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Biology 3592A
Milford Green

Gregor Mendel’s publications paved the way for the development of the field of population genetics. It has three main goals: 1. To explain the origin and maintenance of genetic variation 2. To explain the patterns and organization of genetic variation 3. To understand the mechanisms that cause changes in allele frequencies in populations Different forms of a gene are called alleles and 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 makes up the gene pool; which produces the phenotypic traits on which natural selection acts. • Most populations are genetically variable Nearly all populations have genetic variation for many characters. The study of the genetic basis of natural selection is difficult because genotypes alone do not uniquely determine all phenotypes. Different phenotypes can be produced by a given genotype depending on the environment encountered during development. • Evolutionary change can be measured by allele and genotype frequencies Allele frequencies are usually estimated in locally interbreeding groups within a geographic population  Mendelian population. An allele’s frequency is calculated using the following formula: p = numberof copiesof theallelein the population Refer to Page 491 & 492 sumof allelesin the population • A locally interbreeding group within a geographic population is called a Mendelian population. • Genetic variation: the relative proportions, or frequencies, of all alleles in a population • Biologists can estimate allele frequencies for a given locus by measuring numbers of alleles in a sample of individuals from a population. • Sum of all allele frequencies at a locus is equal to 1, so measures of allele frequency range from 0 to 1 • If there is only one allele at a locus, its frequency = 1. The population is monomorphic at that locus; the allele is said to be fixed. The population is said to be polymorphic at a locus, if there are more than one allele at that locus. • The frequencies of different alleles at each locus and the frequencies of different genotypes in a Mendelian population describe that population’s genetic structure o Allele frequencies measure the amount of genetic variation in a population; genotype frequencies show how a population’s genetic variation is distributed among its members 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, its frequency will not increase even if the allele is dominant • A population of sexually reproducing organisms in which allele and genotype frequencies do not change from generation to generation is said to be at Hardy–Weinberg equilibrium. Genotype frequencies can be predicted from allele frequencies. • Five assumptions must be made in order to meet Hardy–Weinberg equilibrium. o Mating is random. o Population size is very large. (larger the population, the smaller will be the effect of genetic drift-random(chance) fluctuations in allele frequencies) o There is no migration either into or out of the populations. o There is no mutation. No change to alleles A and a, and no new alleles are added to change the gene pool. o Natural selection does not affect the survival of particular genotypes. There is no differential survival of individuals with different genotypes. • If the conditions of the Hardy–Weinberg equilibrium are met, two results follow. o The frequencies of alleles at a locus will remain constant from generation to generation. o After one generation of random mating, the genotype frequencies will not change. 2 2 • The Hardy–Weinberg equation: p + 2pq + q = 1.  Genotype: AA Aa aa  Frequency: p2 2pq q2 • Populations in nature never fit the conditions for Hardy-Weinberg equilibrium. Two reasons why this model is considered important for the study of evolution: o It is useful in predicting genotype frequencies from allele frequencies; and o Since the model describes conditions that would result in no evolution, patterns of deviation from the model help identify specific mechanisms of evolution. 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 m
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