23- Evolution of Populations Text

8/3/2019 23- Evolution of Populations Text

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Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

PowerPoint Lectures for

Biology, Seventh Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero

Chapter 23Chapter 23

The Evolution of Populations


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Overview: The Smallest Unit of Evolution

One common misconception about evolution is

that individual organisms evolve, in the

Darwinian sense, during their lifetimes

Natural selection acts on individuals, but

populations evolve


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Genetic variations in populations

Contribute to evolution

Figure 23.1


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Concept 23.1: Population genetics provides a

foundation for studying evolution

Microevolution

Is change in the genetic makeup of apopulation from generation to generation

Figure 23.2


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The Modern Synthesis

Population genetics

Is the study of how populations change

genetically over time

Reconciled Darwins and Mendels ideas


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The modern synthesis

Integrates Mendelian genetics with the

Darwinian theory of evolution by natural

selection

Focuses on populations as units of evolution


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Gene Pools and Allele Frequencies

A population

Is a localized group of individuals that are capable of

interbreeding and producing fertile offspring

MAP

AREA

Fairbanks

Whitehorse

Fortymile

herd range

Figure 23.3


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The Hardy-Weinberg Theorem

The Hardy-Weinberg theorem

Describes a population that is not evolving

States that the frequencies of alleles and

genotypes in a populations gene pool remain

constant from generation to generation

provided that only Mendelian segregation and

recombination of alleles are at work


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Preservation of Allele Frequencies

In a given population where gametes contribute

to the next generation randomly, allelefrequencies will not change


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Hardy-WeinbergEquilibrium

Hardy-Weinberg equilibrium

Describes a population in which random

mating occurs

Describes a population where allele

frequencies do not change


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A population in Hardy-Weinberg equilibrium

Figure 23.5

Gametes for each generation are drawn at random from

the gene pool of the previous generation:

80%CR(p = 0.8) 20% CW (q = 0.2)

SpermCR

(80%)CW

(20%)

p2

64%

CRCR16%

CRCW

16%

CRCW4%

CWCWqp

CR

(80%)

Eggs

CW

(20%)

pq

If the gametes come together at random, the genotype

frequencies of this generation are in Hardy-Weinberg equilibrium:

q2

64% CRCR, 32% CRCW, and 4% CWCW

Gametes of the next generation:

64% CR from

CRCRhomozygotes

16% CR from

CRCWhomozygotes+ = 80% CR= 0.8 =p

16% CWfrom

CRCWheterozygotes+ = 20% CW= 0.2 = q

With random mating, these gametes will result in the same

mix of plants in the next generation:

64% CRCR, 32% CRCWand 4% CWCWplants

p2

4% CWfrom

CWCWhomozygotes


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Ifp and q represent the relative frequencies of

the only two possible alleles in a population ata particular locus, then

p2 + 2pq + q2 = 1

Andp2 and q2represent the frequencies of the

homozygous genotypes and 2pq represents

the frequency of the heterozygous genotype


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Conditions forHardy-WeinbergEquilibrium

The Hardy-Weinberg theorem

Describes a hypothetical population

In real populations

Allele and genotype frequencies do changeover time


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The five conditions for non-evolving

populations are rarely met in nature

Extremely large population size

No gene flow

No mutations

Random mating

No natural selection


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Population Genetics andHuman Health

We can use the Hardy-Weinberg equation

To estimate the percentage of the human

population carrying the allele for an inherited

disease


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Concept 23.2: Mutation and sexual

recombination produce the variation thatmakes evolution possible

Two processes, mutation and sexual

recombination

Produce the variation in gene pools that

contributes to differences among individuals


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Mutation

Mutations

Are changes in the nucleotide sequence of DNA

Cause new genes and alleles to arise

Figure 23.6


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Point Mutations

A point mutation

Is a change in one base in a gene

Can have a significant impact on phenotype

Is usually harmless, but may have an adaptiveimpact


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Mutations That AlterGene NumberorSequence

