The Evolution of Populations

Chapter 23

Overview: The Smallest Unit of Evolution

• One misconception is that organisms evolve during their lifetimes

• Natural selection acts on individuals, but only populations evolve

• Consider, for example, a population of medium ground finches on Daphne Major Island

– During a drought, large-beaked birds were more likely to crack large seeds and survive

– The finch population evolved by natural selection

Figure 23.1

Figure 23.2

1976(similar to theprior 3 years)

1978(after

drought)

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

• Three mechanisms cause allele frequency change:– Natural selection– Genetic drift– Gene flow

• Only natural selection causes adaptive evolution

• Variation in heritable traits is a prerequisite for evolution

• Mendel’s work on pea plants provided evidence of discrete heritable units (genes)

Concept 23.1: Genetic variation makes evolution possible

Genetic Variation

• Genetic variation among individuals is caused by differences in genes or other DNA segments

• Phenotype is the product of inherited genotype and environmental influences

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

Humans appeared late in Earth’s history.

Humans share a common ancestor with other primates.

• Primates are mammals with flexible hands and feet, forward-looking eyes and enlarged brains.

• Primates evolved into prosimians and anthropoids.

– Prosimians are the oldest living primates.– They are mostly small and nocturnal.

– They are subdivided into the New World monkeys, Old World monkeys, and hominoids.

– Anthropoids are humanlike primates.

– Homonoids are divided into hominids, great apes, and lesser apes.

– Hominids include living and extinct humans.

• Bipedal means walking on two legs.

– foraging– carrying infants and food– using tools

• Walking upright hasimportant adaptiveadvantages.

There are many fossils of extinct hominids.

• Most hominids are either the genus Australopithecus or Homo.

• Australopithecines were a successful genus.• The Homo genus first evolved 2.4 million years ago.

Modern humans arose about 200,000 years ago.

• Homo sapiens fossils date to 200,000 years ago.• Human evolution is influenced by a tool-based culture.• There is a trend toward increased brain size in hominids.

Australopithecusafarensis

Homo habilis Homo neanderthalensis

Homo sapiens

Lucy – 4 mybp 1.5 mybp 196,000 thousand ybp

• Many species evolve from one species during adaptive radiation.

– ancestral species diversifies into many descendent species

– descendent speciesusually adapted towide range ofenvironments

Evolutionary biology today

Evolution unites all fields of biology

"Nothing in Biology Makes Sense Except in the Light of Evolution" -Theodosius Dobzhansky 1973

What is the study of Paleontology?

• Heritability is the ability of a trait to be passed down.

• There is a struggle for survival due to overpopulation and limited resources. Populations would grow geometrically if resources were unlimited.

• Darwin proposed that adaptations arose over many generations.

• Natural selection is a mechanism by which individuals that have inherited beneficial adaptations produce more offspring on average than do other individuals.

• There are four main principles to the theory of natural selection.– Variation: the heritable differences in each population

are the basis of natural selection.

Natural selection explains how evolution can occur.

– Overproduction: having many offspring increases the

chances of survival but creates competition

– Adaptation: a certain variation that allows an organism to

survive better than organisms it competes against.

– Descent with modification: heritability of adaptations.

• Fitness is the measure of survival ability and ability to produce more offspring.

A population shares a common gene pool.

Variation Between Populations

• Most species exhibit geographic variation, differences between gene pools of separate populations

• For example, Madeira is home to several isolated populations of mice

– Chromosomal variation among populations is due to drift, not natural selection

Genetic variation comes from several sources.• Mutation is a random change in the DNA of a gene.

• Recombination forms new combinations of alleles.

– can form new allele– can be passed on to

offspring if in reproductive cells

– usually occurs during meiosis – parents’ alleles

arranged in new ways in gametes

Figure 23.5

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Sources of Genetic Variation

• New genes and alleles can arise by mutation or gene duplication

Formation of New Alleles

• A mutation is a change in nucleotide sequence of DNA

• Only mutations in cells that produce gametes can be passed to offspring

• 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 genes can be neutral because of

redundancy in the genetic code

Rapid Reproduction

• Mutation rates are low in animals and plants• The average is about one mutation in every 100,000

genes per generation• Mutations rates are often lower in prokaryotes and

higher in viruses

Natural selection acts on distributions of traits.

