CHAPTER 17Evolution of Populations

17.1 Genes and VariationGenetics Joins Evolutionary Theory

• Heritable traits are controlled by genes• Variation is the raw material for natural

selection

Genotype and Phenotype in Evolution

• Alleles: specific forms of a gene• Genotype is the particular combination of

alleles• Natural selection acts directly on phenotype,

not genotype

Populations and Gene Pools

• Members of a population interbreed, creating gene pools

• Gene pool consists of all genes• Allele frequency: number of times an allele

occurs in a gene pool

Sources of Genetic Variation

• Genetics enable us to understand how heritable variation is published

• 3 sources of genetic variation: mutation, genetic recombination, lateral gene transfer

Mutations

• Mutation: change in genetic material of a cell• Some mutations may be lethal• Mutations move from generation to

generation

Genetic Recombination in Sexual Reproduction

• Crossing-over is another way genes are recombined

• Crossing-over increases the number of genotypes

• Mutations aren’t only source of heritable variation

Lateral Gene Transfer

• Lateral gene transfer: passing genes from organism to organism that isn’t offspring

• Can increase genetic variation• Important to evolution of antibiotic resistance

in bacteria

Single-Gene and Polygenic Traits

• Genes control phenotypes in different ways• Number of phenotypes on how many genes

control the trait

Single-Gene Traits

• Trait controlled by only one gene• May have just two or three distinct

phenotypes• Controlled by dominant and recessive alleles

Polygenic Traits

• Controlled by two or more genes• Often has two or more alleles• Creates a bell-shaped curve

17.2 Evolution as Genetic Change in populationsHow Natural Selection Works

• Passes copies of its genes to its offspring• Genetically controlled traits

Natural Selection on Single-Gene Traits

• Can lead to changes in allele frequencies• Lead to changes in phenotype frequencies• Mutation will help them survive and adapt

Natural Selection on Polygenic Traits

• Affect relative fitness of phenotypes• Produce three types of selection• Directional selection, stabilizing selection,

disruptive selection

Genetic Drift

• Random change in allele frequency• Natural selection isn’t only source of

evolutionary change

Genetic Bottlenecks

• Change in allele frequency following dramatic reduction in population size

• Sharply reduce population’s genetic diversity• Different alleles than original population

The Founder Effect

• Occur when few individuals colonize a new habitat

• New gene pool is different than the parent gene pool

• Change in allele frequency by migration of small subgroup

Evolution Versus Genetic Equilibrium

• Allele frequencies don’t change• Population is not evolving

Sexual Reproduction and Allele Frequency

• Gene shuffling during sexual reproduction produces gene combinations

• Meiosis and fertilization don’t change allele frequencies

• Populations would remain at genetic equilibrium

The Hardy-Weinberg Principle

• Allele frequencies should remain constant unless factors cause it to change

• Makes predictions like Punnet squares• Predict frequencies of genotypes

17.3 The process of speciationIsolating Mechanisms

• Speciation: formation of a new species• Gene pool can split

Behavioral Isolation

• Differences in courtship rituals• Other behavioral differences can occur• Use different song to attract mates (east

meadowlark v. west meadowlark)

Geographic Isolation

• Geographic barriers• Separate gene pools form• Barriers don’t always guarantee isolation

Temporal Isolation

• Reproduce at different times• Orchids in the same rain forest are examples• Can’t pollinate with each other

Speciation in Darwin’s Finches

• Occurred by founding of a new population • Galapagos Islands

Founders Arrive

• Caused by founder effect• Allele frequencies differed from parent allele

frequencies• New species formed

Geographic Isolation

• Combination of founder effect, geographic isolation, and natural selection

• Group of finches moved to another island• Another gene pool formed on that island

Changes in Gene Pools

• Adapted to its local environments• New phenotypes occur as well• Not only birds, but plants too

Behavioral Isolation

• Different evolution causes them not to be attracted to each other

• Could lead to reproductive isolation• Gene pools remain isolated

Competition and Continued Evolution

• Compete for food• More specialized birds have less competition• Evolution of species increases over time

17.4 Molecular EvolutionTiming Lineage Splits: Molecular Clocks

• Molecular clock uses mutation rates in DNA• Compare stretches of DNA to mark

evolutionary time

Neutral Mutations as “Ticks”

• Molecular clock relies on a repeating process• Causes slight changes in sequence of DNA• Under powerful pressure of natural selection

Calibrating the Clock

• Many different clocks• Different “ticks” at different rates• Some genes accumulate faster than others

Gene Duplication

• Genes evolve through duplication• Modern genes descended from smaller

number of genes

Copying Genes

• Carry several options of various genes• Can carry two copies of the same gene• An entire genome can be duplicated

Duplicate Genes Evolve

• Undergo mutations to change their function• Evolve without effecting original gene function• Undergo copies

Gene Families

• Produce hemoglobins• Focus on Hox genes• Group of related gene called gene family

Developmental Genes and Body Plans

• “evo-devo”• Produce evolutionary changes we see in the

fossil record

Hox Genes and Evolution

• Determine which parts of an embryo develop• Control size and shape• Can produce large changes in adults

Timing is Everything

• Starts to grow at certain time• Grows for a specific time• Stops growing at a specific time