Pangenesis
|
|
Pangenesis was Charles Darwin's hypothetical mechanism for heredity. He presented this 'provisional hypothesis' in his 1868 work The Variation of Animals and Plants Under Domestication and felt that it brought 'together a multitude of facts which are at present left disconnected by any efficient cause'.
The theory itself is now seen as deeply flawed and not supported by observation, yet it represents Darwin's attempt to explain such diverse phenomona as
- atavisms,
- the intermediate nature of hybrids (blending inheritance),
- Lamarckian use and disuse, and
- limb regeneration.
Simply put, the theory holds that body cells shed 'gemmules' which collect in the reproductive organs prior to fertilization. Thus every cell in the body has a 'vote' in the constitution of the offspring. Atavisms arise due to the awaking of long-dormant gemmules while limbs regenerate due to the activation of gemmules from the missing limb which circulate in the main part of the body.
In his later work, The Descent of Man, Darwin elaborated further on the model. In a section on the "Laws of inheritance," Darwin specified that two elements in particular were most important: the transmission and the development of inherited characteristics. Darwin's insights were that characteristics could be transmitted which were not at the time of transmission actually being manifest in the parent organism, and that certain traits would manifest themselves at the same point of development (say, old age) in both the parent and child organisms. In order to make sense of his theory of sexual selection, he also stipulated that certain traits could be passed through organisms but would only develop depending on the sex of the organism in question.
While the theory was of little use for biologists, it proved exceedingly useful to the nascent science of statistics, and was taken up in a major way by his cousin Francis Galton in his development of the "biometric" approach to heredity. Galton eventually discarded the notion that somatic cells (and thus, acquired characteristics) could contribute to an individual's heredity, but appreciated that an individual embryo would contain many more gemmules than would actually be expressed. This is now interpretted as a crude, but insightful, first approach to the question of inherited, but unexpressed, characteristics. So useful to the biometricians was the theory of pangenesis that it continued to be used for some time after the "rediscovery" of Mendel's laws completely replaced it in the biological community. The two approaches were later merged in the 1930s by R.A. Fisher in what eventually became known as the modern synthesis.
External link
- On-line Facsimile Edition of The Variation of Animals and Plants Under Domestication (http://www.esp.org/books/darwin/variation/facsimile/title3.html) from Electronic Scholarly Publishing