References Cited

Charles Darwin used information from several disciplines in developing his theory of evolution. He was particularly impressed by the amount of variation that occurs within living species, especially in domestic animals, and he spent a great deal of time studying breeding programs. Even in Darwin’s day, the human effort in breeding variants of domestic animals had resulted in many breeds of dogs, cats, horses, sheep, and cattle. As an example, consider the tremendous variation in domestic dogs. The Chihuahua and the Saint Bernard are about as different  in size, shape, hair length, and other features as one could imagine; yet both breeds are domestic dogs with the scientific name Canis familiaris. The differences between them were produced by human-engineered selective breeding programs. Artificial selection is the term for what we do when we choose plants and animals with desirable features and breed them to produce or enhance these features in their offspring. As different as they look, Chihuahuas and Saint Bernards ... and Poodles, Pomeranians, Pekinese...all domestic dogs share the same gene pool. This shared gene pool means that all dogs have the ability to interbreed, and this is why all domestic dogs are placed in one species. The common gene pool of dogs also allows for the great variation we see in “man’s best friend.” A standard definition of species in animals is the ability to interbreed and produce fertile offspring.

Darwin gathered data and honed his theory for 20 years before publishing his well-known book in 1859, The Origin of Species by Means of Natural Selection, or The Preservation of Favoured Races in the Struggle for Life. Darwin and his fellow naturalist Alfred Wallace independently came to the conclusion that geologically older species of life gave rise to geologically younger and different species through the process of natural selection.

Darwin’s theory of evolution can be summarized in four statements.

1. Variation exists among individuals within species. Anyone who looks at their friends and relatives, or their pets, can see variation. Breeders of animals and plants use these diverse characteristics to establish new varieties of dogs, cats, pigeons, wheat, cotton, corn, and other domesticated organisms. Scientists who name and classify plants and animals are acutely aware of variation in natural populations. For example, the level of resistance to insecticides varies among individuals within species of insects. This variation enables some individuals to survive application of insecticides and produce offspring that inherit this resistance to these insecticides.
   
2. Organisms produce more offspring than the environment can support. All living things produce more individuals than can survive to maturity. Think of the thousands of acorns that one mature oak tree produces every year. A female salmon produces about 28,000,000 eggs when spawning. One oyster can produce 114,000,000 eggs in a single spawning. Darwin calculated that in elephants, which are among the slowest breeding land mammals, if all of the potential young of a single female survived and reproduced at the same rate, after 750 years the descendants of this single mother could number 19,000,000! Clearly, if all of these seeds, eggs, and young survived to become adults who also reproduced, the world would soon be overrun with oak trees, salmon, oysters, and elephants.
   
   
   
   
3. Competition exists among individuals. Regardless of the rate of reproduction in a species, all of the young do not survive to become reproducing adults. This fact indicates that large numbers of offspring somehow are eliminated from the population. Some certainly die by accident. But most of them succumb to competition with other individuals. The most intense competition may be among individuals of the same species who compete for nearly identical environmental requirements. Competition may be as simple as a race to get a rabbit — the first fox there gets lunch; the others go hungry. Competition may involve obtaining a choice nesting site, or being able to find the last available hiding hole when a bigger fish comes looking for dinner. Those individuals who catch the rabbit or find the hiding hole survive to pass on their genes to the next generation.
   
4. The organisms whose variations best fit them to the environment are the ones who are most likely to survive, reproduce, and pass those desirable variations on to the next generation. Many of the natural variations we observe in species do not seem to be either particularly helpful or particularly harmful to an individual in its struggle for survival. Hair and eye color may be such neutral variations in human beings. Some variations certainly lower the chances of survival, such as hemophilia in mammals, albinism in many wild animals, or an unusually thin shell in clams living where there are numerous hungry snails.
   

Some variations are helpful. For example, any variation that increases an antelope’s speed may help it elude predators. Any variation that increases water retention in a desert plant will favor survival of that plant to reach maturity. Those animals and plants that survive to maturity and are able to reproduce become the parents of the next generation, passing on the genes for the successful variation.

Darwin called the process by which favorable variations are passed from generation to generation natural selection. He made many important observations on the relationship of individual variation to survival. During his stay in the Galapagos Islands, Darwin noted that the populations of tortoises on each island had physical features so distinctive that people could often tell from which island an animal came simply by looking at it.

We commonly hear natural selection referred to as “survival of the fittest.” This popular phrase has a very specific biological meaning. “Fittest” means that organisms must not only survive to adulthood, they must actually reproduce. If they do not reproduce, their genes are not passed on to the next generation. Evolution occurs only when advantageous genetic variations are passed along and become represented with increasing frequency in succeeding generations. 

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