Fig. 22-5

NORTHAMERICA

EUROPE

AFRICA

AUSTRALIA

GREATBRITAIN

SOUTHAMERICA

ATLANTICOCEAN

PACIFICOCEAN Cape of

Good Hope

Tierra del Fuego

Cape HornTasmania

NewZealand

An

des

Equator

TheGalápagosIslands

Pinta

MarchenaGenovesa

SantiagoDaphneIslands

PinzónFernandina

IsabelaSan

Cristobal

SantaFe

SantaCruz

Florenza Española

Fig. 22-6

(a) Cactus-eater (c) Seed-eater

(b) Insect-eater

Fig. 23-1

Chapter 23The Evolution of Populations

1977, Drought

G. Fortis 180/1200 survive

Fig. 23-11

Overview: The Smallest Unit of Evolution

• One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes

• Natural selection acts on individuals, but only populations evolve

• Genetic variations in populations contribute to evolution

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

Fig. 23-2

(a) (b)

1.Variation in individual genotype leads to variation in individual phenotype

2.Not all phenotypic variation is heritable

Oak leaves Oak Twig

Variation Within a Population

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

• Discrete characters can be classified on an either-or (非此即彼 ) basis,

– by a single gene locus with different alleles that produces distinct characters

• Quantitative characters vary along a continuum within a population

– Heritable quantitative variation usually results from the influence of two or more genes on a single phenotypic character.

• Polymorphisms in a population

– by determining the amount of heterozygosity at the whole- gene level (gene variability) and molecular levels (nucleotide variability)

• gene variability

– Average heterozygosity

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

• 1920 loci /13,700 genes of fruit fly (heterozygous/ Total)

– By surveying the protein product

• Nucleotide variability

– is measured by comparing the DNA sequences of pairs of individuals

13.17 19 XX10.169.128.11

1 2.4 3.14 5.18 6 7.15

9.10

1 2.19

11.12 13.17 15.18

3.8 4.16 5.14 6.7

XX

Most species exhibit geographic variation, differences between gene pools of separate populations or population subgroups

Variation Between Populations

Fig. 23-4

1.0

0.8

0.6

0.4

0.2

046 44 42 40 38 36 34 32 30

GeorgiaWarm (21°C)

Latitude (°N)MaineCold (6°C)

Ldh

-B b

alle

le f

req

uen

cy

•Some examples of geographic variation occur as a cline,

•which is a graded change in a trait along a geographic axis

A cline determined by temperature

Cline

From Wikipedia, the free encyclopedia

Cline can refer to

Term meaning "gradual change" (from the Greek [klīnein], "to lean") – Cline (biology), a gradual change of a character or

feature (phenotype) in a species over a geographical area.

– "Cultural cline" is a gradual change of a cultural characteristic or feature over a geographical area.

– Thermocline, a layer within a body of water or air where the temperature changes with depth.

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

• Mutations That Alter Gene Number or Sequence

• Mutation Rates

• The average is about one mutation in every 100,000 genes per generation

Sexual Reproduction

• Sexual reproduction can shuffle existing alleles into new combinations

• In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible

Concept 23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving

• To test whether evolution is occurring in a population

– Is to clarify what we mean by a population

Gene Pools and Allele Frequencies

A gene pool consists of all the alleles for all loci in a population

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

• 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

– homozygous genotypes: p2 and q2

– heterozygous genotype:2pq

Fig. 23-7-4

Gametes of this generation:

64% CR CR, 32% CR CW, and 4% CW CW

64% CR    +     16% CR    =   80% CR  = 0.8 = p

4% CW      +    16% CW    =  20% CW = 0.2 = q

64% CR CR, 32% CR CW, and 4% CW CW plants

Genotypes in the next generation:

SpermCR

(80%)

CW

( 20%

)

80% CR ( p = 0.8)

CW (20%)

20% CW (q = 0.2)

16% ( pq) CR CW

4% (q2) CW CW

CR

(80%

)

64% ( p2) CR CR

16% (qp) CR CW

Eg

gs

Conditions for Hardy-Weinberg Equilibrium

• The Hardy-Weinberg theorem describes a hypothetical population

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

• 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

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

• The occurrence of PKU is 1 per 10,000 births

– q2 = 0.0001

– q = 0.01

• The frequency of normal alleles is

– p = 1 – q = 1 – 0.01 = 0.99

• The frequency of carriers is

– 2pq = 2 x 0.99 x 0.01 = 0.0198

– or approximately 2% of the U.S. population

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

– Natural selection

– Genetic drift

– Gene flow

Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population

Natural Selection

• Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions

• D. melanogaster: an allele that confers resistance to several insecticide DDT)

