Chapter 23

Evolution of Populations

Question?• Is the unit of evolution the

individual or the population?

• Answer = while evolution effects individuals, it can only be tracked through time by looking at populations

So what do we study?• We need to study populations,

not individuals• We need a method to track the

changes in populations over time• This is the area of Biology called

‘population genetics’

Population Genetics• The study of genetic variation in

populations.• Represents the reconciliation of

Mendelism and Darwinism.• Modern Synthesis uses

population genetics as the means to track and study evolution• Looks at the genetic basis of

variation and natural selection

Population• A localized group of individuals

of the same species.

Species• A group of similar organisms.• A group of populations that could

interbreed.

Gene Pool• The total aggregate of genes in a

population.• If evolution is occurring, then

changes must occur in the gene pool of the population over time.

Microevolution• Changes in the relative

frequencies of alleles in the gene pool.

Overview• A common misconception is that

organisms evolve, in the Darwinian sense, during their lifetimes• Natural selection acts on

individuals, but only populations evolve

• Individuals are selected; populations evolve!

The Smallest Unit of Evolution

• Genetic variations in populations contribute to evolution• Microevolution is a change in

allele frequencies in a population over generations

Is this finch evolving by natural selection?

Concept: Mutation and sexual reproduction produce the genetic

variation that makes evolution possible

• Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals

Genetic Variation • Variation in individual genotype

leads to variation in individual phenotype

• Not all phenotypic variation is heritable

• Natural selection can only act on variation with a genetic component

Nonheritable variation?

Nonheritable variation?

Variation Within a Population

• Both discrete and quantitative characters contribute to variation within a population

• Discrete characters can be classified on an either-or basis

• Quantitative characters vary along a continuum within a population

• Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levels

• Average heterozygosity measures the average percent of loci that are heterozygous in a population

• Nucleotide variability is measured by comparing the DNA sequences of pairs of individuals

Variation Between Populations• Most species exhibit geographic

variation, differences between gene pools of separate populations or population subgroups

Geographic variation in isolated mouse populations on Madeira

Fig. 23-5 Porcupine herd

Porcupineherd range

Beaufort Sea

NORTHWEST

TERRITORIES

MAPAREA

ALA S

KA

CAN

ADA

Fortymileherd range

Fortymile herd

ALAS

KAYU

KON

Ldh-B

b alle

le

freq

uenc

y1.0

0.8

0.6

0.4

0.2

046 44 42 40 38 36 34 32 30

GeorgiaWarm (21°C)

MaineCold (6°C)

A cline determined by temperatureLatitude

(°N)

Mutation• Mutations are changes in the

nucleotide sequence of DNA• Mutations cause new genes and

alleles to arise• Only mutations in cells that

produce gametes can be passed to offspring

Point mutations• A point mutation is a change in

one base in a gene• 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

• The effects of point mutations can vary:–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

Point mutations

Mutations That Alter Gene Number or Sequence

• Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful

• Duplication of large chromosome segments is usually harmful

Mutations That Alter Gene Number or Sequence

• Duplication of small pieces of DNA is sometimes less harmful and increases the genome size

• Duplicated genes can take on new functions by further mutation

Hardy-Weinburg

Hardy-Weinberg Theorem• Developed in 1908. • Mathematical model of gene pool

changes over time.

• The frequency of an allele in a population can be calculated–For diploid organisms, the total number of alleles at a locus is the total number of individuals x 2 –The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles

• By convention, if there are 2 alleles at a locus, p and q are used to represent their frequencies

• The frequency of all alleles in a population will add up to 1–For example, p + q = 1

Basic Equation• p + q = 1• p = % dominant allele• q = % recessive allele

Expanded Equation• p + q = 1• (p + q)2 = (1)2

• p2 + 2pq + q2 = 1

Genotypes

• p2 = Homozygous Dominants2pq = Heterozygousq2 = Homozygous Recessives

• The Hardy-Weinberg principle describes a population that is not evolving

• If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving

• The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation

• In a given population where gametes contribute to the next generation randomly, allele frequencies will not change

