Chapter 6: The Evolution of Populations

• Organisms

  • They do not evolve during their lifetimes
  • Natural selection acts on individuals, but only populations evolve
• Population of medium ground finches on Daphne Major Island


  • During a drought, large-beaked birds were more likely to crack large seeds and survive
  • The finch population evolved by natural selection
• Microevolution

  A change in allele frequencies in a population over generations
• Mechanisms that cause allele frequency change

  • Natural selection
  • Genetic drift
  • Gene flow
• Only natural selection causes adaptive evolution
• Variation in heritable traits is a prerequisite for evolution
• Mendel's work on pea plants provided evidence of discrete heritable units (genes)
• Genetic variation


  • It is caused by differences in genes or other DNA segments
  • Phenotype is the product of inherited genotype and environmental influences
  • Natural selection can only act on variation with a genetic component
• Mutation

  A change in nucleotide sequence of DNA
• Only mutations in cells that produce gametes can be passed to offspring
• Point mutation

  A change in one base in a gene
• Effects of point mutations

  • Mutations in noncoding regions of DNA are often harmless
  • Mutations to genes can be neutral because of redundancy in the genetic code
  • Mutations that result in a change in protein production are often harmful
  • Mutations that result in a change in protein production can sometimes be beneficial
• Chromosomal mutations

  • They delete, disrupt, or rearrange many loci and are typically harmful
  • Duplication of small pieces of DNA increases genome size and is usually less harmful
  • Duplicated genes can take on new functions by further mutation
• An ancestral odor-detecting gene has been duplicated many times: humans have 1,000 copies of the gene, mice have 1,300
• Mutation rates

  • They are low in animals and plants, about one mutation in every 100,000 genes per generation
  • They are often lower in prokaryotes and higher in viruses
• Sexual reproduction

  • It can shuffle existing alleles into new combinations
  • In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible
• Factors that alter allele frequencies and bring about most evolutionary change

  • Natural selection
  • Genetic drift
  • Gene flow
• Natural selection

  Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions
• Genetic drift

  • It describes how allele frequencies fluctuate unpredictably from one generation to the next
  • It tends to reduce genetic variation through losses of alleles
• Genetic drift

  1. The smaller a sample, the greater the chance of deviation from a predicted result

  2. Genetic drift can lead to a loss of genetic variation within populations

  3. Genetic drift can cause harmful alleles to become fixed
• Founder effect

  • It occurs when a few individuals become isolated from a larger population
  • Allele frequencies in the small founder population can be different from those in the larger parent population
• Bottleneck effect

  • It is a sudden reduction in population size due to a change in the environment
  • The resulting gene pool may no longer be reflective of the original population's gene pool
  • If the population remains small, it may be further affected by genetic drift
• Understanding the bottleneck effect can increase understanding of how human activity affects other species
• Gene flow

  • It consists of the movement of alleles among populations
  • Alleles can be transferred through the movement of fertile individuals or gametes
  • Gene flow tends to reduce variation among populations over time
• Gene flow can decrease the fitness of a population

  • Immigration from the mainland introduces alleles that decrease fitness
  • Natural selection selects for alleles that increase fitness
  • Birds in the central region with high immigration have a lower fitness; birds in the east with low immigration have a higher fitness
• Gene flow tends to reduce variation among populations over time
• Gene flow can decrease the fitness of a population

  Immigration from the mainland introduces alleles that decrease fitness
• Natural selection selects for alleles that increase fitness

  Birds in the central region with high immigration have a lower fitness; birds in the east with low immigration have a higher fitness
• Gene flow can increase the fitness of a population
• Insecticides have been used to target mosquitoes that carry West Nile virus and malaria

  Alleles have evolved in some populations that confer insecticide resistance to these mosquitoes
• The flow of insecticide resistance alleles into a population

  Can cause an increase in fitness
• Gene flow is an important agent of evolutionary change in human populations
• Evolution by natural selection

  Involves both chance (new genetic variations arise by chance) and "sorting" (beneficial alleles are "sorted" and favored by natural selection)
• Only natural selection consistently results in adaptive evolution
• Natural selection

  • Brings about adaptive evolution by acting on an organism's phenotype
• The phrases "struggle for existence" and "survival of the fittest" are misleading as they imply direct competition among individuals</b>
• Reproductive success is generally more subtle and depends on many factors
• Striking adaptations that have arisen by natural selection

  • Cuttlefish can change color rapidly for camouflage
  • The jaws of snakes allow them to swallow prey larger than their heads
• Natural selection increases the frequencies of alleles that enhance survival and reproduction
• Adaptive evolution occurs as the match between an organism and its environment increases