The problem: Every few months, one hears something like this from a prominent politician, journalist, or cultural figure:

"Well, it is a theory. It is a scientific theory only, and it has in recent years been challenged in the world of science -- that is, not believed in the scientific community to be as infallible as it once was." - Ronald Reagan, 1980

Is it true, or have they naively misinterpreted the methods of science, recent advances, or some combination? Let's see.

Beyond Natural Selection
John Merck

Other mechanisms of evolution: Mendel's work on genetics was rediscovered by the scientific community in 1900. When this occurred, biologists began synthesizing the fields of genetics and evolution in a movement called the New Synthesis. In this synthesis, several other evolutionary processes besides natural selection were discovered.

  • Genetic drift. Evolution (descent with modification) can occur as a result of purely random events. Cf. that every individual has a unique combination of genes. When an individual is added to or removed from the gene pool, that gene pool is changed. In large populations, the effect is negligible, but in small populations, the addition or removal of an individual has noticeable effects. Consider the prevalence of brachydactyly among the Amish. This gene was brought to the community by a single member of the founding population but because the Amish population is small and members generally marry within their group, it has, by chance proliferated.

    Now, play the genetic drift game.

  • Genetic linkage: The evolutionary interactions of genes that are close together on a chromosome. Remember meiosis, the process by which gametes (reproductive cells) are created:

    Thus, genetic information gets shuffled during both meiosis and crossing over. Note, however that:

    Suppose that a particular gene is highly favored by natural selection. Genes with no particular selective advantage that are close to it on the same chromosome are likely to be favored simply because they are fellow-travelers of the selectively favored gene. This "genetic parasitism" is termed genetic linkage. An example in humans is the linkage of nail-patella syndrome (a condition causing abnormalities of the limbs and kidney disease) and type-B blood.

  • Sexual selection: Really a subset of natural selection - evolution based on the selective advantage of traits that improve reproductive success, even if otherwise deleterious.

    Genetic linkage sometimes manifests itself during sexual selection. Suppose there were a human population in which females possessed a gene that cause a compulsion to mate with men with red hair? Clearly we would get a general shift across generations toward a kind of genetic linkage:

    And, any individual passing on one gene would likely pass on the other. This particular concatenation of genes can lead to "runaway sexual selection" in which bizarre and otherwise maladaptive structures and behaviors evolve.

  • Pleiotropy: When a single gene has many independent phenotypic manifestations. Not surprising. Genes code for proteins, and a protein may interact differently with different tissues. A recent example in the news involves the linkage of iris shape and personality through the action of the PAX6 gene. Even if one phenotypic effect of a gene is neutral or slightly deleterious, it may be selected for if another is highly advantageous.

  • Heterochrony: Subtle changes over evolutionary time in an organism's developmental timetable are a potent source of overall evolutionary change. This is an idea that has been played around with for well over 150 years, but not really well understood until recently.
  • Last modified: February 17, 2009