• A normal distribution graphs as a bell-shaped curve.

• Traits not undergoing natural selection have a normal distribution.

– highest frequency near mean value

– frequencies decrease toward each extreme value

Natural selection can change the distribution of a trait in one of three ways.

• Microevolution is evolution within a population.– observable change in the allele frequencies

– can result from natural selection

Hardy-Weinberg

• Required conditions are rarely (if ever) met

– Changes in gene pool frequencies are likely

– When gene pool frequencies change, microevolution has occurred

• Deviations from a Hardy-Weinberg equilibrium indicate that evolution has taken place

Genetic variation in a population increases the chance that some individuals will survive.

Where does that variation occur?

Hardy-Weinberg Equilibrium

eggs

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0.20 D 0.80 d

0.20D

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0.04 DD 0.16 Dd

0.16 Dd 0.64 dd

Genotypes:0.04

D = 0.20 d = 0.800.32 0.64Genotype frequencies:

p2 + 2pq + q2 = 1

p2 = frequency of DD genotype (dark-colored) = (0.20)2 =0.04

=0.32

=0.641.00

2pq = frequency of Dd genotype (dark-colored) = 2(0.20)(0.80)

q2 = frequency of dd genotype (light-colored) = (0.80)2

Allele and gamete frequencies:

F1 generation

F2 generation

Genotype frequencies: 0.04 DD + 0.32 Dd + 0.64 dd = 1

Dd dd

Industrial Melanism and Microevolution

Early observation Later observation

36% dark-colored phenotype 64% dark-colored phenotype

Peppered moth (Biston betularia)

• Genetic variation leads to phenotypic variation.• Phenotypic variation is necessary for natural selection.• Genetic variation is stored in a population’s gene pool.

– made up of all alleles in a population– allele combinations form when organisms have offspring

• Allele frequencies measure genetic variation.

– measures how common allele is in population– can be calculated for each allele in gene pool

Conditions for Hardy-Weinberg Equilibrium

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

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

• The five conditions for non-evolving populations are rarely met in nature:

1. No mutations

2. Random mating

3. No natural selection

4. Extremely large population size

5. No gene flow

Genetic variation comes from several sources.

• Hybridization is the crossing of two different species.– occurs when individuals can’t find mate of own species– topic of current scientific research

A zonkey

Populations, not individuals, evolve.

In which of the following scenarios will natural selection (N.S.) most likely occur?

a) Very little genetic variation is present within a speciesb) Harsh environmental conditions result in competition for survivalc) No reproductive isolation barriers exist within a species living in

an area.d) A geographical area has plenty of food to support individuals

within the species living in that area.

• 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

• For example, an allele that confers resistance to DDT increased in frequency after DDT was used widely in agriculture

Genetic Drift

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

• 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

Figure 23.9-1

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

CRCR CRCR

CRCW

CWCW CRCR

CRCW

CRCR CRCW

CRCR CRCW

Figure 23.9-2

5plantsleaveoff-

spring

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

CRCR CRCR

CRCW

CWCW CRCR

CRCW

CRCR CRCW

CRCR CRCW

CRCRCWCW

CRCW

CRCR CWCW

CRCW

CWCW CRCR

CRCW CRCW

Generation 2p = 0.5 q = 0.5

Figure 23.9-3

5plantsleaveoff-

spring

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

CRCR CRCR

CRCW

CWCW CRCR

CRCW

CRCR CRCW

CRCR CRCW

CRCRCWCW

CRCW

CRCR CWCW

CRCW

CWCW CRCR

CRCW CRCW

Generation 2p = 0.5 q = 0.5

2plantsleaveoff-

spring

CRCR

CRCR CRCR

CRCRCRCR

CRCR CRCR

CRCR

CRCR CRCR

Generation 3p = 1.0 q = 0.0

Effects of Genetic Drift: A Summary

1. Genetic drift is significant in small populations2. Genetic drift causes allele frequencies to change at

random3. Genetic drift can lead to a loss of genetic variation

within populations4. 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 variation among populations over time

