Historical Development and Mechanisms of Evolution and Natural Selection
Early Theories of Evolution
The scientific community has always displayed an interest in the understanding of how life began. A rekindled interest emerged in the 1800s as new geologic and biologic discoveries added to the available knowledge.
Jean Baptiste Lamarck, a French scientist, proposed that species change because of the use or disuse of features, such as tails or arms. He purported the “use it or lose it” idea. He believed that excessive use of a feature would cause it to grow and lack of use would invite atrophy. He also stated that an individual's increased or decreased features could be passed on to the individual's offspring.
Lamarck hypothesized that acquired traits, not genes, were passed on to offspring. The Lamarckian concept of evolution appeared 50 years before Darwin. Unfortunately, he did not possess an understanding of gene behavior and heredity. However, his ideas started the discussion of evolution in the scientific community and, combined with the three additional ideas that follow, serve as a starting point for Darwin and others.
- Populations change over time in response to environmental pressure (same as Darwin) and may have evolved from small numbers of starting organisms.
According to Lamarck, giraffes grew long necks because they wanted to reach leaves higher up on the tree.
- Organisms could actively try to change to be more adapted to the environment and then pass that change on to their offspring. For instance, the ancestors of fish saw the advantage of being able to swim faster, so over generations they developed fins instead of legs to increase their speed in water, thereby making them more efficient at getting food and eluding predators. Because this change helped and therefore was desirable to the organism, it was passed on to all offspring.
- The “use it or lose it” concept began with Lamarck, who believed that organisms could reshape their bodies in response to a desire to change. Appendages that were no longer needed would not be conveyed to the next generation, whereas other desirable changes could be added.
Many period biologists agreed with Lamarck that these acquired characteristics were inheritable factors. However, with no understanding of genetics, they could not explain how these things could happen.
Most people in Darwin's time assumed that the earth was relatively young and that mountains, rivers, and other surface features did not change over time, but were formed by a catastrophic event. For instance, lakes might have formed when a meteorite collided with the earth and then rain filled the resulting crater with water.
Several geologists proposed contrary theories stating that the earth is likely at least four billion years old and that surface features normally change slowly over time. In 1788, James Hutton proposed that the mountains and other surface features gradually change, which supported the hypothesis that the earth was much older than the prevailing thought. Charles Lyell expanded on Hutton's work when he added the concept of uniformitarianism, which stated that the events and processes that created the mountains, lakes, and other surface features are still happening and an understanding of these events was required to explain the formation of existing features. Lyell also proposed that the earth itself had internal movements, as evidenced by earthquakes, that provided further opportunities for change. As scientists began to find and study fossils, Lyell's explanations gained credibility. Lyell contributed to Darwin's thinking in two ways:
- Past events must be explained in terms of today's events.
- The earth's history provides ample geologic time for slow change.
Darwin realized that a preferred method of selection had been taking place on a local level for a long time. In his conversations with local farmers, he learned that variability exists in every population and was the basis for enhancing yield and productivity. For instance, in a given wheat field, some plants developed greater seeds than others; likewise, some cows gave more milk than others. Those individuals that contributed the most, the farmers allowed to reproduce; all others were prohibited. In so doing, the farmers were attempting to increase their productivity, and they also selectively favored certain organisms for reproduction. Darwin reasoned that in this form of artificial selection, man could drastically change the appearance of a species in a relatively short period of time from the original wheat or cow to their descendants who were more capable of producing desirable results than their ancestors.
Thomas Malthus was actually an English clergyman who worried about the distribution of wealth and resources. His observations and reasoning became known as the Malthusian dilemma, which states that populations multiply geometrically while food sources multiply arithmetically. If left to follow course, the human population would eventually grow larger than the ability of the environment to feed everyone. He predicted large mass starvations that crossed international borders with the possibility of resource wars, famine, and plague. Oddly enough, wars, famine, and plague were the answer to the dilemma, because they served to reduce the reproductive population, an idea that did not escape Darwin. He expanded the phenomenon to all creatures, not just humans, and understood that more offspring are born than will survive to reproduce. Refer to the illustration The Malthusian dilemma.
At point A on the graph, the amount of available resources is adequate to sustain a stable or slightly growing population. At point B, the growth of the human population has reached exponential rates, but the growth of available resources has not increased as dramatically. As a result, Malthus predicted that the intense competition for the resources would lead to war, famine, and plagues.
Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.