Chap 20

• Evolution is descent with modification. (Descent is inheritance of traits and modification is change in heritable traits)
• Modern definition of Evolution: Genetic change in population over multiple generations
• Two Aspects of Evolution

  • Pattern of Evolutionary
  • Change Observable data from the natural world.
  • Process of Evolution
  • Mechanisms that produce the observed patterns of change
• Genetic Variation and Evolution

  • Genetic variation
  • Differences in alleles of genes found within individuals in a population.
  • Natural populations contain much variation.
  • Evolution
  • How an entity changes through time.
  • Development of modern concept traced to Darwin.
  • “Descent with modification”.
• Pre-Darwin Thinking: In the beginning of the 19th century, it was generally believed that species had remained unchanged since their creation.
• Darwin was influenced by Lyell’s Principles of Geology and thought that the earth was more than 6000 years old
• Darwin never used the word evolution in the first edition of The Origin of Species ; The phrase descent with modification summarized Darwin’s perception of the unity of life
• The Origin of Species

  • Darwin developed three main ideas:
  • Descent with modification explains life’s unity
  • Descent with modification explains life’s diversity
  • How organisms are suited to life in their environments
• Darwin proposed natural selection as the mechanism of evolution

  • Natural selection can lead to change in allele frequencies – frequencies of alleles of a gene from generation to generation
• Genetic variation is required for evolution to occur
• Hardy–Weinberg principle

  Predicts genotype frequencies

  Proportions of genotypes do not change in a population as long as

  1. No mutation takes place

  2. No genes are transferred to or from other sources (no immigration or emigration)

  3. Random mating is occurring

  4. The population size is very large

  5. No selection occurs
• Hardy–Weinberg principle Principle can be written as an equation

  Used to calculate allele frequencies

  p2 + 2pq + q2 = 1
• If all 5 assumptions for Hardy-Weinberg equilibrium are true, allele and genotype frequencies do not change from one generation to the next In reality, most populations will not meet all 5 assumptions.
• What makes populations vary from HardyWeinberg equilibrium?

  • Natural selection might favor homozygotes over heterozygotes.
  • Individuals may choose to mate with genetically similar individuals. Influx of individuals from other populations.
  • Mutation occurring
• 5 agents of evolutionary change (2/5)

  • Mutation
  • Rates generally low ; Other evolutionary processes usually more important in changing allele frequency; Ultimate source of genetic variation.
  • Gene flow
  • Movement of alleles from one population to another; Animal physically moves into new population ; Drifting of gametes or immature stages of plants or animals into an area; Mating of individuals from adjacent populations.
• 5 agents of evolutionary change (4/5)

  • Nonrandom mating
  • Assortative mating - Phenotypically similar individuals mate. Increases proportion of homozygous individuals.
  • Disassortative mating. - Phenotypically different individuals mate; Produces excess of heterozygotes
  • Genetic drift -In small populations, allele frequency may change by chance alone; Population must be large .; Magnitude of genetic drift is inversely related to population size; Can lead to the loss of alleles in isolated populations and uncommon alleles are more vulnerable. Founder effect AND Bottleneck effect.
• 5 agents of evolutionary change (5/5)

  Selection

  • favors some genotypes over others
  • Some individuals leave behind more progeny than others, and the rate at which they do so is affected by phenotype and behavior
  • Artificial selection – breeder selects desired characteristics
  • Natural selection – environmental conditions determine which individuals produce the most offspring
• Founder effect - One or a few individuals disperse and become the founders of a new, isolated population Some alleles are lost, and others change in frequency; Organisms on islands, Self-pollinating plants and Amish populations

  Bottleneck Effect - The bottleneck effect can result from a drastic reduction in population size due to a sudden environmental change; If the population remains small, it may be further affected by genetic drift • Results in loss of genetic variability. (Only some allele are killed, so the ones that survive reproduce alot and create a bunch more) Northern Elephant Seal
• Evolution by natural selection These conditions must be met for evolution by natural selection to occur