• Gene flow can decrease the fitness of a population• Consider, for example, the great tit (Parus major) on

the Dutch island of Vlieland– Mating causes gene flow between the central and

eastern populations– Immigration from the mainland introduces alleles that

decrease fitness– Natural selection selects for alleles that increase

fitness– Birds in the central region with high immigration have

a lower fitness; birds in the east with low immigration have a higher fitness

Figure 23.12

Population in which the surviving females eventually bred

Central

Eastern

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Females born in central population

Females born in eastern population

Parus major

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Central population

NORTH SEA Eastern population

Vlieland, the Netherlands

2 km

Figure 23.12a

Parus major

• Gene flow can increase the fitness of a population• Consider, for example, the spread of alleles for

resistance to insecticides– 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

• Gene flow is an important agent of evolutionary change in human populations

• Evolution by natural selection involves both change and “sorting”

– New genetic variations arise by chance– Beneficial alleles are “sorted” and favored by natural

selection• Only natural selection consistently results in adaptive

evolution

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

Types of Selection• Most traits are polygenic - variations in the trait

result in a bell-shaped curve

• Three types of selection occur:

– (1) Directional Selection

• The curve shifts in one direction

– Bacteria become resistant to antibiotics

– Guppies become more colorful in the absence of predation

• 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

Three Type of Natural SelectionCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Peak narrows.

disruptive selection

Two peaks result.

directional selection

Peak shifts.

Phenotype Range Phenotype Range Phenotype Range

• Natural selection can take one of three paths.

– Directional selection favors phenotypes at one extreme.

– Stabilizing selection favors the intermediate phenotype.

• Natural selection can take one of three paths.

Directional Selection

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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are draband small

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above waterfall

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© Helen Rodd

Types of Selection

• Three types of selection occur (cont):– (2) Stabilizing Selection

• The peak of the curve increases and tails decrease• Ex - when human babies with low or high birth weight

are less likely to survive

– (3) Disruptive• The curve has two peaks• Ex – When Cepaea snails vary because a wide

geographic range causes selection to vary

Stabilizing Selection

Due to stabilizing selection, the average

human birth weight stays steady. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Disruptive SelectionCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

© Bob Evans/Peter Arnold, Inc.

Forestedareas

Low-lyingvegetatio

Natural selection is not the only mechanism through which populations evolve.

Gene flow is the movement of alleles between populations

• Gene flow: movement of alleles from one pop. to another

• Occurs when individuals join new populations and reproduce.

• Gene flow keeps neighboring populations similar.

• Low gene flow increases the chance that two populations will evolve into different species.

bald eagle migration

Genetic drift is a change in allele frequencies due to chance

• Genetic drift: change in allele frequency due to chance• Common in small populations, some alleles may increase

in frequency, while others decrease or disappear• A population bottleneck can lead to genetic drift.

– It occurs when an eventdrastically reducespopulation size.

– The bottleneck effect isgenetic drift that occursafter an event drastically reduces the population.

• The founding of a small population can lead to genetic drift.

– It occurs when a few individuals start a new population.– The founder effect is genetic drift that occurs after start

of new population.

The current population is thought to have descended from only seven females and eight males. One of the early colonists apparently carried a recessive allele for retinitis pigmentosa, a progressive form of blindness that afflicts homozygous individuals. The frequency of the allele that causes this disease is ten times higher on Tristan da Cunha than in the populations from which the founders came.

• Genetic drift has negative effects on a population.

– less likely to have some individuals that can adapt

– harmful alleles can become more common due to chance

(bottle neck and founder effect)

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

Figure 23.15

• 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 increase in frequency

© 2011 Pearson Education, Inc.

Sexual selection occurs when certain traits increase mating success.

