23.3 23.4

23.3 23.4

N A T U R A L SE L E C T I O

N , GE N E T I C

DR I F

T , AN D

G E N E FL O W C

A N ALT E R A

PO P U L A T I O

N ’ S

G E N E T I C C

O M P O S I TI O

N

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Concept 23.3: Natural selection, genetic drift, and gene flow can alter a population’s genetic composition

Three major factors alter allele frequencies and bring about most evolutionary change

– Natural selection

– Genetic drift

– Gene flow


Genetic DriftUnpredictable fluctuation in alleles frequency from one generation to the next because of a population finite size.

• Statistically, the smaller a sample

– The greater the chance of deviation from a predicted result


Natural Selection• Differential success in reproduction

– Results in certain alleles being passed to the next generation in greater proportions


• 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 of10 plantsleaveoffspring

CWCW CRCR

CRCW

CRCR CWCW

CRCW

CWCW CRCR

CRCW CRCW

Only 2 of10 plantsleaveoffspring

CRCR

CRCR CRCR

CRCRCRCR

CRCR

CRCR

CRCR

CRCRCRCR

Generation 2p = 0.5q = 0.5

Generation 3p = 1.0q = 0.0

Generation 1p (frequency of CR) = 0.7q (frequency of CW) = 0.3


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 population’s gene pool

Originalpopulation

Bottleneckingevent

Survivingpopulation

Figure 23.8 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.


• Understanding the bottleneck effect

– Can increase understanding of how human activity affects other species

Figure 23.8 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.


The Founder Effect

– Occurs when a few individuals become isolated from a larger population

– Can affect allele frequencies in a population


Gene Flow

– Results from the movement of fertile individuals or gametes

– Causes a population to gain or lose alleles

– Tends to reduce differences between populations over time


Concept 23.4:Natural selection is the primary mechanism of adaptive evolution

• Natural selection

– Accumulates and maintains favorable genotypes in a population


Genetic Variation – Occurs in individuals in populations of all species

– Is not always heritable

Figure 23.9 A, B

(a) Map butterflies thatemerge in spring:orange and brown

(b) Map butterflies thatemerge in late summer:black and white

Seasonal differences in hormones


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

– The different forms can be called morphs.

• Quantitative characters

– Vary along a continuum within a population


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

– Alleles of several loci affect the character

• height


Measuring Genetic Variation• Population geneticists measure the number of

polymorphisms in a population by determining the amount of heterozygosity – At the gene level

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

– At the molecular level: nucleotide variability

Average heterozygosity tends to be greater than nucleotide variability


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


Some examples of geographic variation occur as a cline: a graded change in a trait along a geographic axis (parallels to the gradient in the environment)

Figure 23.11

EXPERIMENT Researchers observed that the average sizeof yarrow plants (Achillea) growing on the slopes of the Sierra Nevada mountains gradually decreases with increasing elevation. To eliminate the effect of environmental differences at different elevations, researchers collected seeds from various altitudes and planted them in a common garden. They then measured the heights of theresulting plants.

RESULTS The average plant sizes in the commongarden were inversely correlated with the altitudes at which the seeds were collected, although the height differences were less than in the plants’ natural environments.

CONCLUSION The lesser but still measurable clinal variationin yarrow plants grown at a common elevation demonstrates therole of genetic as well as environmental differences.

Mea

n he

ight

(cm

)A

titud

e (m

)

Heights of yarrow plants grown in common garden

Seed collection sites

Sierra NevadaRange

Great BasinPlateau


A Closer Look at Natural Selection

• From the range of variations available in a population, natural selection increases the frequencies of certain genotypes, fitting organisms to their environment over generations


Evolutionary Fitness• The phrases “struggle for existence” and

“survival of the fittest”

– Are commonly used to describe natural selection

– Can be misleading

• Reproductive success– Is generally more subtle and depends on many

factors


• Fitness– Is the contribution an individual makes to the

gene pool of the next generation, relative to the contributions of other individuals

• In a more quantitative way: Relative fitness– the contribution of a genotype to the next

generation as compared to the contributions of alternative genotypes for the same locus


Directional, Disruptive, and Stabilizing Selection• Selection

– Favors certain genotypes by acting on the phenotypes of certain organisms

• Three modes of selection are

– Directional

– Disruptive

– Stabilizing


• 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


The three modes of selection

Directional selection Disruptive selection Stabilizing selection


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 themfrom predators.


Disruptive selection

• favors variantsat 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.


Stabilizing selection

removesextreme variants from the populationand preserves intermediate types.

If the environment consists of rocks ofan intermediate color, both light anddark mice will be selected against.


The Preservation of Genetic Variation• Various mechanisms help to preserve genetic

variation in a population

– Diploidy (Eukaryotes are diploid)

• Maintains genetic variation in the form of hidden recessive alleles


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


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


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

0–2.5%2.5–5.0%5.0–7.5%7.5–10.0%

10.0–12.5%>12.5%

Distribution ofmalaria caused byPlasmodium falciparum(a protozoan)


Frequency-Dependent Selection – The fitness of any morph declines if it becomes

too common in the population


• An example of frequency-dependent selection

Phe

noty

pic

dive

rsity

Figure 23.14

Parental population sample

Experimental group sample

Plain background Patterned background

On pecking a moth imagethe blue jay receives afood reward. If the bird does not detect a mothon either screen, it pecksthe green circle to continueto a new set of images (anew feeding opportunity).

0.06

0.05

0.04

0.03

0.020 20 40 60 80 100

Generation number

Frequency-independent control


Neutral Variation– Is genetic variation that appears to confer no

selective advantage


Sexual Selection

– Is natural selection for mating success

– Sexual dimorphism:

• marked differences between the sexes in secondary sexual characteristics


Intrasexual selection– Is a direct competition among individuals of

one sex for mates of the opposite sex


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 male’s appearance

Figure 23.15


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


• 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

1. Evolution is limited by historical constraints

– Each species has a legacy of descent with modification. (birds 4 legs & wins?)

2. Adaptations are often compromises– Seals swim well, walk not as well

3. Chance and natural selection interact4. Selection can only edit existing variations

Documents

23.3 23.4