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Thread: Population Genetics: the Hardy-Weinberg Principle

  1. #1

    Post Population Genetics: the Hardy-Weinberg Principle

    Population genetics - Hardy-Weinberg Principle

    How microevolution proceeds.
    1. Hardy-Weinberg equilibrium rarely exists in natural populations, but understanding the assumptions behind it gives us a basis for understanding how populations evolve.
    2. Five conditions are required for Hardy-Weinberg equilibrium.
    1. The population is very large.
    2. The population is isolated (no migration of individuals, or alleles, into or out of the population).
    3. Mutations do not alter the gene pool.
    4. Mating is random with respect to the genes involved.
    5. All individuals are equal in reproductive success (no natural selection).
    3. There are five potential causes of microevolution.
    1. When the population is not large.
    1. The effect of a loss of individuals from a population is much greater when there are fewer individuals.
    2. Genetic drift is a change in a gene pool of a small population due to chance.
    3. The bottleneck effect is genetic drift resulting from a disaster that reduces population size .
    4. The founder effect is genetic drift resulting from colonization of a new area by a small number (even one) of individuals.
    2. Gene flow is a gain or loss of alleles from a population due to immigration or emigration of individuals or gametes.
    3. When mutations alter the gene pool.
    1. Mutations are rare events, but they do occur constantly (as often as one per gene locus per 105 gametes in humans).
    2. Mutation provides the raw material on which other mechanisms of microevolution work.
    3. Mutation is rarely, if ever, directly responsible for evolutionary change.
    4. When non-random mating, e.g.. sexual selection, is occurring
    1. Nonrandom mating is more common than random mating
    2. It is the source of characters which allow members of one sex to identify and choose between members of the other sex.
    3. In animals the choice of mates is often an important part of behavior.
    5. In the presence of natural selection.
    1. Differential success in reproduction is probably always the case for natural populations.
    2. The resulting natural selection is the factor that is likely to result in adaptive changes to a gene pool.
    6. Each of the above mechanisms is a deviation from the conditions required for Hardy-Weinberg equilibrium.

    2. Adaptive change results when natural selection upsets genetic equilibrium.
    1. The degree of adaptation that can occur is limited by the amount and kind of genetic variation in the population.
    2. Variation is extensive in most populations.
    1. Variation in a single characteristic can be caused by the effect of one or more genes or from the action of the environment inducing phenotypic change.
    2. A population is polymorphic for a characteristic if two or more morphs (contrasting forms) are noticeably present; these may be visible or biochemical characteristics.
    3. Most populations exhibit geographic variation in the distribution of characteristics; this variation may show stratification or be clinal, varying smoothly across the population.
    3. Two random processes generate variation.
    1. Review: Mutation and meiosis .
    2. Mutation
    1. Mutations normally are harmful, but they may improve an organism's adaptation to an environment that is changing.
    2. Organisms with very short generation spans can evolve by mutation alone .
    3. Recombination.
    1. Sexual recombination shuffles the mixture of alleles in diploid organisms.
    2. Independent assortment, crossing over, and random fertilization of sperm and egg all play a role.
    3. Organisms that reproduce sexually tend to have longer life spans, and sexual recombination is necessary to increase the variation stemming from single mutations.
    4. Overview: How natural selection affects variation.
    1. An ancestral population is varied, with individuals having characteristics suited for many types of environments.
    2. Over successive generations, those individuals with the characteristics best suited for the environment leave more offspring. These characteristics increase in the subsequent generations.
    3. Those individuals with characteristics not suited for the environment leave fewer offspring. These characteristics decrease in subsequent generations.
    4. Effects of Recessive Alleles
    1. The effects of recessive alleles are not often displayed in diploid organisms.
    2. Recessive alleles may be "hidden" from natural selection when they are found in combination with a dominant allele.
    3. Thus, variation is retained in a population subject to selection.
    5. Heterozygote advantage is a situation in which the heterozygote is favored over either homozygote. As a result, variation is maintained in the population.
    5. Endangered species often have reduced variation.
    1. This is becoming more and more of a problem as human activity endangers wild populations, particularly those that are small to begin with.
    2. There is about 0.04% heterozygosity in the gene loci of the South African population of the cheetah, and 1.4% heterozygosity in the East African population. Historically, these animals suffered bottlenecks due to disease, hunting, and drought.
    6. Not all genetic variation may be subject to natural selection.
    1. Some characteristics showing neutral variation (such as human fingerprints) apparently provide no selective advantage.
    2. The frequency of these characteristics may change as a result of genetic drift, but not by natural selection.
    3. It is impossible to demonstrate that an allele brings no benefit to an organism, and it may be that some supposedly neutral variations provide benefits in some environments.

    3. The overall effects of natural selection.
    1. The perpetuation of genes defines evolutionary fitness.
    1. It is the survival of genes, not individual organisms, that is important. It is the genes that survive in time, not the individual organism(s).
    2. Today, fitness is defined as the relative contribution that an individual makes to the gene pool of the next generation.
    2. Natural selection acts on whole organisms and affects genotypes as a result.
    1. Natural selection acts on phenotypes.
    2. Each phenotype is the sum of the effects of an organism's genotype (as well as, for some characteristics, the genotype's interaction with the phenotype).
    3. There is no way for natural selection to select individual gene loci; it culls, or favors, whole genomes.


  2. #2
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    Post Re: Population genetics - Hardy-Weinberg Principle

    I don't understand the organization of that data. It appears to have been randomly assorted.

  3. #3

    Post Re: Population genetics - Hardy-Weinberg Principle

    (p + q) 2 note: p and q are any two alleles
    p2 + 2pq + q2

    Here is the way it works. You fill in the phenotype for the homozygotous recessive (hopefully there is one) at the top. For instance, if you are looking at sickel cell, two alleles s and S, s being sickel cell, then you insert the gene frequency observed in the population for s, let's say it is .5 (actually it is more like .04 at its highest, but the big number makes it simpler). If s = .5 then 1 - .5 = .5 for S, so you have the values for both alleles. Jump to the second set of numbers, this is what you get multiplying out the top figures. So:
    S (.25) + 2Ss (.5) + s (.25) for the gene frequency of the population as a whole. The hetrozygote has the best chance of survival in a malarial area and the homozygous recessive ss is as dead as disco by puberty.

    A shortcut is the find the observed homozygous recessive and take the square root of that number as its phenotype gene frequency. For instance if there were only two eye colors, blue and brown and each had one allele (not in reality but as an example) and we took the square root of the observed blue eyed gene frequency in Ireland or Norway, we could come up with the gene frequency for the population in the homozygous recessive form and the hetrozygote at .7 This means the blue eyed frequency for these areas is actually 70%

    Unfortunately, Hardy-Weinberg will not work for polygenetic inheritence.

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