• Sexual selection: processes in which certain traits increase mating success and therefore become more common in the population.– males produce many sperm

continuously– females are more limited in

potential offspring each cycle

• Females preferentially mate with males that display certain traits, so those traits get passed on to offspring and become more exaggerated each generation.

https://www.youtube.com/watch?v=W7QZnwKqopo

Bird of Paradise mating dance

• There are two types of sexual selection.

– intrasexual selection: competition among males– intersexual selection: males display certain traits to

females

Sexual Selection

• Female Choice– Choice of a mate is serious consideration

• Good genes hypothesis: Females choose mates on the basis of traits that improve the chance of survival.

• Runaway hypothesis: Females choose mates on the basis of traits that improve male appearance.

• Male Competition– Can father many offspring because they continuously

produce sperm in great quantity. – Compete to inseminate as many females as possible.

Sexual Selection

• Sexual selection adaptive changes in males and females to increase ability to secure a mate. – Males - ability to compete – Females choose to select a male with the best

fitness (ability to produce surviving offspring).

Sexual Selection

• The drab females tend to choose flamboyant males as mates.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Sexual Selection: CompetitionCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

a: © Y. Arthus-Bertrand/Peter Arnold, Inc.; b: © Neil McIntre/Getty Images

b.

a.

New species can arise when populations are isolated.

The isolation of populations can lead to speciation.

• Populations become isolated when there is no gene flow. – Isolated populations adapt to their own environments.– Genetic differences can add up over generations.

• Reproductive isolation can occur between isolated populations.

– members of different populations cannot mate successfully

– final step to becoming separate species

• Speciation is the rise of two or more species from one existing species.

Populations can become isolated in several ways.

• Behavioral barriers can cause isolation.– called behavioral isolation– includes differences in courtship or mating behaviors

• Geographic barriers can cause isolation.

– called geographic isolation– physical barriers divide population

• Temporal barriers can cause isolation.– called temporal isolation– timing of reproductive periods prevents mating

Evolution occurs in patterns.

Evolution through natural selection is not random.

• Natural selection can have direction.• The effects of natural selection add up over time.

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

Heterozygote Advantage

Figure 23.17

Distribution of malaria caused byPlasmodium falciparum (a parasitic unicellular eukaryote)

Key

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%

• 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

• For example, frequency-dependent selection selects for approximately equal numbers of “right-mouthed” and “left-mouthed” scale-eating fish

Frequency-Dependent Selection

Why Natural Selection Cannot Fashion Perfect Organisms

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

interact

• Convergent evolution describes evolution toward similar traits in unrelated species.

• Convergent evolution describes the evolution of traits toward similar features.

• Divergent evolution describes evolution toward different traits in closely related species.

How do convergent and divergent evolution illustrate the directional nature of natural selection?

ancestor

red fox kit fox

Species can shape each other over time.

• Two or more species can evolve together through coevolution.– evolutionary paths become connected– species evolve in response to changes in each other

• Coevolution can occur in beneficial relationships.

• Coevolution can occur in competitive relationships, sometimes called evolutionary.

Species can become extinct.

• Extinction is the elimination of a species from Earth.• Background extinctions occur continuously at a very low rate.

– occur at roughly the same rate as speciation

– usually affects a few species in a small area

– caused by local changes in environment

• Background extinctions occur continuously at a very low rate.

– occur at roughly the same rate as speciation– usually affects a few species in a small area– caused by local changes in environment

• Mass extinctions are rare but much more intense.

– destroy many species at global level– thought to be caused by catastrophic events– at least five mass extinctions in last 600 million years

A small portion of a population becomes geographically isolated from the rest of the population. As such, it

runs the risk of decreased…

a) Genetic driftb) Mutation ratec) Natural selectiond) Genetic variation

Can you define these words:Endosymbiosis – Primate – Prosimian – Anthropoid – Hominid – Bipedal –

Evolution through natural selection is not random.

Explain and justify this statement.

Speciation often occurs in patterns.

• A pattern of punctuated equilibrium exists in the fossil record.– theory proposed by Eldredge and Gould in 1972.– episodes of speciation occur suddenly in geologic time.– followed by long periods of little evolutionary change.– revised Darwin’s idea that species arose through

gradual transformations.

REMEMBER!