– 1930--- 0% 1960s---37% adaptive evolution

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 3p = 1.0q = 0.0

CR CR

CR CR

CR CR

CR CR

CR CR

CR CR CR CR

CR CR

CR CR CR CR

Genetic Drift

1.The smaller a sample, the greater the chance of deviation from a predicted result2.Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next3.Genetic drift tends to reduce genetic variation through losses of alleles

The Founder 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

The Bottleneck Effect

• The bottleneck effect is a sudden reduction in population size due to a change in the environment

• The resulting gene pool 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

Fig. 23-9

Originalpopulation

Bottleneckingevent

Survivingpopulation

Fig. 23-10

Numberof allelesper locus

Rangeof greaterprairiechicken

Pre-bottleneck(Illinois, 1820)

Post-bottleneck(Illinois, 1993)

Minnesota, 1998    (no bottleneck)

Nebraska, 1998    (no bottleneck)

Kansas, 1998    (no bottleneck)

Illinois

1930–1960s

1993

Location Populationsize

Percentageof eggshatched

1,000–25,000

<50

750,000

75,000–200,000

4,000

5.2

3.7

93

<50

5.8

5.8

5.3 85

96

99

(a)

(b)

Case Study: Impact of Genetic Drift on

the Greater Prairie Chicken

•caused a severe reduction in the population of greater prairie chickens in Illinois

Loss of prairie habitat

The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched

•Understanding the bottleneck effect can increase understanding of how human activity affects other species

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

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

• 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

新移民

1.Gene flow

2. Can homogenize the gene pools of such populations thereby reducing geographic variation in appearence

秦、漢以前，漢族的血統或許較純，這從出土的秦俑可以得到證明。秦俑臉寬、鼻扁，而且都有

一雙單眼皮的杏眼（圖一），正是典型的蒙古人種

單眼皮，雙眼皮──由仕女圖所引發的觀察和思索

唐代壁畫及唐代雕塑看出端倪。壁畫和雕塑中都出現了凸鼻凹目的胡人（圖二），而且為數相當多。雙眼皮也

出現了，但似乎未見出現漢人臉上（圖三）。

魏晉南北朝的民族大融合

1997年6月330期 作者：張之傑

科學月刊

南宋起，中國的文化中心遷移到長江流域。漢族的向南拓展，使得許多東南亞系統的民族融入漢族。同時，入侵北方的遼（契丹）、金（滿）等異族被漢族同化。

元世祖（忽必烈）是單眼皮，其皇后（徹伯爾，圖五）卻是

雙眼皮； 明太祖、馬皇后（圖六）、明成祖是單眼皮，明宣宗卻是雙眼皮

唐代以後的雙眼皮增多，可能和西域胡人（屬高加索人種）的混入和南方開發有關。唐代大批西域胡人來到中國，不可能不和漢人通婚。另一方面，南方的土著雖屬蒙古人種，但混有地中海人（屬高加索人種）、矮黑人的血統。

1.仕女圖中千篇一律的單眼皮，在晉朝和唐朝可能出於寫實。 2.宋朝以後，將美人畫成單眼皮卻成為一種程式。程式的形成，或出於陳陳相因，或出於長期以北方為文化中心所形成的審美觀的制約。 3.早期月份牌畫所畫的美女，體態較為纖弱，眼型以細長杏眼、單眼皮居多。到了後期，體態普遍較為健美，眼型則以雙眼皮、大眼睛居多。轉變的軌跡清晰可尋。

傳統審美觀的沒落

Fig. 23-12

NON-MINESOIL

MINESOIL

NON-MINESOIL

Prevailing wind direction

Ind

ex o

f co

pp

er t

ole

ran

ce

Distance from mine edge (meters)

70

60

50

40

30

20

10

020 0 20 0 20 40 60 80 100 120 140 160

•Gene flow can decrease the fitness of a population

•Windblown pollen moves these alleles between populations

• 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

• Only natural selection consistently results in adaptive evolution by acting on an organism’s phenotype

• 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

• Relative Fitness

Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution

Directional, Disruptive, and Stabilizing Selection

• Natural selection can alter the frequency distribution of heritable traits in three ways:

• 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

Fig. 23-13

Original population

(c) Stabilizing selection(b) Disruptive selection(a) Directional selection

Phenotypes (fur color)F

req

uen

cy o

f in

div

idu

als

Originalpopulation

Evolvedpopulation

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

• 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

Sexual Selection

• Sexual selection is natural selection

– for mating success

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

– Intrasexual selection

– Intersexual selection (often called mate choice)

Male showiness

Preservation of Genetic Variation (Various mechanisms)

• Diploidy 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

• Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes

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

• Frequency-Dependent Selection

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%

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

You should now be able to:

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

2. List the five conditions of Hardy-Weinberg equilibrium

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

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

5. Explain the role of population size in genetic drift