Chapter 21 Active Reading Guide: The Evolution of Populations

Give four reasons why natural selection cannot produce perfect organisms.

1. Selection can act only on existing variations. 2. Evolution is limited by historical constraints 3. Adaptations are often compromises 4. Change, natural selection, and the environment interact

What is a population?

A population is a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring.

Of the three factors you listed above, only one results in individuals that are better suited to their environment. Which is it?

Natural Selection

Much of the genetic variation that makes evolution possible comes through sexual reproduction. What are the three mechanisms by which sexual reproduction shuffles existing alleles?

Crossing Over Independent assortment of chromosomes Fertilization

Explain two ways in which genetic variation is preserved in a population.

Diploidy: In diploid eukaryotes, a considerable amount of genetic variation is hidden from selection in the form of recessive alleles. Balancing Selection: Natural Selection that maintains two or more phenotypic forms in a population.

Figure 21.13 is important because it helps explain the three modes of selection. Explain what is occurring in each type of selection:

Directional Selection: occurs when conditions favor individuals at one extreme of a phenotypic change, thereby shifting a population 's frequencies curve for the phenotypic character in one direction or the other. Disruptive Selection: occurs when conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes. Stabilizing Selection: Acts against both extremes phenotypes and favors intermediate variants. It reduces variation and tends to maintain the status quo for a particular phenotypic character

Because Darwin did not know about the work of Gregor Mendel, he could not explain how organisms pass heritable traits to their offspring. In looking at genetic variation, what are discrete characters, and what are quantitative characters?

Discrete characters can be classified on an either-or basis. Quantitative characters vary along a continuum within population

Which of the factors above tends to reduce the genetic differences between populations and make populations more similar?

Gene Flow above tends to reduce the genetic differences between populations and make populations more similar

Discuss what is meant by heterozygote advantage, using sickle-cell anemia as an example.

Heterozygote advantage refers to the greater reproductive success of heterozygous individuals compared with homozygotes; tends to preserve variation in a gene pool. In the case of sickle-cell anemia, the homozygous individuals have a certain recessive allele at the locus that causes sickle cell disease, resulting in distorted red blood cells in low-oxygen conditions. These sickled cells cause serious damage to vital organs by clumping together. Heterozygous individuals for the sickle-cell allele are protected against the most severe effect of malaria, which destroys the sickled red blood cells without affecting normal cells. This protection gives heterozygous individuals an advantage in selection, especially in regions with high instances of malaria.

What is the difference between intrasexual selection and intersexual selection? Give an example of each type of selection.

Intrasexual selection is selection in which there is direct competition among individuals of one sex for mates of the other sex. Example: Male red-winged blackbirds display and sing to establish their territories, the quality of which determines the number of mates they will attract. Intersexual selection is selection whereby of individuals of one sex are choosy (mostly females) in selecting their mates from individuals of the other sex, called mate choice. For example the bright plumage of a male peacock does not help it physically overcome rival mates. But female peacocks tend to prefer male peacocks with bright plumage: a brightly colored male peacock has a selective advantage.

What is microevolution?

Microevolution is a change in allele frequencies in a population over generations.

What are the three main mechanisms that can cause changes in allele frequency?

Natural Selection Genetic Drift Gene Flow

What is the only mechanism that is adaptive, or improves the match between organisms and their environment?

Natural Selection consistently improves the match between organisms and their environment.

Which of the factors above results in a random, nonadaptive change in allelic frequencies?

Natural Selection is the only mechanism that consistently causes adaptive evolution.

In evolutionary terms, fitness refers only to the ability to leave offspring and contribute to the gene pool of the next generation. It may have nothing to do with being big, or strong, or aggressive. Define relative fitness.

Relative fitness is the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals in the population.

What is often the result of sexual selection?

Sexual selection can result in sexual dimorphism, a difference in secondary characteristics between males and females of the same species.

