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random walk depicted in Fig. 15.1. A marker on a line changes position as a result of a coin toss. If the coin lands heads, you move the marker one space to the right; if the coin lands tails, you move the marker one space to the left. These are the rules of change unless the marker happens to be at the left-most or the right-most points. If the coin lands tails when the marker is at the extreme left, you simply toss again. Suppose the game begins with the marker placed at the left-most point on the line. Where do you expect the marker to be after 5 or 50 or 500 coin tosses? Probably not at square 1. The line in this game represents complexity, with 1 being the least complex and 100 the most. Selection can be indifferent to simplicity versus complexity and yet evolution by natural selection can be expected to manifest a net increase in complexity (Sober, 1994).

A third important feature of Darwin’s concept is that selection acts on “random” variation. This is a loaded word, apt to mislead. Darwin says in the Origin (1859, p. 131) that “random” just means that the cause of a new variant’s appearance in a population is unknown. However, “random” for Darwin was more than a confession of ignorance. What he meant was that variations do not occur because they would be useful to the organism in which they occur. In The Variation of Animals and Plants Under Domestication, Darwin explains his point in terms of a beautiful analogy:

Let an architect be compelled to build an edifice with uncut stones, fallen from a precipice. The shape of each fragment may be called accidental; yet the shape of each has been determined by the force of gravity, the nature of the rock, and the slope of the precipice,—events and circumstances all of which depend on natural laws; but there is no relation between these laws and the purpose for which each fragment is used by the builder. In the same manner the variations of each creature are determined by fixed and immutable laws; but these bear no relation to the living structure which is slowly built up through the power of natural selection, whether this be natural or artificial selection.

Darwin (1868c, p. 236)


A fourth important feature concerns the level at which Darwin took natural selection to act. In almost all of the examples that Darwin discusses, traits are said to be selected because they help the individual organisms that

FIGURE 15.1 A random walk on a line with 100 locations. Unless the marker is at the left-most or the right-most location, it moves 1 space to the right if the tossed coin lands heads and 1 space to the left if the coin lands tails.

FIGURE 15.1 A random walk on a line with 100 locations. Unless the marker is at the left-most or the right-most location, it moves 1 space to the right if the tossed coin lands heads and 1 space to the left if the coin lands tails.



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