Natural Selection
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[edit] Introduction
Natural selection is the name given by Charles Darwin to the tendency of nature to favor some animals more than others in a population, because of their individual heritable differences. In Darwin's words:
- As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be NATURALLY SELECTED. [1]
Darwin tended to speak of natural selection as selection for those traits useful for survival, as opposed to sexual selection, which is about traits useful for attracting a mate. Nowadays we often find it more convenient to think of natural selection as including sexual selection as a special case, as we shall do in the remainder of this article.
We might state the law of natural selection as follows: the greater the comparative advantage an inherited (i.e genetic) trait confers on its possessor in surviving and reproducing its genes, the greater the likelihood that this gene for this trait will eventually reach fixation in the gene pool.
This might at first seem like a trivial or vacuous statement: that those genes which are good at getting themselves reproduced will in fact get reproduced. Left in the abstract, it would indeed be vacuous. However, our knowledge of the conditions of nature allow us to say which traits are advantageous in which circumstances. For example, we know that streamlining is useful for swimming animals; we know that camouflage is useful to animals which stalk their prey; we know that resistance to any particular new antibiotic will become useful to bacteria when the antibiotic is released onto the market; and so forth. In short: natural selection selects for creatures well-adapted to their environment.
[edit] Natural Selection and Evolution
People were aware of natural selection before Charles Darwin formulated the theory of evolution, but they tended to see it as a conservative force which would tend to keep the gene pool stable by flushing out bad (maladaptive) variations. However, it also acts as a "ratchet", as it were, on mutations which make a lineage better adapted to their environment. If such a useful mutation comes along, natural selection is likely to fix it in the gene pool; generations may come and go, a hundred or a thousand bad mutations may appear in the gene pool, and be weeded out by natural selection; then a second advantageous mutation may come along and be fixed in the gene pool; and so forth, with the adaptive mutations accumulating and the maladaptive mutations being weeded out.
The cumulative effect of this is that the organisms in question become better and better adapted to their environment, so giving the spurious impression that the organisms were designed to fit their environment.
If the species in question is in two different environments (e.g. island and mainland) and especially when (as in that example) it is thus divided into two separate gene pools, then different mutations will arise, and different mutations be favored, in the two gene pools, causing a progressive divergence in the genetic characters of the two groups. The effect of this can be large enough that the two groups become reproductively isolated: that is, speciation has taken place.
[edit] Misconceptions
A number of people have managed to misunderstand the term "natural selection" since Darwin first coined it. As these mistakes don't seem to have changed much in the last century-and-a-half, we may as well just quote Darwin's own correction of the most common errors as it appears in later editions of the Origin Of Species:
- Several writers have misapprehended or objected to the term Natural Selection. Some have even imagined that natural selection induces variability, whereas it implies only the preservation of such variations as arise and are beneficial to the being under its conditions of life. No one objects to agriculturists speaking of the potent effects of man's selection; and in this case the individual differences given by nature, which man for some object selects, must of necessity first occur. Others have objected that the term selection implies conscious choice in the animals which become modified; and it has even been urged that, as plants have no volition, natural selection is not applicable to them! In the literal sense of the word, no doubt, natural selection is a false term; but who ever objected to chemists speaking of the elective affinities of the various elements?--and yet an acid cannot strictly be said to elect the base with which it in preference combines. It has been said that I speak of natural selection as an active power or Deity; but who objects to an author speaking of the attraction of gravity as ruling the movements of the planets? Every one knows what is meant and is implied by such metaphorical expressions; and they are almost necessary for brevity. So again it is difficult to avoid personifying the word Nature; but I mean by nature, only the aggregate action and product of many natural laws, and by laws the sequence of events as ascertained by us. With a little familiarity such superficial objections will be forgotten. --- Darwin, On The Origin Of Species, sixth edition, chapter IV
