"
Natural Selection" is the title of the fourth chapter of
On the Origin of Species by Means of Natural Selection. The first major new
ideas of the book come in this chapter.
Natural selection, to Darwin, was the "preservation of favourable
variations and the rejection of
injurious variations." To put it another way, any
plant or
animal is mostly similar to its parents, but has a certain amount of
random variation. If this variation is useful, it is more likely than most to survive to have children of its own. If this variation is harmful, the plant or animal is less likely than most to pass on its
genes. Useful variations spread throughout a
species, as
descendants continue to be successful, while harmful ones quickly go away, as their possessors
die.
Natural selection does not make up evolution by itself, but without natural selection evolution would not produce better organisms. In Darwin’s Ghost, Steve Jones recounts the classic example of natural selection: As factories sprung up in England, the peppered moth, a gray animal, seemed to turn black. This was natural selection at work. The newly-built factories spewed black smoke into the air. Gray moths, which had been well camouflaged in the past, were now visible to birds while sitting on filthy trees. Many of these moths were killed. Darker moths more often survived to reproduce, and became more and more common. By 1900, in some parts of England, just one peppered moth in fifty was of the gray variety.
This is an example of natural selection that did not progress to speciation, and it is often challenged for this reason. Creationists and others who oppose evolution say things like, "Evolution can make small changes like that, but it can’t create a new species." Darwin argues, however, that no "extreme amount of variability is necessary; as man can certainly produce great results by adding up in any given direction mere individual differences, so could Nature, but far more easily, from having incomparably longer time at her disposal." As humans breed dogs through selection of preferred traits, so does nature evolve all species.
As mentioned in the last chapter, this is done in part through the deaths of animals that do not have the traits needed to survive. It is also done through another mechanism: sexual selection. Jones describes evolution as "an examination with two papers." He continues, "The first involves staying alive long enough to have a chance to breed, while the mark in the second depends on the number of progeny." The second is as important as the first.
In some species, such as giraffes, sexual selection takes place through fighting among the males for the rights to the females. (As Jones explains, the length of giraffes’ necks is primarily due to their use as clubs; males hit each other with their heads when fighting. This explains why the giraffe’s neck is long rather than its legs, which would be just as good for reaching high leaves.) In other species, one gender or the other has a song or plumage that it uses to impress the other. In either case, the result is that animals with certain traits pass on their genes more often than those with other traits, and those genes, and the traits they carry, spread throughout a population.
Natural selection and sexual selection are sometimes in conflict with each other. To quote Jones, "In Uganda in the 1930s, almost every male elephant had tusks, structures evolved (at least in part) as statements of reproductive excellence. Sixty years later, ivory poachers had greatly reduced the number of animals, with those with the largest tusks at greatest risk. Now, a third of adults are tuskless, because the negative effects of ivory on survival outweighed its role in allure." For these elephants, there is a constant balancing act between survival and reproduction.
Natural selection also works against humanity sometimes. Old collections of bacteria, kept in suspended animation, exist going back to 1914. When these bacteria are reanimated, they are susceptible to every antibiotic used today. Contemporary bacteria, however, are not nearly as easy to destroy. Through natural selection, they have evolved resistance to antibiotics. "Twenty years ago," Jones writes, "the drug [penicillin] could kill the bacterium which causes meningitis. In many places—the United States and France included—three quarters can now defy it." At the same time as being a lesson in biology, this is a lesson in caution; if we had not overused antibiotics, this problem would not be nearly so great. Throughout the rest of his chapter, Jones uses examples like this.
Darwin ends his chapter on natural selection by explaining the principle of divergence of character. This section explains several of the toughest parts of the theories of evolution. For instance, why do we get new species at all? Why do perfectly successful species go extinct? The answer to these questions is that, as new varieties of a species evolve through natural selection, they compete vigorously with their parent species for resources. Because the varieties are so similar, they have similar survival needs, and there is simply not enough to go around between the old species or variety and the new one. To quote Darwin, "Hence, all the intermediate forms between the earlier and later states, that is between the less and more improved state of a species, as well as the original parent-species itself, will generally tend to become extinct."
This tendency of newer species to compete with and destroy older species also explains the "family tree" sort of branching chart that is often drawn to explain evolution. If you look at one of these trees, you’ll notice that species generate other species, but that individual species do not appear to survive the entirety of the chart. This type of chart makes sense only if the earlier species die out.
As a result of this constant natural selection, only one in a thousand species that have ever existed are alive today. This is a shocking number when one considers the vast number of species that exist now, but not so shocking when one considers the length of time species have had to evolve and extinguish.
To end our look at this chapter with one more description of natural selection by Darwin, "Natural selection can act only by the preservation and accumulation of infinitesimally small inherited modifications, each profitable to the preserved being; and as modern geology has almost banished such views as the excavation of a great valley by a single diluvial wave, so will natural selection, if it be a true principle, banish the belief of the continued creation of new organic beings, or of any great and sudden modification in their structure."
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Works Cited