Anthropology 9/22

Types of Selection (Review)

• Genetic Drift: A mechanism of evolution in which allele frequencies of a population change over generations due to chance.

• Diversifying or Disruptive Selection: Favors individuals at both extremes of the phenotypic range over intermediate phenotypes.

  • Mechanism: Instead of selecting against extremes, it selects for the extremes and against the average examples.

  • Example: Darwin's Finches: The original finch population had average-sized beaks. Selective pressure created environmental niches where only very large, thick beaks or longer, thinner, more fragile beaks were beneficial.

    • An average-shaped beak would not be effective for cracking specific large seeds (requiring large beaks) nor for reaching insects in small holes (requiring thin beaks).

    • This pressure led to the selection of individuals with slightly smaller or slightly larger beaks, eventually driving the population towards two extremes and potentially creating new species.

Microevolution in Action: The Peppered Moths (Classic Example)

• Location and Time: England, 19th century, during the Industrial Revolution.

• Phenotypes: Peppered moths typically had light coloration (speckled white with black, resembling pepper) which made them blend into lichen-covered trees.

  • Occasionally, darker variants existed, which were less common in clean environments.

• Industrial Revolution Impact: Factories produced vast amounts of soot and smog, coating trees with black soot.

• Environmental Shift and Selection:

  • In the clean environment, light-colored moths were camouflaged against lichen and performed well; dark moths stood out and were preyed upon.

  • After soot covered the trees, the selective pressure shifted dramatically:

    • Light-colored moths now