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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