual selection wikipedia , lookup
The Descent of Man, and Selection in Relation to Sex wikipedia , lookup
Hologenome theory of evolution wikipedia , lookup
Genetics and the Origin of Species wikipedia , lookup
Microbial cooperation wikipedia , lookup
Saltation (biology) wikipedia , lookup
Evolutionary landscape wikipedia , lookup
Transcript
Genes Within Populations Chapter 20 1 Charles Darwin Served as naturalist on mapping expedition around coastal South America. Used many observations to develop his ideas Proposed that evolution occurs by natural selection 2 Voyage of the Beagle 3 1 Darwin’s Evidence Population growth vs. availability of resources -population growth is geometric -increase in food supply is arithmetic 4 Darwin’s Evidence Population growth vs. availability of resources - Darwin realized that not all members of a population survive and reproduce. -Darwin based these ideas on the writings of Thomas Malthus. 5 Genetic Variation and Evolution • Darwin: Evolution is descent with modification • Evolution: changes through time 1. Species accumulate difference 2. Descendants differ from their ancestors 3. New species arise from existing ones 6 2 Natural selection: mechanism of evolutionary change Natural selection: proposed by Darwin as the mechanism of evolution • individuals have specific inherited characteristics • they produce more surviving offspring • the population includes more individuals with these specific characteristics • the population evolves and is better adapted 7 to its present environment Darwin’s theory for how long necks evolved in giraffes 8 Natural selection: mechanism of evolutionary change Inheritance of acquired characteristics: Proposed by Jean-Baptiste Lamarck • Individuals passed on physical and behavioral changes to their offspring • Variation by experience…not genetic • Darwin’s natural selection: variation a result of preexisting genetic differences 9 3 Lamarck’s theory of how giraffes’ long necks evolved 10 Population Genetics • The study of the properties of genes in populations 11 Hardy-Weinberg Principle Godfrey H. Hardy: English mathematician Wilhelm Weinberg: German physician Concluded that: The original proportions of the genotypes in a population will remain constant from generation to generation as long as five assumptions are met 12 4 Hardy-Weinberg Principle Five assumptions : 1. No mutation takes place 2. No genes are transferred to or from other sources 3. Random mating is occurring 4. The population size is very large 5. No selection occurs 13 Hardy-Weinberg Principle • • • • Calculate genotype frequencies with a binomial expansion (p+q)2 = p2 + 2pq + q2 p = individuals homozygous for first allele 2pq = individuals heterozygous for both alleles q = individuals homozygous for second allele because there are only two alleles: p plus q must always equal 1 14 Hardy-Weinberg Principle 15 5 Hardy-Weinberg Principle Using Hardy-Weinberg equation to predict frequencies in subsequent generations 16 A population not in Hardy-Weinberg equilibrium indicates that one or more of the five evolutionary agents are operating in a population Five agents of evolutionary change 17 Agents of Evolutionary Change • Mutation: A change in a cell’s DNA – Mutation rates are generally so low they have little effect on Hardy-Weinberg proportions of common alleles. – Ultimate source of genetic variation • Gene flow: A movement of alleles from one population to another – Powerful agent of change – Tends to homogenize allele frequencies 18 6 19 Agents of Evolutionary Change • Nonrandom Mating: mating with specific genotypes – Shifts genotype frequencies – Assortative Mating: does not change frequency of individual alleles; increases the proportion of homozygous individuals – Disassortative Mating: phenotypically different individuals mate; produce excess of heterozygotes 20 Genetic Drift • Genetic drift: Random fluctuation in allele frequencies over time by chance • important in small populations – founder effect - few individuals found new population (small allelic pool) – bottleneck effect - drastic reduction in population, and gene pool size 21 7 22 Genetic Drift: A bottleneck effect 23 Selection • Artificial selection: a breeder selects for desired characteristics 24 8 Selection • Natural selection: environmental conditions determine which individuals in a population produce the most offspring • 3 conditions for natural selection to occur – Variation must exist among individuals in a population – Variation among individuals must result in differences in the number of offspring surviving – Variation must be genetically inherited 25 Fitness and Its Measurement • Fitness: A phenotype with greater fitness usually increases in frequency – Most fit is given a value of 1 • Fitness is a combination of: – Survival: how long does an organism live – Mating success: how often it mates – Number of offspring per mating that survive 26 Fitness and its Measurement Body size and egg-laying in water striders 27 9 Maintenance of Variation • Frequency-dependent selection: depends on how frequently or infrequently a phenotype occurs in a population – Negative frequency-dependent selection: rare phenotypes are favored by selection – Positive frequency-dependent selection: common phenotypes are favored; variation is eliminated from the population • Strength of selection changes through time 28 Maintenance of Variation • Oscillating selection: selection favors one phenotype at one time, and a different phenotype at another time • Galápagos Islands ground finches – Wet conditions favor big bills (abundant seeds) – Dry conditions favor small bills 29 Maintenance of Variation • Heterozygotes may exhibit greater fitness than homozygotes • Heterozygote advantage: keep deleterious alleles in a population • Example: Sickle cell anemia • Homozygous recessive phenotype: exhibit severe anemia 30 10 Maintenance of Variation • Homozygous dominant phenotype: no anemia; susceptible to malaria • Heterozygous phenotype: no anemia; less susceptible to malaria 31 Maintenance of Variation Frequency of sickle cell allele 32 Maintenance of Variation Disruptive selection acts to eliminate intermediate types 33 11 Maintenance of Variation Directional selection: acts to eliminate one extreme from an array of phenotypes 34 Maintenance of Variation Stabilizing selection: acts to eliminate both extremes 35 12