Chapter 23: The Evolution of Populations

• Microevolution


  evolutionary change below the species level; change in allele frequencies in a population over generations
• 3 Mechanisms that change allele frequency

  1. Natural selection
  2. Genetic drift
  3. Gene flow
• 2 types of characters in a population

  1. Discrete Character
  2. Quantitative Character
• Discrete Character


  classified on an either-or basis (ex: purple or white flowers)
• Quantitative Character


  vary along a continuum w/i a population; influence to multiple genes (ex: height)
• Average Heterozygosity


  the percent, on average, of a population's loci that are heterozygous in members of the population
• Geographic variation


  differences between the gene pools of geographically separate populations or population subgroups
• Cline


  a graded change in a character along a geographic axis
• Sometimes geographic variation occurs as a cline
• The ultimate source of new alleles is mutation
• Mutation


  a change in the DNA sequence
• Duplications are an important source of variation
• 3 Mechanisms that contribute to allele shuffling

  1. Crossing over
  2. Independent assortment of chromosomes
  3. Fertilization
• Population


  a localized group of individuals of the same species that can interbreed, producing fertile offspring
• Gene pool


  the aggregate of all of the alleles for all of the loci in all individuals in a population
• We can use the Hardy-Weinberg equilibrium to assess whether evolution is happening at a particular locus
• Hardy-Weinberg Principle


  the principle that frequencies of alleles & genotypes in a population remain constant from generation to generation, provided that only Mendelian segregation & recombination are at work
• Hardy-Weinberg Equilibrium


  the condition describing a non evolving population
• If the population is in Hardy-Weinberg equilibrium & not evolving the allele & genotype frequencies will remain constant
• p is the dominant allele
• q is the recessive allele
• Conditions of Hardy-Weinberg Equilibrium:

  1. No mutations
  2. Random mating
  3. No natural selection
  4. Extremely large population size
  5. No gene flow
• Adaptive Evolution


  evolution that results in a better match between organisms & their environment
• Genetic drift


  a process in which chance events cause unpredictable fluctuations in allele frequencies from one generation to the next
• Effects of genetic drift are most pronounced in small populations
• Founder effect and bottleneck effect result in genetic drift having a significant impact on a population
• Founder effect


  genetic drift that occurs when a few individuals become isolated from a larger population & form a new population whose gene pool composition is not reflective of that original population
• Bottleneck effect


  genetic drift that occurs when the size of a population is reduced as by natural disaster or human actions
• Summary of Effects of Genetic Drift

  1. Significant in small populations
  2. Can cause allele frequencies to change at random
  3. Can lead to a loss of genetic variation w/i populations
  4. Can cause harmful alleles to become fixed
• Gene flow


  transfer of alleles from one population to another resulting from the movement of fertile individuals & their gametes
• Gene flow, like mutation, can introduce new alleles into a population
• Gene flow can occur at higher rates & is more likely to directly alter allele frequencies
• Relative fitness


  the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals in the population
• Natural selection works on the whole organism, not just its genotype
• Selection acts more directly on the phenotype than the genotype
• Directional selection


  natural selection in which one extreme phenotype on one side of the phenotypic range is favored over other phenotypes

  
• Natural selection can alter the distribution of heritable traits in 3 ways:

  1. Directional selection
  2. Disruptive selection
  3. Stabilizing selection

  
• Disruptive selection


  natural selection that actively selects against intermediate phenotypes and favors both extremes of the phenotypic range

  
• Stabilizing selection


  natural selection in which intermediate phenotypes are favored over extreme phenotypes; maintains the status quo for a character

  
• As the proportion of individuals that have favorable traits increases, the match between a species & its environment improves; adaptive evolution occurs