Brief Overview Of The Early Development Of Evolutionary Concepts

More than 2,300 years ago, Aristotle (384-322 B.C.), although not recognizing any processes of natural selection, did observe grand design in nature and arranged all organisms known to him from the simplest to the most complex in what he called the Scala Naturae (Scale of Nature). Aristotle's arrangement implied that all organisms were static and didn't evolve. These beliefs were widespread until at least the 17 th century and were not extensively challenged before Darwin. Fossils (parts of previously existing organisms preserved in rocks or other substances) had been found many centuries before Darwin but were not properly identified until the 15th century when Leonardo da Vinci (1452-1519) correctly observed that they were parts of previously existing organisms that had become extinct.

Count de Buffon (1707-1788), a French naturalist, spent much of his adult life writing a natural history of 44 volumes in which he described all known plants and animals. In this work, Buffon (whose real name was GeorgesLouis Leclerc) presented evidence of descent with modification in organisms and speculated on the mechanisms involved. However, he provided no theories on how evolution might take place.

Georges Cuvier (1769-1832), a French zoologist and paleontologist (one who studies fossils; fossils and fossiliza-tion are discussed in Chapter 21), used comparative anatomy toward the close of the 18th century to classify animals. Cuvier, however, firmly believed that organisms did not change over time. When geological finds showed apparent evolution of organisms in rock formations of certain regions, Cuvier tried to explain them away with what was called catastrophism. He theorized that mass extinctions or catastrophes had occurred whenever a new geological find revealed a different group of fossils, and the presence of the new fossils was due to repopulation of the region by species migrating in from surrounding areas.

By the end of the 18th century (before the principles of genetics were known), many prominent biologists had come to believe that hereditary changes in populations over long periods of time (evolution) occurred as a result of the inheritance

Evolution

Evolution Giraffes

Figure 15.1 Lamarck and his contemporaries believed that the long neck of a giraffe developed over time as the animals stretched to reach higher leaves and that the little length increases were inherited and became cumulative. This theory of inheritance of acquired characteristics was experimentally disproved and discarded when the true mechanism of gene inheritance became known.

Figure 15.1 Lamarck and his contemporaries believed that the long neck of a giraffe developed over time as the animals stretched to reach higher leaves and that the little length increases were inherited and became cumulative. This theory of inheritance of acquired characteristics was experimentally disproved and discarded when the true mechanism of gene inheritance became known.

of acquired characteristics. One of the more prominent supporters of this widespread idea was Jean Baptiste Lamarck (1744-1829). He believed, for example, that giraffes acquired their long necks over many generations as a result of the gradual increase in neck length as shorter-necked animals stretched to reach leaves on the branches of trees. Slight stretching of the neck was supposed to have been passed on to the offspring as it occurred, and eventually, numerous tiny increases due to individual stretching added up to the present great neck length of giraffes (Fig. 15.1).

If acquired characteristics could be inherited, we should be able to demonstrate it experimentally, and indeed, many workers have attempted to do so, but all have failed. For example, one biologist surgically removed the tails of mice for many successive generations, but the average length of the tails of the last generation was exactly the same as that of the first generation. The experiment demonstrated that repeatedly removing tails in no way affects the hereditary characteristics carried in the genes within the cells. This is also the reason fruit trees that are pruned annually never produce seeds that grow into dwarfed trees, even after many generations.

Charles Darwin Medicine
Figure 15.2 Charles Darwin. (Courtesy National Library of Medicine)

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Essentials of Human Physiology

Essentials of Human Physiology

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