The Genetic Code

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    When Charles Darwin and Alfred Russell Wallace conceived their theories of evolution by means of natural selection, they had only a primitive understanding of heredity. That knowledge came to little more than "children resemble their parents". That doesnít seem strong enough to support the theory of evolution, but it gave Darwin and Wallace probable cause to pursue their hypotheses. Even though they didnít understand how it came about, both men based their hypotheses upon the concept of "descent with modification" (as Darwin put it), what today we would call heredity with mutation.

    Nonetheless, Darwin, at least, understood that if evolution were to stand as a fully tested theory, and not as a mere hypothesis, it would have to include a full accounting of heredity at the subcellular level. Thus Darwin devised yet another hypothesis, which he presented in "The Variation of Animals and Plants Under Domestication" (1868). He called it pangenesis and he presented it as a means to organize the facts of heredity that he had acquired in his career as a naturalist.

    According to pangenesis, the cells in a living body shed minuscule particles called gemmules (also called plastitudes or pangenes). Accumulating in the reproductive organs, the gemmules become the gametes, whose male and female forms unite to become the seed from which a new individual grows. Pangenesis seemed to explain the facts of heredity and even offered the possibility of validating Jean-Baptiste Lamarckís hypothesis of the inheritance of acquired traits, which Darwin believed provided the modifications upon which natural selection worked. But, as it turns out, pangenesis does not correctly describe heredity.

    In fairness to Darwin we should note that just as the theory of evolution must stand on a sound theory of heredity, so the theory of heredity must stand on a sound theory of chemistry. In Darwinís day chemists were only beginning to work out the atomic theory of matter as it applies to chemistry. Recall that Dmitri Mendeleyev published his Periodic Table of the Chemical Elements in 1869, ten years after "The Origin of Species" was published. The chemists of the Nineteenth Century could not have imagined anything as fantastic as DNA and its workings.

    Nonetheless, some of the major pieces of our modern understanding of heredity come from work done in the Nineteenth Century. In the next three essays I will explore how work done in the Nineteenth Century and before led to our current understanding of heredity, the basis for the evolution of life on Earth.


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