The Modern Synthesis: A Historical Approach

This lecture outline does not address creationism directly, but deals with the complaint that evolution is "just a theory". What is a theory? Does disagreement within the scientific community weaken evolutionary theory?

Every theory in mainstream science has been rigorously tested (otherwise they aren't called theories) and almost none of them strongly resemble the hypotheses that gave birth to them.

• In particular, several theories now central to mainstream science were initially proposed without causal mechanisms! By the time the mechanisms were identified, many other hypotheses had been integrated into the theory.
• For example, Charles Darwin's hypothesis of modification of species by natural selection was integrated with Mendel's theory of genetic heredity early in the 20th century, decades after the deaths of both Darwin and Mendel.
  • One of the assumptions of the original Natural Selection hypothesis is that traits are somehow passed down from generation to generation without change. This is a good assumption, as it turns out, although we now know that traits can be hidden or muted for one or more generations even as they continue to be inherited.
  • Prior to the rediscovery of Mendel's work, the major theory (now discredited) for inheritance was that of blended inheritance, according to which not only were the children's traits were always intermediate between those of their parents, but the genetic material which they passed on the next generation was also mixed. So there was no way to pass on "fitness" if the "fit" and unfit types mated freely, the genetic material itself was changed by interbreeding.
  • Mendel's work, still held to be valid over a century later, refuted the idea of blended inheritance and actually showed that genetic material did not change from generation to generation, that genes were discrete, even if phenotypes (traits) were not. Mendel also showed that recessive traits could be hidden for several generations.
• The authors who united Mendel's and Darwin's work also added their own ideas.
  • The Modern Synthesis (also called neo-Darwinism) was the work of many people, combining experimental and theoretical approaches, including Fisher, Haldane, Wright, Dobzhansky, Mayr, Huxley, Simpson, and Williams.
  • According to the Modern Synthesis (more info) (the mainstream theory of biological evolution), change in gene pools (and therefore in populations of organisms) is caused not just by natural selection, but also by mutation, gene flow, and genetic drift.
  • Furthermore, Fisher argued that directional natural selection could not operate continuously, as it eliminates genetic variation for the trait being selected upon but must stop once the population is no longer variable.
• Important new discoveries have been made since the Modern Synthesis was composed, such as the discovery of the structure of DNA, the endosymbiotic origin of mitochondria and chloroplasts, and the role of genetics in developmental biology, but, these have simply been incorporated into the Synthesis.
• Recently Eldredge and Gould's Punctuated Equilibrium model created a dispute, not so much about the mechanisms of evolution, but about the pace. The controveries, however, have largely given way to a more careful examination of evidence, increasing our understanding of speciation.

Resources for Teaching about the Development of the Modern Synthesis