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Evolution (II): Natural selection
Evolution #2:
Natural selection.

Guest Essay by
Brian van der Spuy:

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In the previous article, we saw that it has been established beyond all doubt that evolution occurs and that all life is descended from one or a few original forms. But we had little to say about why and how evolution occurs. That will be the theme for this article. Because information about the subject is so widely available, I'll try to keep it as brief and to the point as possible.

When Darwin developed his "Theory of Natural Selection" in the first half of the nineteenth century, he reasoned more or less as follows:

  1. In nature, far more organisms are born than can possibly survive, because resources such as habitat and food are limited.
  2. In any population of organisms, the individuals do not all look alike; they all display small variations around a mean.
  3. Which of the organisms that are born will eventually survive to reproduce, and even how much offspring is produced by those survivors, will depend on how well they are able to compete for and/or utilize the limited resources in their environment. This in turn will depend on which of the small variations mentioned under (2) an individual possesses.
  4. By and large, variations are heritable, and an organism possessing a favourable variation will pass it on to its offspring.
  5. In this way, populations will slowly change over time and adapt to their environment. Eventually, enough change will have accumalated so that a new species will have originated.

That, essentially, is the theory of natural selection as proposed by Darwin. (Another naturalist, Alfred Russel Wallace, working independently from Darwin, came to much the same conclusions at the same time). Darwin conceived of nature doing something akin to what plant and animal breeders do: select individuals with which to breed further. The only difference is of course that a human breeder usually selects fairly arbitrarily according to his or her personal tastes, whereas in nature the only criterion is how many offspring an individual can produce. For this reason, domestic plants and animals are often ill suited for survival in the wild.
. . That the principle of selection is sound could be easily seen in Darwin's time. One only had to look at the immense variety of organisms that had been produced by artificial selection. But while most biologists quickly accepted the fact of evolution, many remained skeptical about Darwin's proposed mechanism. There were then two main objections:

  1. Breeding experiments could go only so far. While many new traits --some bizarre-- could be bred into organisms, it did not seem possible that the process could go on indefinitely until fundamentally different organisms had been produced. In other words, it seemed as if only a specific amount of variation was present in any species, and Darwinian evolution would require that completely novel variations should arise from time to time.
  2. Dilution of traits: in Darwin's time, it was well known that offspring usually resembled their parents. But the offspring also usually displayed a mixture of the traits of their parents. Suppose that a favorable trait in a population of antelopes is possessing slightly longer legs, for faster running. When an individual with this trait mates with an individual with normal legs, the offspring will have legs intermediate in size between the norm and the long legs of the one parent. When they mate, their offspring's legs will once again be somewhat shorter. After a few generations, the favorable trait will have been diluted to the point where natural selection cannot act on it anymore.

Darwin and his supporters could never come up with a very satisfactory answer, because they knew virtually nothing about the mechanisms and process of genetic inheritance. In the 1860's, Gregor Mendel discovered enough about this to successfully answer at least the second objection, but he published his results in a relatively obscure journal, where it was only rediscovered around 1900. So it was only in the 1930's that natural selection became more widely accepted, when Darwin's formulation of it was combined with the new science of genetics into an essentially modern form known as Neo-Darwinism.

So what were these discoveries in genetics that saved Darwin's theory? The first of these was made by Mendel: inheritance of traits takes place through discrete units known as genes. These units remain discrete through the generations, so they do not become diluted. What an organism looks like physically (its so-called phenotype), depends on interactions between the genes it inherited from its parents (its genetic make-up is known as its genotype). These interactions often work in such a way that an organism displays a mixture of the traits of its parents. But the individual genes remain discrete, even as they spread through a population. When enough of the individuals in a population have a copies of a new gene, the chances become large that in any individual mating, offspring will be produced that have the genes in the right combination to display the favorable trait again. And every time the favorable trait is displayed in an organism, that organism will on average leave more offspring, so that eventually most of the population will possess the trait. It has also become increasingly clear that speciation events often take place when small populations are cut off from the parent population by, for example, a mountain range or river. In such small populations, advantageous genes can spread quite rapidly through the population.

