Dec 18 2010
atural Selection is the process by which certain heritable traits that convey some advantage become more common in a population over successive generations. These advantages typically make the organism more likely to survive and successfully reproduce. The term was introduced by Charles Darwin (1809-1892) in his influential 1859 book On the Origin of Species. In it, Darwin describes natural selection as being analogous to artificial selection, a process by which animals and plants with traits considered desirable by human breeders are systematically favored for reproduction. It is a key mechanism of evolution and a cornerstone of modern biology.
It was Darwin in fact who also advanced one of the earliest systematic discussions of variation and inheritance. That should be no surprise, for when you think about it, in order for natural selection to work, there would have to be significant differences between individuals. Darwin did not really think about variation in terms of heredity. In fact, he tended to think of both heredity and variation as essentially two antithetical processes. This makes a certain amount of common sense. To this day, we tend to think that the job of heredity is to maintain consistency and that of variation to disturb it.
Variation, to Darwin, was a series of continuous acts of disturbance produced before conception and long afterward. However, there was a critical difference between Darwin and most of his contemporaries. They saw variation as the addition of extra stages to the development of an organism from the fertilized egg to its mature form (a process called ontogeny) where Darwin accepted that variation was more of a breakdown in the copying process.
Variation under changed conditions was certainly known before Darwin; most naturalists were familiar with species of butterflies or other insects that appeared in local varieties. Some taxonomists, called “splitters,” tended to reject the idea that there could be significant variation between species. The splitters held that every local variant was actually the beginning of a new species. Others, called “lumpers,” were inclined to class widely divergent variants as members of the same species. For example, lumpers viewed leopard, tigers, and lions as members of the same species.
Darwin instead turned to the study of the literature on animal husbandry, in particular the writings of breeders and their use of artificial selection. He published his ideas in a book entitled Variation of Animals and Plants Under Domestication under a theory called Pangenesis.
Darwin visualized inheritance as a process by which “gemmules” were budded from the various tissues of the body and transmitted to the reproductive organs. Because they had “budded” from the host tissue, they carried the exact same information and the capacity to reproduce the exact same tissue as that which they were derived. Reproduction mixed up the gemmules from each parent, causing a blending of the characteristics. In essence, each cell gets a “vote.”
Sometimes traits revert to more primitive forms called atavisms. Examples of atavisms include hind legs on whales or extra nipples in humans. Atavisms occur because earlier, primordial genes are often preserved in DNA, even though the genes are not expressed in some or most of the organisms possessing them. According to Darwin, atavisms arise due to the awaking of long-dormant gemmules.
According to Darwin, for change to occur the copying process had to be disturbed by some sort of external event. If the parent’s body changed, then the gemmules changed.
Darwin’s concepts of variation could even leave a role for alternate theories of variation, such as those proposed by the French naturalist Jean-Baptiste Lamarck (1744-1829). Lamarck postulated that an organism could pass on characteristics that it acquired during its lifetime to its offspring, usually described as the Theory of Soft Inheritance. However, Darwin’s work with animal breeders showed him that most variation they worked with was not adaptive but more random, with the generation of many otherwise “useless” versions (although not useless to the breeder!)
Pangenesis was not one of Darwin’s better ideas and was quickly disproved by Darwin’s brother-in-law, Francis Galton (1822-1911) via a series of studies involving blood transfusions between rabbits, in which Galton could not show transference of gemmules via the transfused blood.
Eventually pangenesis was separated from Darwin’s theory of Natural Selection, to produce (when combined with Mendel’s studies on genetics and modern population studies) what is called Neo-Darwinism, which, in its most rigid form, holds that evolution occurs solely through natural selection without any mechanism involving the inheritance of acquired characteristics resulting from use or disuse. However, this version of Neo-Darwinism, often called Genetic Determinism, would continue as the reigning paradigm throughout the twentieth century.
Portions excerpted from Fundamentals of Generative Medicine copyright 2010, Drum Hill Publishing, USA.