Chapter 16 How Populations Evolve

Transcription

1 Title Chapter 16 How Populations Evolve Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2 Population Genetics A population is all of the members of a single species occupying a certain area at the same time. Population genetics studies the variation in alleles in a gene pool. Microevolution Evolution that occurs within a population is called microevolution. The gene pool is the total of all the alleles in a population; it is described in terms of gene frequencies.

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4 Sexual reproduction alone cannot bring about a change in genotype and allele frequencies of a population.

5 Also, the dominant allele need not increase from one generation to the next. Dominance does not cause an allele to become a common allele. The potential constancy, or equilibrium state, of gene pool frequencies was independently recognized by G. H. Hardy and W. Weinberg.

6 The Hardy-Weinberg principle states that an equilibrium of gene pool frequencies, calculated using the binomial expression, will remain in effect in each succeeding generation of a sexually reproducing population, as long as five conditions are met No mutation: no allelic changes occur, or changes in one direction are balanced by changes in the other direction. No gene flow: migration of alleles into or out of the population does not occur. Random mating: individuals pair by chance and not according to their genotypes or phenotypes. No genetic drift: the population is large so changes in allele frequencies due to chance are insignificant. No selection: no selective force favors one genotype over another.

7 In real life, conditions of the Hardy-Weinberg law are rarely if ever met, and allele frequencies in the gene pool of a population do change from one generation to the next, resulting in evolution. Microevolution can be detected by noting any deviation from a Hardy-Weinberg equilibrium of allele frequencies in the gene pool of a population. A change of allele frequencies is expected to result in a change of phenotype frequencies.

8 Industrial Melanism

9 The case of the peppered moths provides a case study in a shift in phenotype frequencies under selection. Before trees became coated with soot from air pollution, the percentage of dark-colored moths was 10%. With birds acting as a selective agent, the light colored moths were reduced while dark-colored moths were better adapted to survive on the darkened trees. The last generation observed has 80% dark-colored moths.

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11 Causes of Microevolution Mutation Mutations are permanent genetic changes. Without mutations, there could be no inheritable phenotypic variations among members of a population. Mutations are the primary source of genetic differences among prokaryotes that produce asexually. In sexual reproducing organisms, both mutations and sexual recombination are important in generating phenotypic differences.

12 Nonrandom Mating and Gene Flow Nonrandom mating occurs when certain genotypes or phenotypes mate with one another.

13 Gene flow (gene migration) is the movement of alleles among populations by migration of breeding individuals. When animals move between populations, or when pollen is distributed between species, gene flow has occurred. Continued gene flow decreases diversity among populations, causing gene pools to become similar. Gene flow among populations can prevent speciation from occurring.

14 Genetic Drift Genetic drift refers to changes in allele frequencies of a gene pool due to chance rather than selection by the environment. Therefore, genetic drift does not necessarily result in adaptation to the environment, as does natural selection. Genetic drift occurs in both large and small populations; small populations are more likely to show the effects of drift. Genetic drift occurs when founders start a new population, or after a genetic bottleneck with interbreeding.

15 The bottleneck effect prevents most genotypes from participating in production of the next generation. The bottleneck effect is caused by a severe reduction in population size due to a natural disaster, predation, or habitat reduction. The bottleneck effect causes a severe reduction in the total genetic diversity of the original gene pool.

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19 The founder effect is an example of genetic drift where rare alleles or combinations occur in higher frequency in a population isolated from the general population. Founding individuals could contain only a fraction of the total genetic diversity of the original gene pool. Which alleles the founders carry is dictated by chance alone. As an example, dwarfism is much higher in a Pennsylvania Amish community due to a few German founders.

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22 Maintenance of Diversity The environment includes specific selecting agents that help maintain diversity. We have already seen how insectivorous birds can help maintain the frequencies of both the light-colored and dark-colored moths, depending on the color of background vegetation. Heterozygote Advantage Heterozygote advantage occurs when the heterozygote is favored over the two homozygotes. In this way, heterozygote advantage assists the maintenance of genetic, and therefore phenotypic, diversity in future generations.

23 Sickle-Cell Disease Heterozygotes are more fit in malaria areas because the sicklecell trait does not express unless the oxygen content of the environment is low; but the malaria agent causes red blood cells to die when it infects them (loss of potassium). Some homozygous dominants are maintained in the population but they die at an early age from sickle-cell disease. Some homozygotes are maintained in the population for normal red blood cells, but they are vulnerable to malaria. Cystic Fibrosis In cystic fibrosis the recessive allele causes the person to have a defective plasma membrane protein. The agent that causes typhoid fever can use the normal version of this protein, but not the defective one, to enter cells. Heterozygote superiority caused the recessive allele to be maintained in the population.

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