Darwinian Selection

Darwinian Selection

In 1831 Charles Darwin began a five-year journey as ship's naturalist on the H.M.S. The Beagle. During this time he visited South America, Australia, South Africa, and islands of the Pacific and South Atlantic. He later published his travels in The Voyage of the Beagle where he introduced many themes that later became crucial to the arguments presented the more-familiar The Origin of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life (published in 1859 and more commonly known as "The Origin of Species"). Majors in the sciences and those interested in philosophy should read this monograph. You won't find it easy reading because the language is often archaic and the arguments are sometimes difficult to follow, but it represents one of the most important contributions made to Western culture.

Although Darwin is often referred to as "the father of evolution", he was not the first to introduce the idea of changing species. Maupertuis and Diderot in the mid 18th century, for example, wrote of evolution and the ideas of changing life are part of many religions. Darwin's contribution was to provide a mechanism through which evolution could function. Briefly, the Darwinian argument is as follows:

Variation exists within a species. Although we may consider all houseflies as being more-or-less alike, on closer examination you find that they are nearly as recognizable as one person is from another.

Some of this variation has a genetic basis. Evolution can act only on traits that are passed genetically from one generation to the next. Just as an animal or plant breeder has no interest in non-genetic traits, evolution can not work on differences caused by trauma, parasitism, and other environmental variation.

The reproductive potential of organisms is vast. Darwin calculated that a single pair of elephants could have 19 million descendants within 750 years if each animal lived to be 100 and each pair had six calves. Calculations for other organisms produce similar increases in population size. Elephants are not the most common beasts, the oceans are not overflowing with fish and we aren't nose-deep in ragweed (although it sometimes seems that way). Therefore something must happen to all these extra offspring and, unless species other than man practice birth control, most of the young must die before they reproduce.

Because individuals differ from one another, some should be more capable than others in eluding predators, coping with environmental extremes, or in competing with members of their own or other species. Those that are more capable should leave more offspring to the succeeding generation. Since some aspects of coping must be tied to genetic attributes, the favorable genes are passed on to the next generation. The genetic makeup of the population changes and evolution is said to occur. This varying reproductive success of individuals based on their different genetic constitutions is natural selection.

ns01.gif (1103985 bytes)

Often the concept of natural selection is simplified to "survival of the fittest". Fitness in evolutionary terms has an exact meaning related to the number of surviving offspring produced by an individual in comparison to less well-endowed individuals. Evolutionary fitness is therefore more than just the ability to run quickly or fight off competitors.

Evolution is not a historical process; it is occurring at this moment. Populations constantly adapt in response to changes in their environment and thereby accumulate changes in the genes that are available to the species through its gene pool.