• Mendelian inheritance preserves genetic variation in a population

Fig. 23-6

Frequencies of alleles

Alleles in the population

Gametes producedEach

egg:Each sperm:

80%chance

80%chance

20%chance

20%chance

q = frequency of

p = frequency ofCR allele = 0.8

CW allele = 0.2

Selecting alleles at random from a gene pool

• Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool

• 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 = 1

• -where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

Conditions for Hardy-Weinberg Equilibrium

• The Hardy-Weinberg theorem describes a hypothetical population

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

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

• 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

Example Calculation• Let’s look at a population where:– A = red flowers– a = white flowers

Starting Population• N = 500• Red = 480 (320 AA+ 160 Aa)• White = 20• Total alleles = 2 x 500 = 1000

Dominant Allele• A = (320 x 2) + (160 x 1) = 800 = 800/1000 A = 80%

Recessive Allele• a = (160 x 1) + (20 x 2) = 200/1000 = .20 a = 20%

A and a in HW equation• Cross: Aa X Aa• Result = AA + 2Aa + aa• Remember: A = p, a = q

Substitute the values for A and a

• p2 + 2pq + q2 = 1(.8)2 + 2(.8)(.2) + (.2)2 = 1.64 + .32 + .04 = 1

Dominant Allele• A = p2 + pq = .64 + .16 = .80 = 80%

Recessive Allele• a = pq + q2

= .16 + .04 = .20 = 20%

Importance of Hardy-Weinberg

• Yardstick to measure rates of evolution.

• Predicts that gene frequencies should NOT change over time as long as the HW assumptions hold (no evolution should occur).

• Way to calculate gene frequencies through time.

Example • What is the frequency of the PKU

allele?• PKU is expressed only if the

individual is homozygous recessive (aa).

Applying the Hardy-Weinberg Principle

• We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that:–The PKU gene mutation rate is low–Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele

–Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions –The population is large–Migration has no effect as many other populations have similar allele frequencies

• The occurrence of PKU is 1 per 10,000 birthsq2 = 0.0001q = 0.01

• The frequency of normal alleles isp = 1 – q = 1 – 0.01 = 0.99

• The frequency of carriers is2pq = 2 x 0.99 x 0.01 = 0.0198–or approximately 2% of the U.S. population

PKU Frequency• PKU is found at the rate of 1/10,000

births.• PKU = aa = q2

q2 = .0001 q = .01

Dominant Allele• p + q = 1 p = 1- q p = 1- .01 p = .99

Expanded Equation• p2 + 2pq + q2 = 1(.99)2 + 2(.99x.01) + (.01)2 = 1.9801 + .0198 + .0001 = 1

Final Results• Normals (AA) = 98.01%• Carriers (Aa) = 1.98%• PKU (aa) = .01%

Result• Gene pool is in a state of equilibrium

and has not changed because of sexual reproduction.

• No Evolution has occurred.

AP Problems Using Hardy-Weinberg

• Solve for q2 (% of total).• Solve for q (equation).• Solve for p (1- q).• H-W is always on the national AP Bio

exam (but no calculators are allowed).

Remember Hardy-Weinberg Assumptions

1. Large Population2. Isolation3. No Net Mutations4. Random Mating5. No Natural Selection

If H-W assumptions hold true:

• The gene frequencies will not change over time.

• Evolution will not occur.• But, how likely will natural

populations hold to the H-W assumptions?

Microevolution• Caused by violations of the 5 H-W

assumptions.

