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Chapter 22.doc

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University of Toronto St. George
Human Biology

Chapter 22: The Mechanisms of Evolution 22.1 What Facts Form the Base of Our Understanding of Evolution? • Darwin developed the major features of an explanatory theory for evolutionary change based on two major propositions: o Species are not immutable; they change over time. o The process that produces these changes is natural selection. • Darwin also observed that, although offspring ten to resemble their parents, the offspring of most organisms are not identical to one another or to their parents. • He suggests that slight variations among individuals affect the chances that a given individual will survive and reproduce  natural selection: differential contribution of offspring to the next generation by various genetic types belonging to the same population. • Individuals do no evolve; populations do. • A population is a group of individuals of a single species that live and interbreed in a particular geographic area at the same time. Adaptation has two meanings • Refers both to the processes by which characteristic that appear to be useful to their bearers evolve and to the characteristics themselves. • With respect to characteristics, an adaptation is a phenotypic characteristic that has help an organism adjust to conditions in its environment. Population genetics provides an underpinning for Darwin’s Theory • We cannot directly observe the genetic composition of organisms; what we do see in nature are phenotypes, the physical expression of organism’ genes. • The features of a genotype are its characters (e.g. eye colour). • The specific form of a character (e.g. brown eyes) is a trait. • A heritable trait is a characteristic of an organism that is at least partly determined by its genes. • A population evolves when its individuals with different genotypes survive or reproduce at different rates. • Population genetics has three main goals: o To explain the origin and maintenance of genetic variation o To explain the patterns and organization of genetic variation o To understand the mechanism that cause changes in allele frequencies in populations • Different forms of a gene, alleles, may exist at a particular locus. • At any particular locus, a single individual has only some of the alleles found in the population to which it belongs. • The sum of all copies of all alleles at all loci found in a population constitutes its gene pool. • The gene pool contains the genetic variations that produce the phenotypic traits on which natural selection acts. Most populations are genetically variable • Nearly all populations have genetic variation for many characters. Evolutionary change can be measure by allele and genotype frequencies • Allele frequencies are usually estimated in locally interbreeding groups, Mendelian populations, within a geographic population of a species. • Allele frequency = p = (number of copies of the allele in the pop. / sum of alleles in the pop.) • If only two alleles for a given locus are found among the members of a diploid population, they may combine to form three different genotypes  polymorphic (more than one allele). • The frequencies of different alleles at each locus and the frequencies of different genotypes in a Mendelian population describe that population’s genetic structure. The genetic structure of a population does not change over time if certain conditions exist • If an allele is not advantageous, its frequency remains constant from generation to generation. • The Hardy-Weinberg equilibrium describes a model situation in which allele frequencies do not change across generations and genotype frequencies can be predicted from allele frequencies (must apply to sexually reproducing organisms). Several conditions must exist for a population to be at Hardy- Weinberg equilibrium: o Mating is random. o Population size is infinite. o There is no gene flow. o There is no mutation. o Natural selection does not affect the survival of particular genotypes. 2 2 • The Hardy-Weinberg equation: p + 2pq + q = 1. Deviations from Hardy-Weinberg equilibrium show that evolution is occurring • The patterns of deviation from Hardy-Weinberg equilibrium can help us identify specific mechanisms of evolutionary change. 22.2 What are the Mechanisms of Evolutionary Change? Mutations generate genetic variation • Origin of genetic variation is mutation: any change in an organism’s DNA. • Mutations appear to be random with respect to the adaptive needs of organisms. • Most are harmful to their bearers or neutral, but if environmental conditions change, previously harmful or neutral alleles may become advantageous. • Can restore to populations alleles that other evolutionary processes have removed. • Create and help maintain genetic variation within populations. • Mutation rates are very low for most loci: one in a million is a typical chance. Gene flow may change allele frequencies • Migration of individuals and movements of gametes between populations, gene flow, are common. • If the arriving individuals or gametes survive and reproduce in their new location, they may add new alleles to the gene pool of the population, or they may change the frequencies all alleles already present if they come from a population with different allele frequencies. Genetic drift may cause large changes in small populations • Genetic drift—random changes in allele frequencies—may produce large changes in allele frequencies from one generation to the next. • Populations that are normally large
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