Chapter 23. Population Genetics I m from the shallow end of the gene pool 1
Essential Questions How can we measure evolutionary change in a population? What produces the variation that makes evolution possible? What are the primary mechanisms of adaptive evolution? 2
Population genetics provides a foundation for studying evolution 3
Smallest unit of evolution Individuals are selected Populations evolve Bent Grass growing on mine tailings; only individuals tolerant of toxic heavy metals will grow from the seeds blown in from nearby field Individuals are selected; populations evolve. The Bent Grass (Agrostis tenuis) in the foreground of the photo is growing on the tailings of an abandoned mine. These plants tolerate concentrations of heavy metals that are toxic to other plants of the same species in the pasture beyond the fence. Many seeds from the pasture drift onto the tailings, but only those with genes that enable them to tolerate metallic soil survive and reproduce. 4
Modern Synthesis Evolution since Darwin comprehensive theory of evolution took form in early 1940s integration of natural selection & Mendelian inheritance (genetics) aka Neo-Darwinism R.A. Fisher J.B.S. Haldane Theodosius Dobzhansky Ernst Mayr Sewall Wright George Gaylord Simpson Ledyard Stebbins Mayr Dobzhansky Ernst Mayr and the Evolutionary Synthesis Ernst Mayr helped define the modern synthesis of evolutionary theory, proposing the "Biological Species Concept." In particular, his work on species and speciation helped scientists understand the progress and mechanisms of evolution from one species to another, and the importance of the species unit as "the keystone of evolution." Ironically, one great unsolved problem in Darwin's master work, On the Origin of Species, was just that: How and why do species originate? Darwin and his later followers were faced with a seeming paradox. They described evolution as a continuous, gradual change over time, but species are distinct from each other, suggesting that some process has created a discontinuity, or gap, between them. Credit for doing the most to crack this puzzle goes to Ernst Mayr, perhaps the greatest evolutionary scientist of the twentieth century. Along with Theodosius Dobzhansky, George Gaylord Simpson, and others, Mayr achieved the "modern synthesis" in the 1930s and 1940s that integrated Mendel's theory of heredity with Darwin's theory of evolution and natural selection 5
Populations & gene pools Concepts a population is a localized group of interbreeding individuals gene pool is collection of alleles in the population remember difference between alleles & genes! allele frequency is frequency of allele in a population how many A vs. a in whole population 6
Evolution of populations Evolution implies a change in allele frequencies in a population hypothetical: what would it be like if allele frequencies didn t change? non-evolving population very large population size (no genetic drift) no migration (in or out) no mutation random mating (no competition) no natural selection 7
Hardy-Weinberg equilibrium Hypothetical, non-evolving population preserves allele frequencies Serves as a model natural populations rarely in H-W equilibrium useful model to measure if forces are acting on a population G.H. Hardy mathematician W. Weinberg physician G.H. Hardy (the English mathematician) and W. Weinberg (the German physician) independently worked out the mathematical basis of population genetics in 1908. Their formula predicts the expected genotype frequencies using the allele frequencies in a diploid Mendelian population. They were concerned with questions like "what happens to the frequencies of alleles in a population over time?" and "would you expect to see alleles disappear or become more frequent over time?" 8
Hardy-Weinberg theorem Alleles frequency of dominant allele = p frequency of recessive allele = q frequencies must add to 100%, so: p + q = 1 Individuals frequency of homozygous dominant = p 2 frequency of homozygous recessive = q 2 frequency of heterozygotes = 2pq frequencies must add to 100%, so: p 2 + 2pq + q 2 = 1 9
Calculating frequency of alleles Example: a wildflower population with 2 flower colors allele for red flower color (R) is completely dominant to the allele for white flowers (r) 10
Calculating frequency of alleles Population of 500 plants what is the allele frequency? what % of gene pool is red allele vs. white allele? remember diploid = 1000 alleles 11
Calculating frequency of alleles R RR: 320 x 2 = 640 Rr: 160 x 1 =160 R = 800/1000 = 80% p = 0.8 r rr: 20 x 2 = 40 Rr: 160 x 1 =160 r = 200/1000 = 20% q = 0.2 12
Application of HW theorem What is the frequency of an allele in the population example: what % of the human population carries allele for PKU (phenylketonuria ) ~ 1 in 10,000 babies born in the US is born with PKU, which results in mental retardation & other problems if untreated disease is caused by a recessive allele PKU (phenylketonuria) is a rare, inherited metabolic disease that results in mental retardation and other neurological problems when treatment is not started within the first few weeks of life. When a very strict diet is begun early and well-maintained, affected children can expect normal development and a normal life span. The disease arises from the absence of a single enzyme (phenylalanine hydroxylase). This enzyme normally converts the essential amino acid, phenylalanine, to another amino acid, tyrosine. Failure of the conversion to take place results in a buildup of phenylalanine. Through a mechanism that is not well understood, the excess phenylalanine is toxic to the central nervous system and causes the severe problems normally associated with PKU. PKU is carried through an autosomal recessive gene. The incidence of carriers in the general population is approximately one in fifty people, but the chance that two carriers will mate is only one in 2500. Carrier tests are available only through PKU treatment programs. 13