Causes of Microevolution1. Genetic Drift2. Gene Flow3. Mutations4. Nonrandom Mating5. Natural Selection

Genetic Drift• Changes in the gene pool of a

small population by chance.• Types:–1. Bottleneck Effect–2. Founder's Effect

• The smaller a sample, the greater the chance of deviation from a predicted result

Genetic Drift• Genetic drift describes how

allele frequencies fluctuate unpredictably from one generation to the next

• Genetic drift tends to reduce genetic variation through losses of alleles

Fig. 23-8-1

Generation 1p (frequency of CR) = 0.7

q (frequency of CW ) = 0.3

CW CW

CR CR

CR CW

CR CR

CR CR

CR CR

CR CR

CR CW

CR CW

CR CW

Genetic Drift

Fig. 23-8-2

Generation 1p (frequency of CR) =

0.7q (frequency of CW ) = 0.3

Generation 2 p = 0.5

q = 0.5

CW CW

CR CR

CR CW

CR CR

CR CR

CR CR

CR CR

CR CW

CR CW

CR CW

CR CWCR CW

CR CW

CR CW

CW CW

CW CW

CW CW

CR CR

CR CR

CR CR

Genetic Drift

Fig. 23-8-3

Generation 1

CW CW

CR CR

CR CW

CR CR

CR CR

CR CR

CR CR

CR CW

CR CW

CR CW

p (frequency of CR) = 0.7q (frequency of CW

) = 0.3

Generation 2

CR CWCR CW

CR CW

CR CW

CW CW

CW CW

CW CW

CR CR

CR CR

CR CR

p = 0.5q = 0.5

Generation 3 p = 1.0

q = 0.0

CR CR

CR CR

CR CR

CR CR

CR CR

CR CR CR CR

CR CR

CR CR CR CR

Genetic Drift

Genetic Drift By Chance

Bottleneck EffectLoss of most of the population by disasters.

Bottleneck Effect• The bottleneck effect is a

sudden reduction in population size due to a disaster or a change in the environment• Surviving population may have a

different gene pool than the original population, it may no longer be reflective of the original population’s gene pool

• If the population remains small, it may be further affected by genetic drift• Understanding the bottleneck effect

can increase understanding of how human activity affects other species

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

Fig. 23-10a

Rangeof greaterprairiechicken

Pre-bottleneck(Illinois, 1820)

Post-bottleneck(Illinois, 1993)

(a)

Fig. 23-10b

Numberof allelesper locus

Minnesota, 1998   (no bottleneck)

Nebraska, 1998   (no bottleneck)

Kansas, 1998   (no bottleneck)

Illinois1930–1960s1993

Location Populationsize

Percentageof eggshatched

1000–25,000<50

750,000

75,000–200,0004,000

5.23.7

93<50

5.8

5.8

5.3 85

96

99

(b)

Prairie Chicken• 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%

Importance• Reduction of population size may

reduce gene pool for evolution to work with.

• Ex: Cheetahs

Founder's Effect• The founder effect occurs when

a few individuals become isolated from a larger population

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

Founder's Effect• Genetic drift in a new colony that

separates from a parent population.

• Examples:• Old-Order Amish• A butterfly that gets

blown to an island and lays her eggs

Result of Bottleneck and Founder’s effects

• Genetic variation reduced.• Some alleles increase in frequency

while others are lost (as compared to the parent population).• Very common in islands and other

groups that don't interbreed

Effects of Genetic Drift: A Summary

1. Genetic drift is significant in small populations

2. Genetic drift causes allele frequencies to change at random

3. Genetic drift can lead to a loss of genetic variation within populations

4. Genetic drift can cause harmful alleles to become fixed

Gene Flow• Gene flow consists of the

movement of alleles among populations (in or out of a population)–Immigration–Emigration

• Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen)

Gene Flow• Gene flow tends to reduce

differences between populations over time• Gene flow is more likely than

mutation to alter allele frequencies directly

Fig. 23-11

• Gene flow can decrease the fitness of a population• In Bent grass, alleles for copper

tolerance are beneficial in populations near copper mines, but harmful to populations in other soils• Windblown pollen moves these

alleles between populations• The movement of unfavorable

alleles into a population results in a decrease in fit between organism and environment

Fig. 23-12a

NON-MINESOIL

MINESOIL

NON-MINESOIL

Prevailing wind direction

Inde

x of

cop

per

tole

ranc

e

Distance from mine edge (meters)