Chromosomal mutations that affect many loci

Are almost certain to be harmful

May be neutral and even beneficial


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Gene duplication

Duplicates chromosome segments


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Mutation Rates

Mutation rates

Tend to be low in animals and plants

Average about one mutation in every 100,000

genes per generation

Are more rapid in microorganisms


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Sexual Recombination

In sexually reproducing populations, sexual

recombination

Is far more important than mutation in

producing the genetic differences that make

adaptation possible


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Genetic Drift

Statistically, the smaller a sample

The greater the chance of deviation from a

predicted result


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Genetic drift

Describes how allele frequencies can fluctuate

unpredictably from one generation to the next

Tends to reduce genetic variation

Figure 23.7

CRCR

CRCW

CRCR

CWCW CRCR

CRCW

CRCW

CRCWCRCR

CRCR

Only 5 of

10 plants

leave

offspring

CWCW CRCR

CRCW

CRCR CWCW

CRCW

CWCW CRCR

CRCW CRCW

Only 2 of

10 plants

leave

offspring

CRCR

CRCR CRCR

CRCRCRCR

CRCR

CRCR

CRCR

CRCRCRCR

Generation 2

p = 0.5

q = 0.5

Generation 3

p = 1.0q = 0.0

Generation 1

p (frequency ofCR) = 0.7q (frequency ofCW) = 0.3


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The Bottleneck Effect

In the bottleneck effect

A sudden change in the environment may

drastically reduce the size of a population

The gene pool may no longer be reflective of

the original populations gene pool

Original

population

Bottlenecking

eventSurviving

population

Figure 23.8 A

(a) Shaking just a few marbles through the

narrow neck of a bottle is analogous to a

drastic reduction in the size of a population

after some environmental disaster. By chance,

blue marbles are over-represented in the new

population and gold marbles are absent.


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Understanding the bottleneck effect

Can increase understanding of how human

activity affects other species

Figure 23.8 B

(b) Similarly, bottlenecking a population

of organisms tends to reduce genetic

variation, as in these northern

elephant seals in California that were

once hunted nearly to extinction.


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The FounderEffect

The founder effect

Occurs when a few individuals become

isolated from a larger population

Can affect allele frequencies in a population


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Gene Flow

Gene flow

Causes a population to gain or lose alleles

Results from the movement of fertile

individuals or gametes

Tends to reduce differences between

populations over time


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Concept 23.4: Natural selection is the primary

mechanism of adaptive evolution

Natural selection

Accumulates and maintains favorable

genotypes in a population


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Genetic Variation

Genetic variation

Occurs in individuals in populations of all

species

Is not always heritable

Figure 23.9 A, B

(a) Map butterflies that

emerge in spring:

orange and brown

(b) Map butterflies that

emerge in late summer:

black and white


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Variation Within a Population

Both discrete and quantitative characters

Contribute to variation within a population


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Discrete characters

Can be classified on an either-or basis

Quantitative characters

Vary along a continuum within a population


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Polymorphism

Phenotypic polymorphism

Describes a population in which two or more

distinct morphs for a character are each

represented in high enough frequencies to be

readily noticeable

Genetic polymorphisms

Are the heritable components of characters

that occur along a continuum in a population


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Measuring Genetic Variation

Population geneticists

Measure the number of polymorphisms in a

population by determining the amount of

heterozygosity at the gene level and the

molecular level

Average heterozygosity

Measures the average percent of loci that are

heterozygous in a population


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Variation Between Populations

Most species exhibit geographic variation

Differences between gene pools of separate

populations or population subgroups

1 2.4 3.14 5.18 6 7.15

XX1913.1710.169.128.11

1 2.19 3.8 4.16 5.14 6.7

XX15.1813.1711.129.10Figure 23.10


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Some examples of geographic variation occur

as a cline, which is a graded change in a traitalong a geographic axis

Figure 23.11

EXPERIMENT Researchers observed that the average size

of yarrow plants (Achillea) growing on the slopes of the Sierra

Nevada mountains gradually decreases with increasing

elevation. To eliminate the effect of environmental differencesat different elevations, researchers collected seeds

from various altitudes and planted them in a commongarden. They then measured the heights of the

resulting plants.

RESULTS The average plant sizes in the commongarden were inversely correlated with the altitudes at

which the seeds were collected, although the heightdifferences were less than in the plants natural

environments.

CONCLUSION The lesser but still measurable clinal variationin yarrow plants grown at a common elevation demonstrates the

role of genetic as well as environmental differences.

