Natural Selection

Although we associate evolution with Charles Darwin, he was not the first to realize evolution occurs, nor was he the first to search for the mechanism. Jean Baptiste Lamarck (1744—1829) made the first robust, scientific attempts at explaining the Tree of Life. Much of Darwin's thinking was influenced by the writings of Lamarck and his ideas on the inheritance of "acquired traits."

The classic example of Lamarckian evolution is the explanation for the evolution of the long giraffe neck. Under this view giraffes had short necks originally and during their lives they stretched to reach leaves hanging high on the trees. Their necks became slightly longer than if they had not eaten those out-of-reach leaves. The offspring of the neck stretching proto-giraffes inherited these slightly longer necks and so on and so forth until after many generations the modern giraffe with a long neck was the result. Imagined in human terms, it is easier to see the flaw in Lamarckian evolution. A woman that lifts weights during her lifetime and builds up her muscles will not give birth to muscle-bound children, just as giraffes that stretched their necks will not produce longer-necked offspring. Environmental influences are not passed along to future generations, so unfortunately for Lamarck, without a grasp of genetics and inheritance in those days, his hypothesis fell short.

Although not the first to recognize evolution, Charles Darwin was the first to conceive of a feasible mechanism for it. Strictly speaking, Charles Darwin and Alfred Russel Wallace (1823-1913) independently came up with the theory of natural selection, but Darwin is given the majority of the credit because he published a widely read book. In The Origin of Species (1859), Darwin proposed a mechanism for evolution that— although it is the basis of modern evolutionary science—just cracked the surface for the discoveries that would follow in support of evolution.

Aside from evidence he gathered on the voyage of the H.M.S. Beagle, Darwin's ideas were also influenced by the work of economist Thomas Malthus (1766-1834). In his An Essay on the Principle of Population Malthus described how human population growth is exponential or geometric while food supplies can only grow at a relatively stable, linear, or arithmetic rate. Populations would grow continuously for eternity if limiting resources like food did not keep them in check. A fast growing population that reproduces too quickly for its resources can outstrip those resources and then experience a crash until the population size settles to a level the resources can support.

To preface his theory of natural selection, Darwin postulated the following: (1) The ability of a population to expand is infinite. But the ability of any environment to support a population is finite. He called this a "struggle for existence" and the "economy of nature"; (2) Organisms within populations vary and this variation affects an individual's ability to survive and reproduce; (3) Variation is transmitted from parent to offspring. Darwin called this "descent with modification" but at the time there was still no known mechanism of inheritance.

Put into modern terms, Darwin's theory of natural selection can be broken down into four parts.

Was this article helpful?

0 0

Post a comment