Lecture 15

Topics

• Darwin's sources

• Fundamentals of Selection

Study Sources

• Ricklefs: Chapter 26 (see below)

• Study Guide: Basic Genetics and Selection

The Main Points

• Darwins's Theory of Natural Selection is based upon a unit of organization called a population. While people had been thinking about populations since Aristotle, I think Darwin's work really formalized our thinking about the biology of populations.

• The Elements of Natural Selection - I have paraphrased Darwin's theory as it appears here. The Origin of Species (1859) was a 400+ page "abstract" (as Darwin thought of it) that explores the observations and deductions that follow. Do yourself a favor and read, at least, the last chpater of The Origin of Species. It is remarkable in so many ways. Not only are we treated to Darwin's insights into biology, but we experience his formidable rhetorical style.

• The central Darwinian theory boils down to 3 observations and 2 deductions. Together, they form a nifty piece of logic.

  • Observation 1 - Populations have an innate capacity to grow at a geometric rate. (At the beginning of the 19th century, Malthus had written his Essay on Population, using his own arithmetic to convince readers about the potential of the human population to grow more rapidly than it could produce resources to support itself.) It is likely that Darwin drew upon this well-known idea to anchor his theory.

  • Observation 2 - Populations tend to remain relatively stable in size. (I think Darwin drew upon his own observations of the populations of plants and animals that lived around his home).

  • Deduction 1 - There is a "struggle for existence" and "more are born than can possibly survive." (We think of this "struggle" now as intraspecific [within the population] and interspecific [between species populations] competition). This results because some resource that individuals use in common is in short-supply.

  • Observation 3 - Individuals in a population are all a little different from one another. That is, there is much phenotypic (Darwin didn't use this word) variation in a population. Here, you need to review some fundamentals of Mendelian genetics, especially the mechanisms of meiosis that produce different genotypic combinations. Having done taxonomic work on some of his own collections, Darwin was well aware of the degree of difference among individual specimans.

  • Deduction 2 - There is Natural Selection among all these different individuals; that is, some individuals leave more offspring than others. Darwin talked about "survival of the fittest" to describe the outcome when these different "types" are exposed to a local set of conditions. Some simply "do better" than others.

    For a very long time, in the use of domestic plants and animals, farmers had been "selecting" which "types" would breed and which would not. The desired outcome, of course, was to increase productivity of the crop or the herd. Darwin saw that differential success in nature, would be a kind of "natural selection."

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