7060

5040

30

20

10

020 0 2

00 2

040

60

80

100

120

140

160

Fig. 23-12b

Bent grass

• Gene flow can increase the fitness of a population• Insecticides have been used to

target mosquitoes that carry West Nile virus and malaria• Alleles have evolved in some

populations that confer insecticide resistance to these mosquitoes• The flow of insecticide resistance

alleles into a population can cause an increase in fitness

Result• Changes in gene frequencies within a

population.• Immigration often brings new alleles

into populations increasing genetic diversity.

Mutations• Inherited changes in a gene.

Result of mutations• May change gene frequencies

(small population).• Source of new alleles for

selection.• Often lost by genetic drift.

Nonrandom Mating• Failure to choose mates at

random from the population.

• Inbreeding within the same “neighborhood”.• Assortative mating (like with like).• Results in increases in the number

of homozygous loci.• Does not in itself alter the overall

gene frequencies in the population.

Sexual Mate selection

• Sexual selection is natural selection for mating success

• May not be adaptive to the environment, but increases reproduction success of the individual.

Sexual Mate selection

• This is a VERY important selection type for species.

• It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

Fig. 23-15

Sexual dimorphism

• Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex

• Intersexual selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates

• Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival

• How do female preferences evolve?• The good genes hypothesis

suggests that if a trait is related to male health, both the male trait and female preference for that trait should be selected for

Fig. 23-16a

SC male graytree frog

Female graytree frog LC male

graytree frogSC sperm Eggs LC

sperm

Offspring ofLC father

Offspring ofSC father

Fitness of these half-sibling offspring compared

EXPERIMENT

Fig. 23-16b

RESULTS

1995Fitness Measure 1996

Larval growthLarval survivalTime to metamorphosis

LC better

NSD

LC better(shorter)

LC better(shorter)

NSD

LC better

NSD = no significant difference; LC better = offspring of LC malessuperior to offspring of SC males.

Result• Sexual dimorphism.• Secondary sexual features for

attracting mates.

Comment• Females may drive sexual

selection and dimorphism since they often "choose" the mate.

Natural Selection• Differential success in survival and

reproduction.• Differential success in reproduction

results in certain alleles being passed to the next generation in greater proportions

Natural Selection• Only natural selection consistently

results in adaptive evolution• Natural selection brings about

adaptive evolution by acting on an organism’s phenotype

• Result - Shifts in gene frequencies.

Comment• As the Environment changes, so does

Natural Selection and Gene Frequencies.

Result• If the environment is "patchy",

the population may have many different local populations.

The Key Role of Natural Selection in Adaptive Evolution

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

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

Fig. 23-14a

Color-changing ability in cuttlefish

Fig. 23-14b

Movable jaw bones in snakes

Movable bones

• Because the environment can change, adaptive evolution is a continuous process• 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

Genetic Basis of Variation1. Discrete Characters – Mendelian

traits with clear phenotypes.2. Quantitative Characters –

Multigene traits with overlapping phenotypes.

Polymorphism• The existence of several

contrasting forms of the species in a population.

• Usually inherited as Discrete Characteristics.

Examples of Polymorphism Garter SnakesGaillardia

Human Example• ABO Blood Groups• Morphs = A, B, AB, O

Other examples

Quantitative Characters• Allow continuous variation in the

population.• Result – –Geographical Variation–Clines: a change along a geographical axis

Yarrow and Altitude

Sources of Genetic Variation• Mutations.• Recombination though sexual

reproduction.–Crossing-over–Random fertilization

The Preservation of Genetic Variation

• Various mechanisms help to preserve genetic variation in a population

1. Diploidy - preserves recessives as heterozygotes.

2. Balanced Polymorphisms - preservation of diversity by natural selection.

Diploidy

• Diploidy maintains genetic variation in the form of hidden recessive alleles

•Heterozygote Advantage - When the heterozygote or hybrid survives better (have a higher fitness) than the homozygotes. Also called Hybrid vigor.

