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CHAPTER 23HOW POPULATIONS EVOLVE

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OVERVIEW Natural selection acts on individuals, but only

populations evolve. Genetic variations in populations contribute to

evolution. Microevolution is defined as a change in allele

frequencies in a population over time and represents evolutionary change on its smallest scale.

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I. Concept 23.1: Genetic Variation Makes Evolution Possible

A. Two processes produce the genetic differences that are the basis of evolution:1. mutation2. sexual reproduction

B. Genetic Variation1. Variation in individual genotype leads to variation

in individual phenotype2. Natural selection can only act on variation with a

genetic component.

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NONHERITABLE VARIATIONDifference in caterpillars of same species is due to

dietCaterpillars raised on oak flowers resemble the flowers.

Their siblings raised on oak leaves resembled oak twigs.

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C. Variation Within a Population1. Both discrete and quantitative characters contribute to variation within a population2. Discrete characters can be classified on an either-or

basis (Ex: flower color)3. Quantitative characters vary along a continuum within a population (Ex: polygenic traits like skin color)4. Population geneticists measure genetic variation in a population by determining the amount of heterozygosity at the gene level and the molecular level of DNA (nucleotide variability)

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5. Average heterozygosity measures the average percent of loci that are heterozygous in a population (gene variability)6. Nucleotide variability is measured by comparing the

DNA sequences of base-pairs of individuals in a population

7. Average heterozygosity tends to be greater than nucleotide variability

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D. Variation Between Populations1. Most species exhibit geographic variation which

results from differences in phenotypes or genotypes between populations or between subgroups of a single population that inhabit different areas2. Some examples of geographic variation occur as a

cline, which is a graded change in a trait along a geographic axis based on some environmental variable like temperature

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CLINE

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E. Mutation1. Defined as a change in the nucleotide sequence of

DNA2. Mutations cause new genes and alleles to arise3. Only mutations in cells that produce gametes can

be passed to offspring (germ mutation)4. A point mutation is a change in one base in a gene5. The effects of point mutations can vary:

Mutations in noncoding regions of DNA are often harmless

Mutations in a gene might not affect protein production because of redundancy in the genetic code

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Mutations that result in a change in protein production are often harmful

Mutations that result in a change in protein production can sometimes increase the fit between organism and environment

F. Mutations That Alter Gene Number or Sequence1. Chromosomal mutations that delete, disrupt, or

rearrange many gene loci are typically harmful2. Duplication of large chromosome segments is usually harmful3. Gene duplication can be an important source of

new genetic variation.

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G. Mutation Rates1. Mutation rates are low in animals and plants2. The average is about one mutation in every 100,000 genes per generation3. Mutations rates are often lower in prokaryotes and higher in viruses

H. Sexual Reproduction1. Sexual reproduction can shuffle existing alleles into new combinations2. In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation and evolution possible

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3. Three mechanisms that contribute to the unique combinations of alleles are:a. Crossing overb. Independent assortment of chromosomesc. Fertilization

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II. Concept 23.2: Hardy-Weinberg Equation A. Gene Pools

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

Individuals near the population’s center are, on average, more closely related to one another than to those individuals on the periphery

2. A gene pool consists of all the alleles for all loci in a population (allele frequencies)

3. A locus is fixed if all individuals in a population are homozygous for the same allele4. Population Genetics—study of how populations change genetically over time

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One Species (caribou); Two Populations

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B. Allele Frequencies1. If there are 2 alleles at a locus, p and q are used to

represent their frequencies2. The frequency of all alleles in a population will add

up to 1 p + q = 1

3. Example: (diploid population)Imagine a population of 500 wildflower plants with two alleles (CR and CW) at a locus that codes for flower pigment (incomplete dominance)

20 plants are CW CW—white320 plants are CR CR –red 160 plants are CR CW—pink

What are the allele frequencies for CW and CR?

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Total number of alleles—1000 CR—800 alleles The frequency of CR allele in the gene pool of this

population is 800/1000 = 0.8 or 80%. Therefore frequency of CW = ? 0.2 or 20% WHY????? p = 0.8 q = 0.2

4. Allele and genotype frequencies can be used to test whether evolution is occurring in a population

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C. The Hardy Weinberg Principle1. Describes the gene pool of a population that is not evolving2. If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving 3. The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation unless acted upon by agents other than Mendelian segregation and recombination of alleles4. Such a gene pool is said to be in Hardy-Weinberg equilibrium

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Selecting alleles at random from a gene pool

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5. If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then

p2 + 2pq + q2 = 1AA Aa aa

where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the

frequency of the heterozygous genotype6. This equation can be used to determine the frequencies of the possible genotypes if we know the frequencies of alleles, or we can calculate the frequencies of alleles in a gene pool if we know the

frequencies of genotypes.

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Hardy-Weinberg Principle

Gametes for each generation are drawn at random from the gene pool of the previous generation

If the gametes come together at random, the genotype frequencies of this generation are in Hardy-Weinberg equilibrium.

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7. The Hardy-Weinberg theorem describes a hypothetical population

8. In real populations, allele and genotype frequencies do change over time

9. The five conditions for nonevolving populations are rarely met in nature:

No mutations Random mating No natural selection Extremely large population size No gene flow (migration)

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10. Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci

D. Applying the Hardy-Weinberg Principle1. We can assume the locus that causes phenylketonuria

(PKU) is in Hardy-Weinberg equilibrium. (PKU is recessive) 2. The occurrence of PKU is 1 per 10,000 births

q2 = 0.0001 q = 0.01

3. The frequency of normal allele is p = 1 – q = 1 – 0.01 = 0.99

4. The frequency of carriers is 2pq = 2 x 0.99 x 0.01 = 0.0198 or approximately 2% of the U.S. population

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III. Concept 23.3: Three Mechanisms That Directly Alter Allele FrequenciesA. Any condition that is a deviation from the 5

criteria for the Hardy-Weinberg Theorem has the potential to cause evolution.