  1. Variation must exist among individuals in a population

  2. Variation among individuals must result in differences in the number of offspring surviving in the next generation

  3. Variation must have a genetic basis
• Evolution by natural selection

  Natural selection and evolution are not the same

  • Natural selection is a process.
  • Only one of several processes that can result in evolution.
  • Evolution is the historical record, or outcome, of change through time.
  • Result of evolution driven by natural selection is that populations become better adapted to their environment
• Selection to match climatic conditions

  • Enzyme allele frequencies often vary with latitude
  • Allele frequencies of enzyme lactate dehydrogenase in fish vary geographically
  • Enzymes formed by these alleles function differently at different temperatures
• Fitness

  • Individuals with one phenotype leave more surviving offspring in the next generation than individuals with an alternative phenotype.
  • Relative concept; the most fit phenotype is simply the one that produces, on average, the greatest number of offspring.
• Fitness has many components

  • Survival.
  • Sexual selection – some individuals more successful at attracting mates.
  • Number of offspring per mating.
  • Traits favored for one component may be a disadvantage for others.
  • Selection favors phenotypes with the greatest fitness;Phenotype with greater fitness usually increases in frequency
• Parental Investment

  • Refers to the energy and time each sex invests in producing and rearing offspring
  • Females have a higher parental investment
  • Sexes face very different selective pressures
  • Males best increase their fitness by mating with as many females as possible
  • Females more limited by number of eggs that can be produced, so a female should be choosy in picking the male that can provide greatest benefit
• Sexual selection

  • Intrasexual selection – competitive interactions between members of one sex (Fighting)
  • Intersexual selection – mate choice (includes Sensory exploitation which means evolution in males of a signal that exploits preexisting biases)
  • Secondary sexual characteristics – antlers and horns used to combat other males; long tail feathers and bright plumage used to “persuade” members of opposite sex
• Sexual selection

  Sexual dimorphism – Differences between sexes (males larger than females)

  Sperm competition – selects for features that increase probability that a male’s sperm will fertilize the egg
• Maintenance of variation

  • Negative frequency-dependent selection. Rare phenotypes favored by selection. • Rare forms may not be in “search image”; preyed upon less frequently.
  • Positive frequency-dependent selection. • Favors common form. • Tends to eliminate variation. • “Oddballs” stand out.
• Oscillating selection

  • Selection favors one phenotype at one time and another phenotype at another time
  • Effect will be to maintain genetic variation in the population Medium ground finch of Galápagos Islands
  • Birds with big bills favored during drought. and Birds with smaller bills favored in wet conditions
• Heterozygote advantage

  • Heterozygotes are favored over homozygotes
  • Works to maintain both alleles in the population
  • Sickle cell anemia
  • Hereditary disease affecting hemoglobin.
  • Causes severe anemia.
  • Homozygotes for sickle cell allele usually die before reproducing (without medical treatment).
• Selection acting on traits affected by multiple genes

  • Many traits affected by more than one gene
  • Selection operates on all the genes for the trait
  • Changes the population depending on which genotypes are favored
  • Types of selection
  • Disruptive.
  • Directional.
  • Stabilizing.
• Disruptive selection

  • Acts to eliminate intermediate types
  • Different beak sizes of African black-bellied seedcracker finch
  • • Available seeds fall into 2 categories.
  • Favors bill sizes for one or the other.
  • Birds with intermediate-sized beaks are at a disadvantage with both seed types – they are unable to open large seeds and too clumsy to efficiently process small seeds
• Directional selection

  • Acts to eliminate one extreme
  • Often occurs in nature when the environment changes
  • In Drosophila, eliminated flies that moved toward the light
  • Now fewer in resulting population have that behavior
• Stabilizing selection

  • Acts to eliminate both extremes
  • Makes intermediate more common by eliminating extremes
  • In humans, infants with intermediate weight at birth have the highest survival rate
• Mutations and genetic drift may counter selection

  • In nature, mutation rates are rarely high enough to counter selection.
  • Selection is nonrandom but genetic drift is random
• Gene flow can be

  • Constructive. - Spread beneficial mutation to other populations.
  • Constraining. - Can impede adaptation by continual flow of inferior alleles from other populations.