The Evolution of Populations

Chapter 23
The Evolution of Populations
Overview: The Smallest Unit of Evolution
One common misconception about evolution is that individual organisms evolve, in the Darwinian sense, during their lifetimes
Natural selection acts on individuals, but populations evolve
Genetic variations in populations
Contribute to evolution
Concept 23.1: Population genetics provides a foundation for studying evolution
Microevolution
Is change in the genetic makeup of a population from generation to generation
The Modern Synthesis
Population genetics
Is the study of how populations change genetically over time
Reconciled Darwin’s and Mendel’s ideas
The modern synthesis
Integrates Mendelian genetics with the Darwinian theory of evolution by natural selection
Focuses on populations as units of evolution
Gene Pools and Allele Frequencies
A population
Is a localized group of individuals that are capable of interbreeding and producing fertile offspring
The gene pool
Is the total aggregate of genes in a population at any one time
Consists of all gene loci in all individuals of the population
The Hardy-Weinberg Theorem
The Hardy-Weinberg theorem
Describes a population that is not evolving
States that the 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 population
Preservation of Allele Frequencies
In a given population where gametes contribute to the next generation randomly, allele frequencies will not change
Hardy-Weinberg Equilibrium
Hardy-Weinberg equilibrium
Describes a population in which random mating occurs
Describes a population where allele frequencies do not change
A population in Hardy-Weinberg equilibrium
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
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 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 Human Health
We can use the Hardy-Weinberg equation
To estimate the percentage of the human population carrying the allele for an inherited disease
Concept 23.2: 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
Mutation
Mutations
Are changes in the nucleotide sequence of DNA
Cause new genes and alleles to arise
Point Mutations
A point mutation
Is a change in one base in a gene
Can have a significant impact on phenotype
Is usually harmless, but may have an adaptive impact
Mutations That Alter Gene Number or Sequence
Chromosomal mutations that affect many loci
Are almost certain to be harmful
May be neutral and even beneficial
Gene duplication
Duplicates chromosome segments
Mutation Rates
Mutation rates
Tend to be low in animals and plants
Average about one mutation in every 100,000 genes per generation
Are more rapid in microorganisms
Sexual Recombination
In sexually reproducing populations, sexual recombination
Is far more important than mutation in producing the genetic differences that make adaptation possible
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
Natural Selection
Differential success in reproduction
Results in certain alleles being passed to the next generation in greater proportions
Genetic Drift
Statistically, the smaller a sample
The greater the chance of deviation from a predicted result
Genetic drift
Describes how allele frequencies can fluctuate unpredictably from one generation to the next
Tends to reduce genetic variation
The Bottleneck Effect
In 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
Original
population
Bottlenecking
event
Surviving
population
Understanding the bottleneck effect
Can increase understanding of how human activity affects other species
The Founder Effect
The founder effect
Occurs when a few individuals become isolated from a larger population
Can affect allele frequencies in a population
Gene Flow
Gene flow
Causes a population to gain or lose alleles
Results from the movement of fertile individuals or gametes
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
Genetic variation
Occurs in individuals in populations of all species
Is not always heritable
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
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
Measuring Genetic Variation
Population geneticists
Measure the number of polymorphisms in a population by determining the amount of heterozygosity at the gene level and the molecular level
Average heterozygosity
Measures the average percent of loci that are heterozygous in a population
Variation Between Populations
Most species exhibit geographic variation
Differences between gene pools of separate populations or population subgroups
Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis
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
Relative fitness
Is 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
The Preservation of Genetic Variation
Various mechanisms help to preserve genetic variation in a population
Diploidy
Diploidy
Maintains genetic variation in the form of hidden recessive alleles
Balancing Selection
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
Frequency-Dependent Selection
In frequency-dependent selection
The fitness of any morph declines if it becomes too common in the population
An example of frequency-dependent selection
Phenotypic diversity
Neutral Variation
Neutral variation
Is genetic variation that appears to confer no selective advantage
Sexual Selection
Sexual selection
Is natural selection for mating success
Can result in 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
The Evolutionary Enigma of Sexual Reproduction
Sexual reproduction
Produces fewer reproductive offspring than asexual reproduction, a so-called reproductive handicap
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
Evolution is limited by historical constraints
Adaptations are often compromises
Chance and natural selection interact
Selection can only edit existing variations