chapter 17 evolution of populations. 17.1 genes and variation genetics joins evolutionary theory...
TRANSCRIPT
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