BIOL1951 W9-13

• Four mechanisms/forces of evolution

  Gene flow, genetic drift, mutation, and selection (natural and sexual)

  
• Mutation


  Any change in sequence in the genome of an organism
• Evolutionary Consequences of Mutations

  • Neutral for an organism's survival
  • Hurt survival
• If mutation is the only force occurring in a population (i.e., no natural selection)

  The health of the population will decrease over time
• If both mutation and selection are occurring

  The most likely outcome is the population's health will be maintained
• Genetic Drift

  Changes in frequencies of alleles or genotypes due to random chance
• Characteristics of Genetic Drift

  • Drunkards walk
  • Smaller populations experience more drift
  • Allele frequencies can reach fixation or loss
  • Drift is more powerful than selection in small populations
  • Drift can cause loss of variation
• Genetic Drift vs. Natural Selection

  Genetic drift can cause allele frequencies to change even in the absence of selection

  
• Migration


  Movement of individuals & gametes
• Characteristics of Gene Flow

  • Prevents populations from diverging
  • Can counteract the effects of selection and drift
• Gene Flow vs. Natural Selection & Genetic Drift

  • Gene flow can prevent local adaptation
  • Migration can counteract the effects of selection and drift
• Directional Selection

  An extreme phenotype is favored over all other phenotypes
• Disruptive Selection

  Deviant trait values (traits far from the mean) have highest fitness and intermediate trait values (traits near the mean) does the worst
• Stabilizing Selection

  Selection favors the intermediate trait value over the extreme values
• Negative Frequency-Dependent Selection

  Fitness of a phenotype decreases as it becomes more common
• The experiment with Heliconius butterflies

  Suggests negative frequency-dependent selection is occurring
• Hardy-Weinberg Equilibrium


  A population exists in equilibrium when there is no evolution occurring
• Mutation

  • Ultimate source of genetic variation
  • Some mutations alter the phenotype, others do not
• Mutations can create

  1. New alleles by altering existing alleles

  2. New genes through duplication events

  3. Divergence of the new gene allows new functions to arise
• Evolutionary consequences of mutations

  • Only mutations that are heritable matter evolutionarily
  • Somatic mutations cannot be passed to the next generation, so has little evolutionary consequence
  • The higher the mutation rate the more genetic variation in a population, the faster evolution could proceed
• Each human zygote has on average ~36 new point mutations (one base pair change) in their 3.2 billion base pairs in their genome
• Vast majority of mutations are neutral and then deleterious mutations
• Most new beneficial mutations are lost through chance because initially only occurs in one individual
• Mutation

  1. Leads to evolution

  2. Results in change frequencies in an increase of alleles or genotype
• If mutation is better fit to the environment

  It leads to evolution if the mutation is heritable
• Mutation standing variation in one population

  • It will increase, introduces new variation. sometimes a mutation may have no affect or it will become widespread or get rid of because it is random
• Mutation similarity between two populations

  They will become less similar because it expands diversity and usually unique to one population
• Mutation expands diversity and is usually unique to one population
• Migration

  Leads to evolution through the change of allele frequencies which can have widespread genetic variation if better or worse suited for the environment
• Mutation

  Sometimes/it depends if better fit to the environment because sometimes a new genotype is better but sometimes it is not specific to the environment
• Standing variation in one population with mutation

  Increases/it depends because transferring to a new environment with new genotypes and new alleles may increase variation, unless the same alleles
• Similarity between two populations with mutation

  They will become more similar because the new population become more similar to original population
• Genetic Drift

  Leads to evolution through the increase expression in genotype and phenotype at pure chance and strong in small population
• Genetic drift is rarely better fit to the environment because it is pure chance
• Standing variation in one population with genetic drift

  Decreases because it will favor a genotype by chance
• Similarity between two populations with genetic drift

  They will become less similar because its random its not likely two populations would do the same thing
• Natural Selection

  Leads to evolution through the one genotype is more favored and has better reproductive success
• Natural selection is always a better fit to the environment because it has a better genotype that has a better fitness and more success killing of others without the genotype in constant environment
• Standing variation in one population with natural selection

  Decreases/it depends because the environment favors the genotype(directional)
• Similarity between two populations with natural selection

  It depends if they become more or less similar because it selects for one or multiple alleles in one environment with extremes