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BOAG CH22.docx

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Queen's University
BIOL 103
Virginia K Walker

BOAG CH. 22: POPULATION GENETICS pop'n genetics: study of genes + genotypes in a pop'n gene pool: all the genes in a pop'n -each individual receives genes from its parents, members of the gene p ool pop'n: a group of individuals of the same species that can interbreed w/ one another -certain species occupy a wide geographic range; divided into discrete pop'ns -dynamic; changes from 1 gen to the next: #, geographic location, and genetic composition Genes in Natural Pop'ns are usually Polymorphic Polymorphism: refers to phenomenon that many traits display variation in phenotypes -lots of polymorphism in colours + patterns of individuals -caused by 2+ alleles that influence the phenotype of the individual inheriting them -also describes the variation in genes; polymorphic gene: a gene that commonly exists as 2+ alleles in a pop'n -monomorphic gene: 1 allele in a pop'n -can involve various types of changes: deletion of a gene region, duplication of a region, and single-nucleotide polymorphism: change in a single nucleotide --> SNPs are smallest, most common type of genetic change in a given gene -->large, healthy pop'ns have high levels of genetic variation + SNPs in most genes ALLELE + GENOTYPE FREQUENCIES Allele frequency: (# copies of a specific allele @ a specific locus in a pop'n) (total # of all alleles for that gene in a pop'n) *hetero only carries one allele copy, while homo carries 2 copies of the same allele *write in example from page 523 Genotype frequency: (# of individuals w/ a particular genotype in a pop'n) (total # of individuals in a pop'n) frequency from 0 to 1 monomorphic: frequency = 1 polymorphic: allele r frequency + allele R frequency = 1 HARDY-WEINBERG EQUILIBRIUM -relates allele and genotype frequencies when they ARE NOT changing p + q = 1 --> all the allele frequencies of a polymorphic pop'n must = 1 for a gene existing in 2 alleles: (p + q)^2 = 1 --> ^2 from 2 alleles, one from each parent therefore p^2 + 2pq + q^2 = 1 (HARDY-WEINBERG EQUATION) -->p^2 = genotype frequency of RR -->2pq = genotype frequency of Rr -->q^2 = genotype frequency of rr eg. if allele frequency of R is 70% and r is 30%, then expected genotype frequency of RR is 49%, Rr is 42%, and rr is 9% -eq'n reflects the way gametes combine randomly to produce offspring -if p and q aren't co-dominant, but one is dominant while the other is recessive, the hetero will be phenotypically indistinguishable from the homo dominant HARDY-WEINBERG predicts an eq'l of unchanging allele and genotype frequencies in a pop'n -if a pop'n is in eq'l: not adapting and evolution is NOT occuring -prediction is valid only is certain conditions are met in the pop'n -usually, pop'ns are in disequilibrium: evolutionary mechanisms affecting the pop'n EVOLUTIONARY MECHANISMS + EFFECTS ON POP'NS Microevolution: changes in a pop'ns gene pool from generation to generation; rooted in: 1. introduction of new genetic variation in a pop'n -new alleles of pre-existing genes can arise from random mutation -new genes can be introduced into a pop'n by gene duplication, exon shuffling, and horizontal gene transfer -provides continual source of new variation in pop'ns -mutations are the raw material for evolution, but do not constitute evolution by itself -mutations supply new genetic variation to a pop'n -but mutations occur at low rates; NOT a major force in promoting widespread changes in pop'n 2. Action of evolutionary mechanisms that alter the prevalence of a given allele or genotype in a pop'n -changes in frequency of existing alleles/genotypes -natural selection, random genetic drift, migration, non-random mating -collective contributions of the mechanisms over lots of gen promote widespread genetic changes in a pop'n -these can also affect the frequencies of new genes that arise in a pop'n by gene duplication, exon shuffling, and horiz gene transfer NATURAL SELECTION -only a certain percentage of the offspring produced by a species will survive -struggle for survival results in selective survival of individuals w/ certain genotypes -such genotypes promote reproductive success -natural selection acts on 2 aspects of reproductive success: 1. certain traits make organisms better adapted to their environ + more likely to survive to reprod age; favours individuals w/ adaptations that provide a survival advantage 2. favours individuals that produce viable offspring; traits that enhance ability of individuals to reproduce are favoured -natural selection occurs on individuals, evolution occurs w/ a pop'n 1. in a pop'n, allelic variation comes from random mutations that causes differences in DNA sequences; a mutation that makes a new allele can change the a.acid sequence of the encoded protein, altering protein f'n 2. some alleles may encode proteins that enhance an individual's survival/reprod