Various theories of evolution were formulated during the latter part of the nineteenth century. Karl Wilhelm Nageli (1817-1891), who devoted much of his life to the microscopical study of plants, proposed the presence of an inner-directed force that guides the course of evolution. He coined the word
Figure 38-1 Charles Darwin (1809-1882) laid the foundation for our concepts of evolution. (Illustration by Donna Mariano)
idioplasm for the material of heredity. Theodor Eimer was an advocate of so-called straight line evolution and a phenomenon called orthogenesis, wherein evolution proceeds in an undeviating manner toward a particular outcome that has nothing to do with environment. Hugo DeVries (1848-1935) contributed significantly to our understanding of evolution by his theory of mutations (those sudden, unexpected changes in organisms that cannot be accounted for by the laws of heredity and that can be used to explain the abrupt appearances of new species). DeVries' greatest contribution to science was likely his use of scientific method, rather than the older method of observation and inference, in the study of evolution. He also was one of those who rediscovered Mendel's work in 1900. These theories emerged before any
M Notes HL detailed knowledge of the laws of heredity. These laws once uncovered by Gregor Mendel had to wait yet another thirty-four years before finding a widespread audience and acceptance in the scientific community.
The work of Gregor Mendel made possible the application of proven science to what had previously been educated speculation. The labors of William Bateson (1861-1926), who extended our understanding of the principles of segregation and gene interaction, and Thomas Hunt Morgan (1866-1945), who demonstrated that genes are located on the chromosomes, while technically falling under the realm of genetics, significantly reinforced the Darwinian concepts of evolution.
But how does evolution occur? Charles Darwin's theory of the tendency toward overpopulation; competition; gradual, cumulative changes; and the survival of the fittest is a highly favored one. Yet much can be added to facilitate our understanding of this process. Evolution is not caused by urge, seeking, or need. How organisms are able to accommodate to environment and how they relate to each other is important. Many questions are yet unanswered. Does evolution progress at a steady rate, or do evolutionary changes occur in bursts? Is evolution triggered by challenge? An understanding of genetics is helpful in understanding the presumed mechanism of evolution. (For those interested in review, Gregor Mendel's work is discussed in chapter 5, whereas DNA and its role in genetics are discussed in chapter 6.)
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