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Charles Darwin's pangenesis theory postulated that every part of the body emits tiny particles called gemmules which migrate to the gonads and are transferred to offspring. Gemmules were thought to develop into their associated body parts as offspring matures. The theory implied that changes to the body during an organism's life would be inherited, as proposed in Lamarckism.

Pangenesis was Charles Darwin's hypothetical mechanism for heredity, in which he proposed that each part of the body continually emitted its own type of small organic particles called gemmules that aggregated in the gonads, contributing heritable information to the gametes.[1] He presented this 'provisional hypothesis' in his 1868 work The Variation of Animals and Plants under Domestication, intending it to fill what he perceived as a major gap in evolutionary theory at the time. The etymology of the word comes from the Greek words pan (a prefix meaning "whole", "encompassing") and genesis ("birth") or genos ("origin"). Pangenesis mirrored ideas orignally formulated by Hippocrates and other pre-Darwinian scientists, but built off of new concepts such as cell theory, explaining cell development as beginning with gemmules and accounting for regeneration and the Lamarckian concept of the inheritance of acquired characteristics, as a body part altered by the environment would produce altered gemmules. Gemmules were also specified to be necessary for the occurrence of new growths in an organism, both in initial development and regeneration.[2] This hypothesis was made effectively obsolete after the 1900 rediscovery among biologists of Gregor Mendel's theory of the particulate nature of inheritance.

Contents

Early historyEdit

Pangenesis was similar to ideas put forth by Hippocrates, Democritus and other pre-Darwinian scientists in proposing that the whole of parental organisms participate in heredity (thus the prefix pan).[3] Darwin wrote that Hippocrates' pangenesis was "almost identical with mine—merely a change of terms—and an application of them to classes of facts necessarily unknown to the old philosopher."[4]

Science historian Conway Zirkle wrote that:

The hypothesis of pangenesis is as old as the belief in the inheritance of acquired characters. It was endorsed by Hippocrates, Democritus, Galen (?), Clement of Alexandria, Lactantius, St. Isidore of Seville, Bartholomeus Anglicus, St. Albert the Great, St. Thomas of Aquinas, Peter of Crescentius (?), Paracelsus, Jerome Cardan, Levinus Lemnius, Venette, John Ray, Buffon, Bonnet, Maupertius, von Haller and Herbert Spencer.[3]

A theory put forth by Pierre Louis Maupertuis in 1745 called for particles from both parents governing the attributes of the child, although some historians have called his remarks on the subject cursory and vague.[5][6]

In 1749, French naturalist Georges-Louis Leclerc, Comte de Buffon developed a hypothetical system of heredity much like Darwin's pangenesis, wherein 'organic molecules' were transferred to offspring during reproduction and stored in the body during development.[6][7] Commenting on Buffon's views, Darwin stated, "If Buffon had assumed that his organic molecules had been formed by each separate unit throughout the body, his view and mine would have been very closely similar."[3]

In 1801, Erasmus Darwin advocated a hypothesis of pangenesis in the third edition of his book Zoonomia.[8] In 1809, Jean-Baptiste Lamarck in his Philosophie Zoologique first proposed the idea that characteristics acquired during the lifetime of an organism may be passed on to the offspring. Charles Darwin first had significant contact with Lamarckism during his time at the University of Edinburgh Medical School in the late 1820s, both through Robert Edmond Grant, whom he assisted in research, and in Erasmus's journals.[9] Darwin's his first known writings on the topic of Lamarckian ideas as they related to inheritance are found in a notebook he opened in 1837, also entitled Zoonomia.[10] Historian Johnathan Hodge states that the theory of pangenesis itself first appeared in Darwin's notebooks in 1841.[11]

In 1861, Irish physician Henry Freke developed a variant of pangenesis in his book Origin of Species by Means of Organic Affinity.[12] Freke proposed that all life was developed from microscopic organic agents which he named granules, which existed as 'distinct species of organizing matter' and would develop into different biological structures.[13]

In 1864, four years before the publication of Variation, Herbert Spencer in his book Principles of Biology proposed a theory of "physiological units" similar to Darwin's gemmules, which likewise were said to be related to specific body parts and responsible for the transmission of characteristics of those body parts to offspring.[4] He also supported the Lamarckian idea of transmission of acquired characteristics.

Darwin had debated whether to publish a theory of heredity for an extended period of time due to its highly speculative nature. He decided to include pangenesis in Variation after sending a manuscript to his close friend and supporter Thomas Huxley in May 1865, who responded advising Darwin to publish, writing: "Somebody rummaging among your papers half a century hence will find Pangenesis & say 'See this wonderful anticipation of our modern Theories—and that stupid ass, Huxley, prevented his publishing them'"[14] Darwin's initial version of pangenesis appeared in the first edition of Variation in 1868, and was later reworked for the publication of a second edition in 1875.

