lesson 5

• Evolution


  Descent with modification - changes in heritable traits from generation to generation
• Evolution


  • Acts on populations, not individuals
  • Occurs when allele frequencies change from one generation to the next
• Allele frequency


  Calculated as: # of copies of an allele / Total # of alleles for the same gene in the population
• Gene pool is the entire collection of genes and alleles in a population
• Gene pools differ from population to population, even for the same species
• If Swedes migrate to Asia and interbreed with locals

  Allele frequencies in the gene pool will change, and evolution has occurred
• Evolutionary thinkers


  • Aristotle
  • Buffon
  • Lamarck
  • Lyell
  • Darwin
  • Wallace
• Fossils of extinct species suggest that living organisms are descended from common ancestors
• Charles Darwin's voyage on the HMS Beagle


  • Provided a wealth of evidence for evolution
  • Observations on the Galapagos Islands were especially influential
• Darwin described 14 distinct types of finch on the Galapagos Islands, each different from the birds on the mainland yet sharing some features
• The beak shape of the finches varied depending on the food supply on each island
• Darwin thought the 14 finch species had probably descended from a single ancestral type of finch
• Pondering the variety of organisms in South America and their relationships to fossils and geology led Darwin to think these were clues to how new species originate
• Artificial selection


  Selective breeding where humans choose desired features and allow only individuals that best express those qualities to reproduce
• Natural selection


  Environmental factors cause the differential reproductive success of individuals with particular genotypes
• Principles of natural selection

  • Individuals with the traits best suited to the prevailing conditions tend to leave more surviving, fertile offspring
  • Traits that increase survival and reproduction in the current generation will be more common in the next generation
• Natural selection operates on the variation present in a population, and more individuals are born than resources can support, so the struggle to survive is inevitable
• Some individuals in a population are better than others at surviving and reproducing
• Natural selection does not create camouflage alleles. Instead, it strongly selects for camouflage alleles that arise by chance.
• Natural selection


  • Operates on the variation present in a population
  • Since more individuals are born than resources can support, the struggle to survive is inevitable
• Some individuals in a population are better than others at surviving and reproducing.
• Adaptations


  Features that provide a selective advantage because they improve an organism's ability to survive and reproduce
• Bacteria that are resistant to antibiotics have an adaptive trait that non-resistant bacteria lack. When antibiotics are administered, resistant bacteria are strongly selected for.
• Antibiotics can not create a resistance allele. The variation in resistance was already present in the population; the presence of antibiotics caused the resistance allele frequency to shift.
• As environmental conditions change, the phenotypes that natural selection favors will also change. Adaptations that seem "perfect" in one environment would be completely wrong in another.
• The orchid and its wasp pollinator have evolved alongside one another for long enough that no other animal can pollinate the flower.
• The orchid does not evolve in order to be better-pollinated by the wasp. Neither the orchid nor natural selection has foresight.
• Instead, the orchids best-suited to wasp pollination are the most likely to reproduce, so their alleles get passed to the next generation most often.
• If none of the ferns already have the ability to reproduce without water, the ferns might go extinct during a prolonged drought.
• Scientists test evolution against a null hypothesis, which states that allele frequencies do not change from one generation to the next.
• Hardy-Weinberg equilibrium


  The unlikely situation in which allele frequencies do not change between generations
• Assumptions for Hardy-Weinberg equilibrium

  • Natural selection does not occur
  • No mutations
  • The population is large enough to eliminate random changes in allele frequencies
  • Individuals mate at random
  • No migration
• p


  Frequency of the dominant allele
• q


  Frequency of the recessive allele
• Hardy-Weinberg equilibrium is a useful model for converting known allele frequencies to genotype frequencies (or vice versa), but in real populations, the assumptions of Hardy-Weinberg are always violated.
• Three modes of natural selection—directional, disruptive, and stabilizing—are distinguished by their effects on the phenotypes in a population.
• Directional selection


  One phenotype is favored over another
• Disruptive selection


  Extreme phenotypes are favored over an intermediate phenotype
• Stabilizing selection


  An intermediate phenotype is favored over the extreme phenotypes
• However, these three models do not explain why natural selection maintains some harmful phenotypes in a population.