Only one question remained to be answered: what is the source for new variations beyond that which the population already possesses? By the early 1950's, the structure of the genetic material, DNA, was known, and it could be worked out in detail what genes are and how they work. DNA is a long molecule built up of basic subunits, and the order in which these subunits are placed, spell out a code for the manufacture in the cell of proteins. These proteins mostly act as enzymes, that is, catalysts for the chemical reactions in the cell. But every now and then, because of various influences such as radiation or errors in copying of the DNA when cells divide (see my article on cell division), one of these subunits will be replaced by another. Such a spontaneous, random change in the DNA is known as a mutation, and mutations are the source of new genes and therefore variations. Of course, many or most mutations are harmful or even lethal, and will not spread through populations. But some are beneficial, or fairly neutral, and such mutations can spread through populations and serve as raw material for natural selection.

A well-known example of this is that many disease-causing bacteria are increasingly becoming resistant to the effects of antibiotics. The resistance can almost always be traced to mutations that changed the bacterium's biochemistry in such a way that it can neutralize the effects of the antibiotic. A similar phenomenon is found in many agricultural insect pests that have acquired resistance to pesticides.

By around the middle of the twentieth century, most biologists accepted the principle of natural selection. Only one problem remained: it was difficult to demonstrate that any instance of evolution observed in the wild was due to natural selection and not some other mechanism. But studies done during the 1970's and 1980's by various researchers have demonstrated conclusively that in at least some cases, natural selection is by far the most important mechanism for evolution. A very good popular account of some of these studies can be found in a book titled The beak of the finch, by Jonathan Weiner.

The above is just a very brief introduction to natural selection, and leaves many questions unanswered. When I have the time, I shall maybe write a third article in this evolution series, in which I'll try to address some of these questions. For the moment, I'll just summarize the conclusions of the first two articles:

  1. Natural selection can be demonstrated to be responsible for at least some evolution.
  2. Instances where new species arise through evolution can be, and have been, observed.
  3. Various lines of evidence demonstrate beyond reasonable doubt that modern organisms are interelated and descended from one or a few original ancestors.
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Why do men have nipples?
. . Andrew M. Simons, a professor of biology at Carleton University in Ottawa, Ontario, explains. [In Sci American mag]
. . Evolution is a process constrained by many factors including history, chance, and the mechanisms of heredity, which also explains why particular attributes of organisms are not as they would be had they been "designed" from scratch. Nipples in male mammals illustrate a constrained evolutionary result.
. . A human baby inherits one copy of every gene from his or her father and one copy of every gene from his or her mother. Inherited traits of a boy should thus be a combination of traits from both his parents. Thus, from a genetic perspective, the question should be turned around: How can males and females ever diverge if genes from both parents are inherited? We know that consistent differences between males and females (so-called sexual dimorphisms) are common--examples include bird plumage coloration and size dimorphism in insects. The only way such differences can evolve is if the same trait (color, for example) in males and females has become "uncoupled" at the genetic level. This happens if a trait is influenced by different genes in males and females, if it is under control of genes located on sex chromosomes, or if gene expression has evolved to be dependent on context (whether genes find themselves within a male or a female genome). The idea of the shared genetic basis of two traits (in this case in males and females) is known as a genetic correlation, and it is a quantity routinely measured by evolutionary geneticists. The evolutionary default is for males and females to share characters through genetic correlations.
. . The uncoupling of male and female traits occurs if there is selection for it: if the trait is important to the reproductive success of both males and females but the best or "optimal" trait is different for a male and a female. We would not expect such an uncoupling if the attribute is important in both sexes and the "optimal" value is similar in both sexes, nor would we expect uncoupling to evolve if the attribute is important to one sex but unimportant in the other. The latter is the case for nipples. Their advantage in females, in reproductive success, is clear. But because the genetic "default" is for males and females to share characters, the presence of nipples in males is probably best explained as a genetic correlation that persists through lack of selection against them, rather than selection for them.
. . Interestingly, though, it could be argued that the occurrence of problems associated with the male nipple, such as carcinoma, constitutes contemporary selection against them. In a sense, male nipples are analogous to vestigial structures such as the remnants of useless pelvic bones in whales: if they did much harm, they would have disappeared.
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