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Lecture 13

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Western University
Biology 1001A
Tom Haffie

October 28, 2011 Biology Chapter 17.3-17.4 (Lecture 13-14) Dominance affects evolution rate and outcome • Selection weeds out dominant harmful alleles • Beneficial dominant alleles increase in frequency more than beneficial recessive because recessive alleles are not expressed as much Why is Genetic Variation important? • Very little genetic variation leads to inbreeding and reduced fitness • Quantifying Genetic Variation o Of an individual proportion of heterozygous loci, inbreeding coefficient o Of a population proportion of polymorphic loci, alleles per locus 17.3-The Agents of Microevolution Agent Definition Effect on Genetic Variation Mutation A heritable change in DNA Introduces new genetic variation into population (new alleles) Gene Flow Change in allele frequencies May introduce genetic as individuals join a variation from another population and reproduce population Genetic Drift Random changes in allele Reduces genetic variation, frequencies caused by chance especially in small events populations; can eliminate alleles Natural Selection Differential survivorship or One allele can replace reproduction of individuals another or allelic variation can with different genotypes be preserved Non-random Mating Choice of mates based on Does not directly affect allele their phenotypes and frequencies, but usually genotypes prevents genetic equilibrium Mutation • Heritable change in DNA • Introduces new genetic variation into population • Major source of heritable variation, because it is a mechanism through which entirely new genetic variations arise • A mutation may be passed to the next generation and ultimately influence the gene pool • Deleterious Mutations-alter an individual’s structure, function, or behaviour in harmful ways • There are also neutral, lethal and advantageous mutations Gene Flow • Change in allele frequencies as individuals join a population and reproduce • Introduces novel genetic variants into populations • The evolutionary importance of gene flow depends on the degree of genetic differentiation between populations and the rate of gene flow between them o If two gene pools are very different, a little gene flow may increase genetic variability within the population that receives immigrants, and it will make the two populations more similar Genetic Drift • Random changes in allele frequencies caused by chance events • Reduces genetic variability within populations • Dramatic effects on small populations o Genetic drift is particularly common in small populations because only a few individuals contribute to the gene pool and because any given allele is present in very few individuals • Generally leads to the loss of alleles and reduced genetic variability. Two general circumstances, population bottlenecks and founder effects, often foster genetic drift Population Bottlenecks Founder Effect -An evolutionary event that occurs when a -An evolutionary phenomenon in which a stressful factor (disease, starvation, drought) population that was established by just a few reduces population size greatly and eliminates colonizing individuals has only a fraction of the some alleles from a population genetic diversity seen in the population from -Greatly reduces genetic variation even if the which it was derived population numbers later rebound Natural Selection • Differential survivorship or reproduction of individuals with different genotypes • Shapes genetic variability by favouring some traits over others • Although natural selection can change allele frequencies, it is the phenotype of an individual organism, rather than any particular allele, that is successful or not. o Relative fitness-number of offspring that an individual produces compared with the number left by others in the population o A particular allele will increase in frequency in the next generation if individuals carrying that allele leave more offspring than individuals carrying other alleles. o Differences in the relative success of individuals are the essence of natural selection • Biologists measure the effects of natural selection on phenotypic variation of characters over time o Three modes of natural selection have been identified: directional selection, stabilizing selection, and disruptive selection Directional Selection -A type of selection in which individuals near one end of the phenotypic spectrum have the highest relative fitness -shifts a trait away from the existing mean and toward the favoured extreme -After selection, the trait’s mean value is higher or lower than before Stabilizing Selection -A type of natural selection in which individuals expressing intermediate phenotypes have the highest relative fitness -opposing forces of directional selection can sometimes produce an overall pattern of stabilizing selection Disruptive Selection -A type of natural selection in which extreme phenotypes have higher relative fitness than intermediate phenotypes -Alleles producing extreme phenotypes become common, promoting polymorphism -under natural conditions this is much less likely to occur • Polymorphism: the existence of discrete variants of a character among individuals in a population Sexual Selection • A form of natural selection established by male competition for access to females and by the females’ choice of mates • Pushes phenotypes towards one extreme Intersexual Selection Intrasexual Selection -Selection based on the interactions between -Selection based on the interactions between males and females members of the same sex -males produce useless structures simply as a -males use their large body size, antlers, or result of females finding them irresistibly tusks to intimidate, injure, or kill rival males. attractive in the past -In many species, sexual selection is the most probable cause of sexual dimorphism, differences in the size or appearance of males and females Non-random Mating • Choice of mates based on their phenotypes and genotypes • Inbreeding is a special form of non-random mating in which individuals that are genetically related mate with each other o Other organisms that that live in small, relatively closed populations often mate with related individuals. o Relatives often carry the same alleles, therefore inbreeding often generally increases the frequency o
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