On the Inheritance of Acquired Traits and the Theory of Use and Disuse.
Charles Darwin’s On the Origin of Species described in great detail a means to explain the theory of evolution through natural selection. Within his work he makes many observations in relation to the heritability of acquired characteristics. As he describes the effects of artificial selection, he dawns on the topic of “The effects of habit and of the use or disuse of parts; correlated variation; inheritance”(Darwin, p10). Darwin makes the observation, when speaking of domesticated mammals, that “not one of our domesticated animals can be named which has not in some country drooping ears...the drooping is due to the disuse of muscles of the ear, from the animals being seldom much alarmed”(Darwin, p10). Darwin argues that the increased use or disuse of parts in an animals lifetime result in heritable variation that can be passed on to their offspring. This variation he argues, is the basis for which animals develop advantageous traits and lose deleterious ones. Darwin bases his theory of heritable variation on the theory of acquired heritability, and the use and disuse principle, which was proposed by Lamarck. Being the predominant idea at the time the theory of Use and disuse states that, “use would cause the structure to increase in size over several generations, whereas disuse would cause it to shrink or even disappear”(Waggoner 1996). His second principle, or the idea of acquired heritability states that, “all such changes were heritable”(Waggoner 1996). Using these principles Darwin attempted to describe the relationships between organisms, the variation that resulted in speciation, and the evolutionary implications of those postulates. Lamarck’s theories provided a mechanism by which Darwin could explain natural selection and evolution, and in this respect they were invaluable. These theories however, are now known to be incorrect. The first principle of use and disuse can be dis-proven by the modern evolutionary principle that natural selection can act only on variability already present within a population. Genetic variation is already present in natural populations and selection acting on this variation results in evolution. The second principle of acquired heritability can be disproved under this same idea. Any particular advantageous trait(s) that an organism may develop during their lifetime, is a result of a genetic basis for that trait already present in the population or that may have arisen through mutation. The fitness increase caused by such a trait would result in the fixation of that trait in a population, so it would seem that an advantageous trait was inherited as a result of the parent organism developing that trait through use or disuse. Without a genetic background to explain heritability Lamarck’s theories made the most sense at the time. Since Darwin’s elaboration on the theory of evolution and heritability, numerous genetic experiments have taken place to attempt to provide a better understanding of the laws of heritability. Darwin makes several assertions within the first chapter of On the Origin of Species that defend his ideas to this day. His idea of correlated variation, or the idea that certain traits are associated with each other is explained by the statement that, “if man goes on selecting, and thus augmenting any peculiarity, he will almost certainly modify unintentionally other parts of the structure, owing to the mysterious laws of correlation”(Darwin, p11). What Darwin was referring to, although he didn’t know it, was idea of genetic linkage, or that certain traits are associated with others through genetic linkage on a genome. Darwin used the domestication of the canine as an example of artificial selection to aid in his explanation of heritable variation. He focuses on the idea that while the domestic dog is one species, a significant degree of variation can be seen within that species...
References: 1. Darwin, Charles. (1859). On the Origin of Species. London: John Murray.
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