capability compared w/ others of the pop'n 3. individuals w/ beneficial alleles are more likely to survive + contribute their alleles to the gene pool of the next gen 4. over many gen, allele frequencies of many genes may change through natural selection, altering the traits of the pop'n -->net result of natural selection is a pop'n better adapted to its environ + more successful @ reprod Darwinian fitness: the relative likelihood that a genotype will contribute to the gene pool of the next generation, compared to other genotypes -measure of reprod success -eg. AA is more fit than aa, b/c it produced 5 offspring vs 4 -variations in fitness occur b/c individuals w/ certain genotypes have more reprod success -these genotypes are more fit then others -natural selection acts on phenotypes from one's genotype Mean fitness of pop'n: average reprod success of member of a pop'n -over gen, as individuals' fitness increases, natural selection also increases the mean fitness of pop'n -natural selection results in a pop'n that is well adapted to native environ + successful @ reprod DIFFERENT PATTERNS OF NATURAL SELECTION DIRECTIONAL SELECTION -favours individuals consistently above/below the mean/median of a phenotypic distribution that have greater reprod success in a particular environ -selects for a phenotype at one end of the spectrum -can occur if a new allele is introduced into a pop'n by mutation -->if new allele gives higher fitness -->homozygote carrying the favoured, new allele has highest fitness, natural selection will favour the allele -->allele will eventually become predominant in the pop'n (might even become monomorphic) -can occur if a pop'n is exposed to a prolonged change in environ -->under new conditions, relative fitness values change to favour one genotype, promoting elimination of other genotypes Directional selection.This pattern of natural selection selects for a phenotype at one end of the spectrum that confers the highest fitness in the population's environment. (a) In this example a mutation for darker fur arises in a population of mice. This new genotype confers higher Darwinian fitness, because mice with dark fur can evade predators and are more likely to survive and reproduce. Over many generations, directional selection will favour the prevalence of darker individuals. (b) These graphs show the change in fur colour phenotypes in this mouse population before and after directional selection. STABILIZING SELECTION -favours the survival of individuals w/ intermediate/average phenotypes -extreme values (ends of the spectrum) selected against -decreases genetic diversity -eg. laying too many eggs makes the next too crowded (not enough resources) and laying too little doesn't contribute enough individuals to the next gen -->must lay an average amount of eggs to be the more successful Stabilizing selection.In this pattern of natural selection, the extremes of a phenotypic distribution are selected against. Those individuals with intermediate traits have the highest fitness. These graphs show the results of stabilizing selection on clutch size in a population of collared flycatchers (Ficedula albicollis). This process results in a population with less diversity and more uniform traits. DISRUPTIVE SELECTION -favours survival of 2+ different genotypes that produce different phenotypes -fitness values of a particular genotype are higher in one environ and lower in a different environ -fitness values of the other genotype vary in an opposite manner -likely to occur in pop'ns that occupy diverse environ so that some members of the species will survive in each type of environmental condition -->heterogeneous environ can be geographically continuous: members of pop'n interbreed freely -->heterogeneous environ geographically isolated from each other: given enough time, disruptive selection caused by the differing environ can eventual lead to evolution of 2+ different species Disruptive selection.This pattern of natural selection selects for two different phenotypes, each of which is most fit in its particular environment. (a) In this example mutations have created metal-resistant alleles in colonial bentgrass (Agrostis tenuis) that allow it to grow on soil contaminated with high levels of heavy metals, such as copper. These alleles provide high fitness where the soil is contaminated, but they confer low fitness where the soil is not contaminated. Because both metal- resistant and metal-sensitive alleles are maintained in the population, this situation is an example of disruptive selection caused by heterogeneous environments. (b) These graphs show the change in phenotypes in this bentgrass population before and after disruptive selection. BALANCING SELECTION -maintains genetic diversity in a pop'n -natural selection doesn't always eliminate the weaker/less fit alleles -over many gen, can create balanced polymorphism: 2+ alleles are kept in bal
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