Geographic variation may be shown in a graded manner along a geographic axis known as a cline. What external factors might produce a cline? Why does the existence of a cline suggest natural selection?

Some clines are produced by a gradation in an environment variable, such as temperature. The existence of a cline suggests natural selection because of the close association between the environmental variable and the frequency of the allele.

The Hardy-Weinberg principle is used to describe a population that is not evolving. What does this principle state?

This principle states that the frequencies of alleles and genotypes in a population will remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work.

What is translocation? How could it be beneficial?

Translocation is an aberration in chromosome structure resulting from attachment of a chromosomal fragment to a nonhomologous chromosome. Translocation can be beneficial for example, the translocation of a part of one chromosome to a different chromosome could link DNA segments in a way that results in a positive effect.

What is the relative fitness of a sterile mule?

Zero

Explain what happens in each of these examples of genetic drift:

founder effect: Genetic drift that occurs when a few individuals become isolated from a larger population and form a new population whose gene pool composition is not reflective of that of the original populations bottleneck effect: Genetic drift that occurs when the size of a population is reduced as by natural disaster or human actions. Typically the surviving population is no longer genetically representative of the original population.

What is a gene pool?

A gene pool consists of all copies of every type of allele at every locus in all members of the population.

Chromosomal changes that delete, disrupt, or rearrange many loci at once are usually harmful. How does gene duplication occur? How might it play a role in evolution?

Gene duplication can occur due to errors in meiosis, slippage during DNA replication or the activities of transposable elements. Duplications that do not severe affects can persist over generations, allowing mutations to accumulate . The result is an expanded genome with new genes that may take on new functions playing a major role in evolution.

There are several sources of genetic variation. We will look at several. What is the ultimate source of new alleles?

The ultimate source of new alleles is mutation.

In a population of plants, 64% exhibit the dominant flower color (red), and 36% of the plants have white flowers. What is the frequency of the dominant allele?

0.6 40%

First, let's try to summarize the big idea from this section. Scan through the entire concept to pull out this information. Three major factors alter allelic frequency and bring about evolutionary change. List each factor, and give an explanation.

1. Natural Selection: results in alleles being passed to the next generation in proportions that differ from those in the present generation. 2. Genetic Drift: The process in which chance events cause allelic frequencies to fluctuate unpredictably from one generation to the next 3. Gene Flow: The transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes.

Conditions for Hardy-Weinberg Equilibrium

1. No mutations: The gene pool is modified if mutations alter alleles or if entire genes are deleted or duplicated. 2. Random mating: If individuals tend to mate within a subset of the population, such as their neighbors or close relatives, random mixing of gametes does not occur, and genotype frequencies change 3. No natural selection: Differences in the survival and reproductive success of individuals carrying different genotypes can alter allele frequencies. 4. Extremely large population size: The smaller the population, the more likely it is that allele frequencies will fluctuate by chance from one generation to the next 5. No gene flow: By moving alleles into or out of populations, gene flow can alter allele frequencies

Using the techniques of molecular biology, what are the two ways of measuring genetic variation in a population?

1. Survey the protein products of genes using gel electrophoresis 2. Compare the DNA sequences of two individuals in a population.

In a plant population, suppose that red flowers (R) are dominant to white flowers (r). In a population of 500 individuals, 25% show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous for this trait?

125 individuals 250 individuals

Mutations are any change in the nucleotide sequence of an organism's DNA. These mutations provide the raw material from which new traits may arise and be selected. What occurs in a point mutation? Why do not all point mutations result in a change of phenotype?

A point mutation reflects a change in one base in a gene. Not all point mutations result in a change of phenotype because so much of the DNA in eukaryotic genomes does not code for protein products and because of the redundancy in the genetic code.

The greater the number of fixed alleles, the lower the species' diversity. What does it mean to say that an allele is fixed?

An allele is said to be fixed if only one allele exists for a particular locus in a population.