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section 22.2.doc

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McGill University
Biology (Sci)
BIOL 111
Christa Scholtz

22.2- What Are the Mechanisms of Evolutionary Change? • Hardy-Weinberg equilibrium is a null hypothesis that assumes evolutionary forces are absent. • Known evolutionary mechanisms: • Mutation • Nonrandom mating • Gene flow • Natural selection • Genetic drift Mutations Generate Genetic Variation • Origin of genetic variation is mutation; mutation is any change in an organism’s DNA • Most mutations are harmful to their bearers or are neutral, but if environmental conditions change, previously harmful or neutral alleles may become advantageous • Mutations can restore to populations alleles that other evolutionary processes have removed • Most mutations appear to be random and are harmful or neutral to their bearers. • Some mutations can be advantageous. • Mutation rates are low; one out of a million loci is typical. • Although mutation rates are low, they are sufficient to create considerable genetic variation. • Rates as high as one mutation per locus in a thousand zygotes per generation are rare; one in a million is more typical • One condition for Hardy–Weinberg equilibrium is that there is no mutation. • Although this condition is never met, the rate at which mutations arise at single loci is usually so low that mutations result in only very small deviations from Hardy–Weinberg expectations. • If large deviations (from H-W expectations) are found, it is appropriate to dismiss mutation as the cause and look for evidence of other evolutionary agents. Gene flow may change allele frequencies • Gene flow results when individuals migrate to another population and breed in new locations.  Immigrants • No immigration is allowed for a population to be in Hardy–Weinberg equilibrium. Genetic drift may cause large changes in small populations • Genetic drift is the random loss of individuals (and their alleles)-may produce large changes in allele frequencies from one generation to the next • In very small populations, genetic drift may be strong enough to influence the direction of change of allele frequencies even when other evolutionary agents are pushing the frequencies in a different direction. • Organisms that normally have large populations may pass through occasional periods when only a small number of individuals survive (a population bottleneck). Genetic variation can be reduced by genetic drift. o Population bottlenecks occur when only a few individual survive a random event, resulting in a shift in allele frequencies within the population • Founder effect- random changes in allele frequencies resulting from establishment of a population by a very small number of individuals o When a few pioneering individuals colonize a new region, the resulting population will not have all the alleles found among members of the source population. Nonrandom Mating Changes Genotype Frequencies • Nonrandom mating occurs when individuals mate either more often with individuals of the same genotype or more often with individuals of a different genotype. • The resulting proportions of genotypes in the following generation differ from Hardy– Weinberg expectations. • If individuals mate preferentially with other individuals of the same genotype, homozygous genotypes are overrepresented and heterozygous genotypes are underrepresented in the next generation. • Conversely, individuals may mate preferentially with individuals of a different genotype • Self-fertilization (selfing) is another form of nonrandom mating that is common in many organisms, especially plants. • Selfing reduces the frequencies of heterozygous individuals below Hardy–Weinberg expectations and increases the frequencies of homozygotes, without changing allele frequencies, and thus not result in adaptation • Sexual selection- is a particularly important form of nonrandom mating that does change allele frequencies and often results in adaptations 22.3- What Evolutionary Mechanisms Result in Adaptation? • Recall: for adaptation (and evolution) to occur, individuals that differ in heritable traits must survive & reproduce with different degrees of success. • When some individuals contribute more offspring to the next generation than others, allele frequencies in the population change in a way that adapts individuals to the environments that influenced their success: natural selection. • The reproductive contribution of a phenotype to subsequent generations, relative to the contributions of other phenotypes, is called its fitness. • The fitness of a phenotype is determined by the average rates of survival and reproduction of individuals with that phenotype. Natural Selection Produces Variable Results • Most characters (traits) are influenced by alleles at more than one locus and are more likely to show quantitative rather then qualitative variation. • For example, the body size of individuals in a population is influenced by genes at many loci, and distribution of body sizes is likely to be a bell-shaped curve. Quantitative variation: avg body size of a population may increase or decrease as a result of selection. • Natural selection can act on characters with quantitative variation in three ways:  Stabilizing selection-preserves the average characteristics of a population by favoring average individuals  Directional selection-changes the characteristics of a population by favoring individuals that vary in one direction from the mean of the population  Disruptive selection- changes the characteristics of a population by favoring individuals that vary in opposite directions from the mean of the population • Stabilizing selection favors average individuals.  the extremes of a population con
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