Lecture 10

• Artificial selection


  Genetically improving stocks by plant and animal breeders
• Artificial selection has been essential to civilization
• Evolution of domestic stocks from wild stocks

  • Evolution of domestic tomato from wild tomato
  • Evolution of cabbage, broccoli, Brussels sprouts, and kohlrabi from wild cabbage
• Unintended artificial selection


  Humans have changed selective pressures on some populations, unintentionally causing evolutionary changes in the wild
• Examples of rapid evolution caused by unintentional artificial selection

  • Increase in insecticide-resistant pests
  • Reduction in age at maturity and body size of Atlantic cod
  • Reduction of horn size in at least one population of bighorn sheep
• Natural selection


  A process like artificial selection that happens automatically in nature
• Conditions necessary and sufficient for a trait to evolve by natural selection

  • The trait varies among individuals
  • Fitness varies among individuals
  • Differences in the trait are correlated with differences in fitness
  • Variation in the trait is heritable
• Intraspecific variation


  • Variation is caused by genetic variation, environmental variation, and genotype-by-environment interactions
• Among medium ground finches, some have beaks that are only half as deep as those of others</b>

  
• If the environment changes, natural selection might favour individuals with shallower beaks or deeper beaks
• Fitness


  More individuals are produced than can survive, causing a "struggle for existence" (i.e., competition to survive)
• Variation in fitness is common in natural populations: some individuals die without leaving any offspring; others leave many more than average
• In the case of medium ground finches, a drought in 1977 caused a decline in food availability and most finches died

  
• Birds better able to survive the drought by cracking open large, hard seeds would have had a fitness advantage
• Variation between the trait and fitness may be correlated, depending on environment
• In the case of medium ground finches, individuals who survived the drought had larger, deeper beaks than the pre-drought average

  
• They had a fitness advantage because they were better at cracking open large, hard seeds
• Variation in many traits is at least partially heritable
• In the case of medium ground finches, beak depths of parents and offspring are highly correlated, suggesting that this variation is heritable

  
• Because variation in the trait is heritable, natural selection can cause evolution
• Natural selection


  Occurs within generations (i.e., individuals die or survive)
• Evolution


  Occurs between generations (i.e., "change in allele frequencies over generations")

  
• Beak depth evolved because of the drought: the average beak depth changed across generations

  
• Their deeper beak appears to be an adaptation for feeding on hard seeds: it seems to have evolved by natural selection for this function

  
• Selection for


  Causation of fitness
• Selection of


  Correlation with fitness
• Adaptive explanations for traits should often be accepted only tentatively
• Demonstrating "selection for" (i.e., causation of fitness) is much harder than demonstrating "selection of" (i.e., correlation with fitness)
• Traits may have various non-adaptive explanations

  • Physical/chemical necessity
  • Evolution by non-adaptive mechanisms (such as drift)
  • Genetic correlation with a trait undergoing selection (due to pleiotropy or genetic linkage)
  • The trait might have evolved for other reasons and just happens to be beneficial for some function today
• Ways to recognize adaptations

  • Adaptive complexity – only selection can explain complex features that seem designed for a particular function
  • Experiments – putative adaptations can be altered to determine how function and fitness are changed
  • Comparative method – comparing traits and environments across taxa
• Special statistical methods are often needed for the comparative method because species are not independent data points (due to shared ancestry)
• Example of using the comparative method

  • To test the hypothesis that large testes are an adaptation for sperm competition in primates, testes weights of monogamous and polygamous species were compared
• Natural selection acts on individual phenotypes within a generation, but its evolutionary consequences are allele frequency changes in a population over generations

  
• Natural selection acts on existing traits, but new traits can evolve
• A trait that originated with one fitness benefit (e.g., rudimentary feathers for insulation) may be "exapted" and evolve a new function (e.g., flight feathers)
• Natural selection does not lead to perfection
• Natural selection has no foresight, and it has no goals
• Adaptations have not evolved by natural selection "for the good of the species"
• Adaptations have evolved because they have had individual fitness benefits
• Summary of key points about natural selection

  • Natural selection is a correlation between differences in a trait and differences in fitness
  • Four conditions are necessary—and sufficient—for a trait to evolve (i.e., change over generations) by natural selection
  • Evolution by natural selection has been observed directly many times
  • An adaptation is a trait that has evolved by natural selection for a particular function