Chapter 23The Evolution of Populations

Overview: The Smallest Overview: The Smallest Unit of EvolutionUnit 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

: Population genetics: : Population genetics: foundation for studying foundation for studying

evolutionevolution Microevolution is change in the genetic makeup of a population

from generation to generation

Population genetics is the study of how populations change genetically over time

Population genetics integrates Mendelian genetics with the Darwinian theory of evolution by natural selection

This modern synthesis focuses on populations as units of evolution

Gene Pools and Gene Pools and Allele FrequenciesAllele Frequencies

• A population is a localized group of individuals capable of interbreeding and producing fertile offspring

The gene pool is the total aggregate of genes in a population at any one time

The gene pool consists of all gene loci in all individuals of the population

The Hardy-Weinberg The Hardy-Weinberg TheoremTheorem

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

It states that frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work

Mendelian inheritance preserves genetic variation in a populationHardy-Weinberg equilibrium describes a population in which random mating occurs

It describes a population where allele frequencies do not change

Preservation of Allele Preservation of Allele FrequenciesFrequencies

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

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 And p2 and q2 represent the frequencies of the homozygous

genotypes and 2pq represents the frequency of the heterozygous genotype

LE 23-5Gametes for each generation are

drawn at random from the gene poolof the previous generation:

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

SpermCR

(80%)CW

(20%)

pqp2

16%CRCW

64%CRCR

Eg

gs

CW

(20%

)C

R

(80%

)

16%CRCW

qp4%

CWCW

q2

Conditions for Hardy-Conditions for Hardy-Weinberg EquilibriumWeinberg Equilibrium

The Hardy-Weinberg theorem describes a hypothetical population

In real populations, allele and genotype frequencies do change over timeThe 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

Population Genetics and Population Genetics and Human HealthHuman Health

We can use the Hardy-Weinberg equation to estimate the percentage of the human population carrying the allele for an inherited disease

MutationMutation

Mutation and sexual recombination produce the variation that makes evolution possible

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

Mutations are changes in the nucleotide sequence of DNA

Mutations cause new genes and alleles to arise

Animation: Genetic Variation from Sexual Recombination

MutationsMutations

A point mutation is a change in one base in a gene It is usually harmless but may have significant impact on

phenotype

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

Gene duplication is nearly always harmful

Mutation rates are low in animals and plants

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

Mutations are more rapid in microorganisms

Sexual Recombination Sexual Recombination

Sexual recombination is far more important than mutation in producing the genetic differences that make adaptation possible

Natural selection, genetic drift, and gene flow Natural selection, genetic drift, and gene flow

can alter a population’s genetic compositioncan alter a population’s genetic composition

Three major factors alter allele frequencies and bring about most evolutionary change: Natural selection Genetic drift Gene flow Differential success in reproduction results in certain alleles being

passed to the next generation in greater proportions

Genetic DriftGenetic 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

Animation: Causes of Evolutionary Change

LE 23-7

CRCR CRCR CWCW CRCR

CRCW

CRCR

CRCW

CWCW

CWCW

CRCW CRCW

CRCRCRCW

CRCWCRCR

CRCR

CRCW

CWCW

CRCW

CRCR

Only 5 of10 plantsleaveoffspring

Only 2 of10 plantsleaveoffspring

CRCR

CRCR

CRCR CRCR

CRCR

CRCR CRCR

CRCR

CRCR

CRCR

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 EffectThe Bottleneck Effect

The bottleneck effect is a sudden change in the environment that may drastically reduce the size of a population

The resulting gene pool may no longer be reflective of the original population’s gene pool

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

LE 23-8

Originalpopulation

Bottleneckingevent

Survivingpopulation

The Founder Effect/Gene The Founder Effect/Gene FlowFlow

The founder effect occurs when a few individuals become isolated from a larger population

It can affect allele frequencies in a population

Gene flow consists of genetic additions or subtractions from a population, resulting from movement of fertile individuals or gametes

Gene flow causes a population to gain or lose alleles

It tends to reduce differences between populations over time

Natural selection is the primary Natural selection is the primary

mechanism of adaptive evolutionmechanism of adaptive evolution

Natural selection accumulates and maintains favorable genotypes in a population

Genetic Variation Genetic Variation Genetic variation occurs in individuals in populations of all

species

It is not always heritable

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

LE 23-9

Map butterflies thatemerge in spring:orange and brown

Map butterflies thatemerge in late summer:black and white

PolymorphismPolymorphism

Phenotypic polymorphism describes a population in which two or more distinct morphs for a character are 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

Measuring Genetic Measuring Genetic VariationVariation

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.

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

LE 23-10

1 2.4 3.14 5.18 6 7.15

8.11 9.12 10.16 13.17 19 XX

1

9.10 11.12 13.17 15.18 XX

2.19 3.8 4.16 5.14 6.7

• Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis

Heights of yarrow plants grown in common garden

Seed collection sites

A Closer Look at Natural A Closer Look at Natural SelectionSelection

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

Evolutionary FitnessEvolutionary Fitness The phrases “struggle for existence” and “survival of the

fittest” are commonly used to describe natural selection but 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

Relative fitness is the contribution of a genotype to the next generation, compared with contributions of alternative genotypes for the same locus

Directional, Disruptive, Directional, Disruptive, and Stabilizing Selectionand Stabilizing Selection

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

Three modes of selection: Directional=Directional selection favors individuals at one end of

the phenotypic range Disruptive-Disruptive selection favors individuals at both extremes

of the phenotypic range Stabilizing-Stabilizing selection favors intermediate variants and

acts against extreme phenotypes

LE 23-12

Originalpopulation

Evolvedpopulation

Phenotypes (fur color)

Original population

Directional selection Disruptive selection Stabilizing selection

Fre

qu

ency

of

ind

ivid

ual

s

The Preservation of Genetic The Preservation of Genetic

VariationVariation

Various mechanisms help to preserve genetic variation in a population 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

Balancing selection leads to a state called balanced polymorphism

Some individuals who are heterozygous at a particular locus have greater fitness than homozygotes=HETEROZYGUS ADVANTAGE-SICKLE CELL AMEMIA

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

LE 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)

SelectionSelection

In frequency-dependent selection, the fitness of any morph declines if it becomes too common in the population

Neutral variation is genetic variation that appears to confer no selective advantage

Sexual selection is natural selection for mating success It can result in sexual dimorphism, marked differences between the

sexes in secondary sexual characteristics

LE 23-14

Parental population sample

Experimental group sample

On pecking a mothimage the blue jayreceives a food reward.If the bird does notdetect a moth oneither screen, it pecksthe green circle tocontinue a new setof images (a newfeeding opportunity).

Plain background Patterned background

0.6

0.5

0.4

0.3

0.2

0 20 40 60 80 100

Generation number

Frequency-independent control

Ph

eno

typ

icva

riat

ion

Intrasexual selection is 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

Selection may depend on the showiness of the male’s appearance

The Evolutionary Enigma of The Evolutionary Enigma of Sexual ReproductionSexual Reproduction

Sexual reproduction produces fewer reproductive offspring than asexual reproduction, a so-called “reproductive handicap”

Sexual reproduction produces genetic variation that may aid in disease resistance

LE 23-16

Asexual reproduction

Female Generation 1

Generation 2

Generation 3

Generation 4

Sexual reproduction

Female

Male

Why Natural Selection Why Natural Selection

Cannot Fashion Perfect Cannot Fashion Perfect

OrganismsOrganisms

Evolution is limited by historical constraints

Adaptations are often compromises

Chance and natural selection interact

Selection can only edit existing variations