B. Three major factors that alter allele frequencies and bring about evolutionary change are:

1. Natural selection (adaptive)2. Genetic drift (nonadaptive)3. Gene flow (nonadaptive)

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C. Natural SelectionDifferential success in reproduction results in certain

alleles being passed to the next generation in greater proportions

D. Genetic Drift 1. Defined as changes in the gene pool (allele frequencies) of a small population due to chance2. The smaller a sample, the greater the chance of

deviation from a predicted result3. Genetic drift tends to reduce genetic variation through losses of alleles

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GENETIC DRIFT

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GENETIC DRIFT

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GENETIC DRIFT

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4. May occur in small populations (less than 100) as the result of two situations:

Bottleneck effectFounder effect

5. Bottleneck effectDefined as a sudden reduction in population size due to a change in the environment (Ex: disaster)

Alleles may be lostNew gene pool may differ drastically from original

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BOTTLENECK EFFECT

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Case Study: Impact of Genetic Drift on the Greater Prairie Chicken Loss of prairie habitat caused a severe reduction in

the population of greater prairie chickens in Illinois The surviving birds had low levels of genetic variation,

and only 50% of their eggs hatched Researchers used DNA from museum specimens to

compare genetic variation in the population before and after the bottleneck

The results showed a loss of alleles at several loci Researchers introduced greater prairie chickens from

population in other states and were successful in introducing new alleles and increasing the egg hatch rate to 90%

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6. Founder effect Occurs when a few individuals colonize a new

habitat or become isolated from a larger population

Allele frequencies in the small founder population can be different from those in the larger parent population

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7. Effects of Genetic Drift Summarya. Genetic drift is significant in small

populationsb. Genetic drift causes allele frequencies to change at randomc. Genetic drift can lead to a loss of genetic

variation within populationsd. Genetic drift can cause harmful alleles to become fixed

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E. Gene Flow1. Defined as the transfer of alleles among

populations due to the migration of fertile individuals or gametes (pollen)

2. Tends to reduce differences between populations over time

3. More likely than mutation to alter allele frequencies directly

4. Can increase or decrease the fit between between organism and environment

5. Can introduce new alleles into a population

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IV. Concept 23.4: Natural Selection Only natural selection leads to the adaptation of an

organism to its environment (adaptive evolution) Natural selection brings about adaptive evolution by acting

on an organism’s phenotype A. Relative Fitness

1. Defined as the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals2. Natural selection acts on the genotype indirectly

by how the genotype affects the phenotype 3. How an organism benefits from a particular allele depends on the genetic and environmental contexts in which it is expressed

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B. Three Modes of Selection:

Depends on which phenotypes in a population are favored.

1. Directional SelectionFavors phenotype of one extremeMost common when species migrate to new and different habitats or during major environmental change

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2. Disruptive SelectionFavors individuals at both extremes of the phenotypic range

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3. Stabilizing SelectionFavors intermediate variants and acts against extreme phenotypes

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THREE MODES OF NATURAL SELECTION

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C. Key Role of Natural Selection in Adaptive Evolution

1. Natural selection increases the frequencies of alleles that enhance survival and reproduction

2. Adaptive evolution occurs as the match between an organism and its environment increases

3. Because the environment can change, adaptive evolution is a continuous process4. Genetic drift and gene flow do not consistently lead

to adaptive evolution as they can increase or decrease the match between an organism and its environment

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D. Sexual Selection1. Defined as natural selection for mating success2. Can result in sexual dimorphism (marked differences between the sexes in secondary sexual

characteristics not directly associated with reproduction)3. Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex4. Intersexual selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates

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SEXUAL SELECTION

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E. Mechanisms for Preserving Genetic Variation in a Population

1. Diploidy Maintains genetic variation in the form of hidden recessive alleles2. Balancing selection

Occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population (polymorphism)

Two mechanisms that help maintain balanced polymorphism

a. Heterozygote advantage --Occurs when heterozygotes have a higher fitness

than do both homozygotes

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--Natural selection will tend to maintain two or more alleles at that locus

--The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance

--Defined in terms of the genotype, not the phenotype

b. Frequency-dependent selection--the fitness of a phenotype declines if it

becomes too common in the population--Selection can favor whichever phenotype is less

common in a population

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c. Neutral Variation --genetic variation that appears to confer no

selective advantage or disadvantageF. Why Natural Selection Cannot Fashion Perfect

Organisms1. Selection can act only on existing variations2. Evolution is limited by historical constraints3. Adaptations are often compromises4. Chance, natural selection, and the environment

interact

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You should now be able to:1. Explain why the majority of point mutations are

harmless2. Explain how sexual recombination generates

genetic variability3. Define the terms population, species, gene pool,

relative fitness, and neutral variation4. List the five conditions of Hardy-Weinberg

equilibrium5. Apply the Hardy-Weinberg equation to a population

genetics problem

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6. Explain why natural selection is the only mechanism that consistently produces adaptive change

7. Explain the role of population size in genetic drift8. Distinguish among the following sets of terms:

directional, disruptive, and stabilizing selection; intrasexual and intersexual selection

9. List four reasons why natural selection cannot produce perfect organisms