Lecture 4

• Darwinism


  • Evolution - organismal characteristics change over time
  • Common descent - species have diverged from common ancestors (and can be portrayed as a tree)
  • Gradualism - evolution happens by small steps, not by leaps ("saltations")
  • Populational change - evolution is a change in the proportions of different variants in a population
  • Natural selection - differential survival and reproduction of individuals caused by differences in phenotype; accounts for adaptations
• Common descent


  Species share a common ancestor from which they have descended, with modification. They can be portrayed as a tree.
• Natural selection


  Natural selection is a mechanism whereby differences in the phenotypes of individuals cause some of them to survive and reproduce more effectively than others. There is variation among individuals in a population, at least some of which is heritable. Some variants have greater fitness than others, resulting in differential reproductive success. Natural selection accounts for adaptations.
• Contemporary objections to Darwinism that will be addressed today are marked in red. Those that will be addressed in the future are marked in black.
• Lamarckism


  The process in which organisms had agency to influence evolution, as opposed to Darwinism's dependence on random variation and natural selection.

  
• Weismann's experiments determined there was a "barrier" that prevented inheritance of acquired characteristics, and that hereditary information in the germ plasm (genes) contributes to the development of the soma (body), but information never flows in reverse.
• Blending inheritance


  The offspring's phenotype is the average of the parental phenotypes, like mixing white paint and black paint to make grey paint. This is incompatible with natural selection.
• Particulate inheritance


  Traits are inherited as discrete units (genes), which do not blend but are passed down intact.
• Mutationism


  The view that large mutations were the primary drivers of evolutionary change, leading to sudden changes and the rise of new species ("hopeful monsters").
• Selectionism


  The view that emphasized the role of natural selection acting on small variations.
• Modern synthesis


  The reconciliation of Darwinism and Mendelism in the 1930s and 1940s, primarily through the development of population genetics and quantitative genetics, which showed how particulate inheritance can cause continuously varying traits subject to gradual natural selection.
• The modern evolutionary theory based on the modern synthesis is sometimes called "Neo-Darwinism".
• Common descent was widely accepted by scientists soon after On the Origin of Species was published, but natural selection was not widely accepted until the modern synthesis of the 1930s and 1940s.