Meanheight(cm)

Atitude(m)

Heights of yarrow plants grown in common garden

Seed collection sites

Sierra NevadaRange

Great BasinPlateau


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A Closer Look at Natural Selection

From the range ofvariations available in a

population

Natural selection increases the frequencies of

certain genotypes, fitting organisms to their

environment o

ver generations


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Fitness

Is the contribution an individual makes to the

gene pool of the next generation, relative to

the contributions of other individuals

Relative fitness

Is the contribution of a genotype to the next

generation as compared to the contributions of

alternative genotypes for the same locus


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Directional, Disruptive, and StabilizingSelection

Selection

Favors certain genotypes by acting on the

phenotypes of certain organisms

Three modes of selection are

Directional

Disruptive

Stabilizing


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The three modes of selection

Fig 23.12 AC

(a) Directional selection shifts the overall

makeup of the population by favoring

variants at one extreme of the

distribution. In this case, darker mice are

favored because they live among dark

rocks and a darker fur color conceals them

from predators.

(b) Disruptive selection favors variants

at both ends of the distribution. These

mice have colonized a patchy habitat

made up of light and dark rocks, with the

result that mice of an intermediate color are

at a disadvantage.

(c) Stabilizing selection removes

extreme variants from the population

and preserves intermediate types. If

the environment consists of rocks of

an intermediate color, both light and

dark mice will be selected against.

Phenotypes (fur color)

Original population

Originalpopulation

Evolved

population


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The Preservation ofGenetic Variation

Various mechanisms help to preserve genetic

variation in a population


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Diploidy

Diploidy

Maintains genetic variation in the form of

hidden recessive alleles


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alancing Selection

Balancing selection

Occurs when natural selection maintains

stable frequencies of two or more phenotypic

forms in a population

Leads to a state called balanced polymorphism


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Heterozygote Advantage

Some individuals who are heterozygous at a

particular locus

Have greater fitness than homozygotes

Natural selection

Will tend to maintain two or more alleles at that

locus


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The sickle-cell allele

Causes mutations in hemoglobin but also

confers malaria resistance

Exemplifies the heterozygote advantage

Figure 23.13

Frequencies of thesickle-cell allele

02.5%

2.55.0%

5.07.5%

7.510.0%

10.012.5%

>12.5%

Distribution ofmalaria caused by

Plasmodium falciparum

(a protozoan)


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Frequency-Dependent Selection

In frequency-dependent selection

The fitness of any morph declines if it becomes

too common in the population


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An example of frequency-dependent selection

Phenotypicd

iversity

Figure 23.14

Parental population sample

Experimental group sample

Plain background Patterned background

On pecking a moth image

the blue jay receives afood reward. If the bird

does not detect a moth

on either screen, it pecks

the green circle to continue

to a new set of images (a

new feeding opportunity).

0.06

0.05

0.04

0.03

0.02

0 20 40 60 80 100Generation number

Frequency-

independent control

N l V i i


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NeutralVariation

Neutral variation

Is genetic variation that appears to confer no

selective advantage

S l S l ti


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Sexual Selection

Sexual selection

Is natural selection for mating success

Can result in sexual dimorphism, marked

differences between the sexes in secondary

sexual characteristics


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Intrasexual selection

Is a direct competition among individuals of

one sex for mates of the opposite sex


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Intersexual selection

Occurs when individuals of one sex (usually

females) are choosy in selecting their mates

from individuals of the other sex

May depend on the showiness of the malesappearance

Figure 23.15

Th E l ti E i f S l R d ti


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The Evolutionary Enigma of Sexual Reproduction

Sexual reproduction

Produces fewer reproductive offspring than asexual

reproduction, a so-called reproductive handicap

Figure 23.16

Asexual reproduction

Female

Sexual reproduction

Female

Male

Generation 1

Generation 2

Generation 3

Generation 4


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If sexual reproduction is a handicap, why has it

persisted?

It produces genetic variation that may aid in

disease resistance

Why Natural Selection Cannot Fashion Perfect Organisms


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Why Natural Selection Cannot Fashion Perfect Organisms

Evolution is limited by historical constraints

Adaptations are often compromises


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Chance and natural selection interact

Selection can only edit existing variations

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