Lecture 9: Natural Selection on the Phenotype


  1. What is Natural Selection?
    1. Natural selection is differential survival or reproduction of individuals with different phenotypes
      1. High mortality or low reproduction within a population is not natural selection unless it is differential across individuals with different phenotypes [when food is in low supply everyone will suffer; there may not be selection]
      2. Evolution occurs when natural selection causes changes in relative frequencies of alleles in the gene pool


  2. Natural selection acts directly on the phenotype, not the genotype
    1. If a single locus has a clear influence on the phenotype, then we can track the effects of differential survival or reproduction on gene frequencies
      1. Industrial melanism — a case of rapid evolution (dramatic change in allele frequencies in 50 years); example of selection for or against a particular allele
    2. Sometimes natural selection acts to maintain genetic variation
      1. Sickle cell anemia: heterozygote advantage; balanced polymorphism


3. Modes of selection: Natural selection acts in 3 ways on the distribution of phenotypes (Fig. 23.11)

  1. Stabilizing selection: an intermediate phenotype is favored → reduced phenotypic variation
  2. Directional selection: one end of a phenotypic distribution is favored → shifts the mean phenotype
  3. Diversifying selection: favors phenotypes of opposite extremes; disfavors intermediate phenotypes → expands the phenotypic distribution and can lead to 2 peaks in the frequency distribution


  1. No trait is adaptive in all environments — eg. brain size
    1. The way that natural selection acts on any trait depends on the environment and the particular organism
    2. Differential environmental situations favor different phenotypes


5. Kin selection: selection for traits which increase the survival of close relatives (survival and reproduction of close relatives increases one’s inclusive fitness)