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

Heterozygote Advantage

Heterozygote Advantage• Can't bred "true“ and the diversity of

the population is maintained.• Example of hybrid vigor – Sickle Cell

Anemia• The sickle-cell allele causes mutations

in hemoglobin but also confers malaria resistance

Fig. 23-17

0–2.5%

Distribution ofmalaria caused byPlasmodium falciparum(a parasitic unicellular eukaryote)

Frequencies of thesickle-cell allele

2.5–5.0%

7.5–10.0%

5.0–7.5%

>12.5%10.0–12.5%

• In 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

Frequency-Dependent Selection

Fig. 23-18a

“Right-mouthed”

“Left-mouthed”

Fig. 23-18

“Right-mouthed”

1981

“Left-mouthed”

Freq

uenc

y of

“lef

t-m

outh

ed”

indi

vidu

als

Sample year

1.0

0.5

0’82

’83

’84

’85

’86

’87

’88

’89

’90

Comment

• Population geneticists believe that ALL genes that persist in a population must have had a selective advantage at one time.

• Ex – Sickle Cell and Malaria, Tay-Sachs and Tuberculosis

Fitness - Darwinian• The relative contribution an

individual makes to the gene pool of the next generation.

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

• Selection favors certain genotypes by acting on the phenotypes of certain organisms

Relative Fitness• Contribution of one genotype to

the next generation compared to other genotypes.

• The phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals

• Reproductive success is generally more subtle and depends on many factors

Rate of Selection• Differs between dominant and

recessive alleles.• Selection pressure by the

environment.

Modes of Selection• Three modes of selection:–Directional selection favors individuals at one end of the phenotypic range–Disruptive selection favors individuals at both extremes of the phenotypic range–Stabilizing selection favors intermediate variants and acts against extreme phenotypes

Stabilizing• Selection toward the average

and against the extremes.• Ex: birth weight in humans

Directional Selection

• Selection toward one extreme.• Ex: running speeds in race

animals.• Ex. Galapagos Finch beak size

and food source.

Diversifying• Selection toward both extremes

and against the norm.• Ex: bill size in birds

Comment• Diversifying Selection - can split

a species into several new species if it continues for a long enough period of time and the populations don’t interbreed.

Fig. 23-13

Original population

(c) Stabilizing selection

(b) Disruptive selection

(a) Directional selection

Phenotypes (fur color)

Freq

uenc

y of

in

divi

dual

sOriginalpopulation

Evolvedpopulation

Balancing Selection

• Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population

Neutral Variation

• Neutral variation is genetic variation that appears to confer no selective advantage or disadvantage• For example, –Variation in noncoding regions of DNA–Variation in proteins that have little effect on protein function or reproductive fitness

Why Natural Selection Cannot Fashion Perfect Organisms

1. Selection can act only on existing variations

2. Evolution is limited by historical constraints

3. Adaptations are often compromises

4. Chance, natural selection, and the environment interact

Fig. 23-19

Question• Does evolution result in perfect

organisms?

Answer - No

1. Historical Constraints2. Compromises3. Non-adaptive Evolution

(chance)4. Available variations – most

come from using a current gene in a new way.

Summary• Know the difference between a

species and a population.• Know that the unit of evolution is

the population and not the individual.

Summary• Know the H-W equations and

how to use them in calculations.• Know the H-W assumptions and

what happens if each is violated.

Summary• Identify various means to

introduce genetic variation into populations.

• Know the various types of natural selection.

You should now be able to:

1. Explain why the majority of point mutations are harmless

2. Explain how sexual recombination generates genetic variability

3. Define the terms population, species, gene pool, relative fitness, and neutral variation

4. List the five conditions of Hardy-Weinberg equilibrium

5. Apply the Hardy-Weinberg equation to a population genetics problem

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

7. Explain the role of population size in genetic drift

8. 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

End of Chapter 23!