Application of HW theorem PKU frequency of homozygous recessive individuals (q 2 ) = 1 in 10,000 or 0.0001 frequency of recessive allele (q): q 2 0.0001 = 0.01 frequency of dominant allele (p): p = 1 q 1 0.01 = 0.99 frequency of carriers, heterozygotes (2pq): 2 x (0.99 x 0.01) = 0.0198 or ~2% ~2% of the US population carries the PKU allele 14
Hardy-Weinberg equilibrium Implications of HW theorem in H-W population, all alleles remain at the same frequencies if allele frequencies change, then population is not in equilibrium & evolution is occurring population biologists measure & study sampling of individuals & genetic testing measure from year to year 15
Using H-W theorem Microevolution generation to generation change in a population s allele frequencies Measuring changes in population from generation to generation 16
Mutation & sexual recombination produce the variation that makes evolution possible 17
Mutation Mutation creates variation new genes & new alleles originate only by mutation only mutations to sex cells can be passed on Mutation changes DNA sequence changes amino acid sequence changes protein change structure? change function? changes in protein may change phenotype & therefore change fitness most mutations are deleterious Every individual has hundreds of mutations 1 in 100,000 bases copied 3 billion bases in human genome But most happen in introns, spacers, junk of various kind Not every mutation has a visible effect. Some effects on subtle. May just affect rate of expression of a gene. 18
Types of mutations Point mutations sickle cell anemia Duplications hemoglobin chains, fetal hemoglobin olfactory receptors immunoglobulins trnas & rrnas Rearrangements translocations Beneficial increases in gene number appear to have played a major role in evolution. For example, the remote ancestors of mammals carried a single gene for detecting odors that has been duplicated through a variety of mutational mechanisms. As a result, modern humans have close to 1,000 olfactory receptor genes, and mice have 1,300. About 60% of human olfactory receptor genes have been inactivated by subsequent mutations, whereas mice have lost only 20% of theirs a remarkable demonstration that a versatile sense of smell is more important to mice than it is to us! 19
Sexual recombination Sex spreads variation sex causes recombination segregation & independent assortment offspring have new combinations of traits = new phenotypes Sexual reproduction recombines alleles into new arrangements in every offspring 20
Selection & Variation Natural selection requires a source of variation within the population there have to be differences some individuals are more fit than others Genetic variation is the substrate for natural selection 21
Types of selection The effect of selection depending on what is fit 22
Directional selection Environment favors one extreme Directional selection for beak size in Galápagos population of medium ground finch Drier years = thicker shelled seeds = select stronger billed birds 23
Diversifying selection Environment favors extremes small billed soft seeds large billed hard seeds 24
Variation Discrete vs. quantitative characters red vs. white flower color = discrete human height = quantitative Polymorphic morphs distinct types in a population Geographic variation clines 25
Polymorphic 26
Clines Plant height varies with altitude, but still same population 27
Preserving variation Diploidy genetic variation even lethal alleles are hidden in heterozygotes Balancing Selection balanced polymorphism maintaining 2 or more phenotypes through selection heterozygote advantage frequency-dependent selection 28
Heterozygote advantage heterozygotes have a greater fitness maintain both alleles in population sickle cell anemia heterozygotes are protected severest effects of malaria & do not develop sickle cell disease 29
Frequency-dependent selection Fitness of any morph decrease if it becomes too common selection against more abundant phenotype consider action of both predators & parasites 30
Sexual selection Natural selection for mating success competition amongst males for females ritual displays & battles between males female choice courtship displays to attract females Blue Footed Booby courtship display 31
Female choice rules animal kingdom! Sexual dimorphism 32
Males may go to extremes 33
How can such a male evolve? 34
Limitations of Natural Selection Natural selection cannot fashion perfect organisms evolution is limited by genetic constraints legacy of ancestral genes existing variations may not be ideal adaptations are often compromises adaptation for one situation may be limitation for another chance & natural selection interact the founders may not be the fittest 35
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