TheoryEdit

DarwinEdit

Darwin's pangenesis theory attempted to explain the process of sexual reproduction, inheritance of traits, and complex developmental phenomena such as cellular regeneration in a unified mechanistic structure.[15][16] Mechanistically, he proposed pangenesis to occur through the transfer of organic particles which he named 'gemmules.' Gemmules, which he also sometimes referred to as plastitudes,[17] pangenes, granules,[18] or germs, were supposed to be shed by the organs of the body and carried in the bloodstream to the reproductive organs where they accumulated in the germ cells or gametes..[19] Each gemmule was said to be specifically related to a certain body part- as described, they did not contain information about the entire organism. [18] The different types were assumed to be dispersed through the whole body, and capable of self-replication given ‘proper nutriment'. When passed on to offspring via the reproductive process, gemmules were thought to be responsible for developing into each part of an organism and expressing characteristics inherited from both parents. [18] Darwin thought this to occur in a literal sense: he explained cell proliferation to progress as gemmules to bind to more developed cells of their same character and mature. In this sense, the uniqueness of each individual would be due to their unique mixture of their parents’ gemmules. [18]

 
Darwin's description of cell proliferation using pangenesis theory in The Variation of Animals and Plants under Domestication

Some gemmules were thought to remain dormant for generations, whereas others were routinely expressed by all offspring. Every child was built up from selective expression of the mixture of the parents and grandparents' gemmules coming from either side. Darwin likened this to gardening: a flowerbed could be sprinkled with seeds "most of which soon germinate, some lie for a period dormant, whilst others perish."[20] He did not claim gemmules were in the blood, although his theory was often interpreted in this way. Responding to Fleming Jenkin's review of On the Origin of Species, he argued that pangenesis would permit the preservation of some favourable variations in a population so that they wouldn't die out through blending.[21]

Darwin thought that environmental effects that caused altered characteristics would lead to altered gemmules for the affected body part. The altered gemmules would then have a chance of being transferred to offspring, since they were assumed to be produced throughout an organisms life.[2] Thus, pangenesis theory allowed for the Lamarckian idea of transmission of acquired characteristics. Accidentally gemmule development in incorrect parts of the body could explain deformations and the 'monstrosities' Darwin cited in Variation.[2]

De VriesEdit

Hugo de Vries characterized his own version of pangenesis theory in two propositions, of which he only accepted the first:

I. In the cells there are numberless particles which differ from each other, and represent the individual cells, organs, functions and qualities of the whole individual. These particles are much larger than the chemical molecules and smaller than the smallest known organisms; yet they are for the most part comparable to the latter, because, like them, they can divide and multiply through nutrition and growth. They are transmitted, during cell-division, to the daughter-cells: this is the ordinary process of heredity.
II. In addition to this, the cells of the organism, at every stage of development, throw off such particles, which are conducted to the germ-cells and transmit to them those characters which the respective cells may have acquired during development.[22]

Other variantsEdit

Science historian Janet Browne points out that while Herbert Spencer and Carl von Nägeli also put forth ideas for systems of inheritance involving gemmules, their version of gemmules differed in that it contained "a complete microscopic blueprint for an entire creature."[23]

She goes on to say that Darwin believed specifically in gemmules for each body part because they might explain how environmental effects could be passed on as characteristics to offspring.[23]

CollapseEdit

Galton's experiments on rabbitsEdit

Darwin's half-cousin Francis Galton conducted wide-ranging inquiries into heredity which led him to refute Charles Darwin's hypothetical theory of pangenesis. In consultation with Darwin, he set out to see if gemmules were transported in the blood. In a long series of experiments from 1869 to 1871, he transfused the blood between dissimilar breeds of rabbits, and examined the features of their offspring. He found no evidence of characters transmitted in the transfused blood.[24]

Galton was troubled because he began the work in good faith, intending to prove Darwin right, and having praised pangenesis in Hereditary Genius in 1869. Cautiously, he criticized his cousin’s theory, although qualifying his remarks by saying that Darwin's gemmules, which he called "pangenes," might be temporary inhabitants of the blood that his experiments had failed to pick up.[25]

Darwin challenged the validity of Galton's experiment, giving his reasons in an article published in Nature where he wrote:[26]

Now, in the chapter on Pangenesis in my Variation of Animals and Plants under Domestication, I have not said one word about the blood, or about any fluid proper to any circulating system. It is, indeed, obvious that the presence of gemmules in the blood can form no necessary part of my hypothesis; for I refer in illustration of it to the lowest animals, such as the Protozoa, which do not possess blood or any vessels; and I refer to plants in which the fluid, when present in the vessels, cannot be considered as true blood." He goes on to admit: "Nevertheless, when I first heard of Mr. Galton's experiments, I did not sufficiently reflect on the subject, and saw not the difficulty of believing in the presence of gemmules in the blood.[26]

After the circulation of Galton's results, the perception of pangenesis quickly changed to severe skepticism if not outright disbelief.[27]

WeismannEdit

 
August Weismann's germ plasm theory. The hereditary material, the germ plasm, is confined to the gonads. Somatic cells (of the body) develop afresh in each generation from the germ plasm. The implied Weismann barrier between the germ line and the soma prevents Lamarckian inheritance.

August Weismann's idea, set out in his 1892 book Das Keimplasma: eine Theorie der Vererbung (The Germ Plasm: a Theory of Inheritance),[28] was that the hereditary material, which he called the germ plasm, and the rest of the body (the soma) had a one-way relationship: the germ-plasm formed the body, but the body did not influence the germ-plasm, except indirectly in its participation in a population subject to natural selection. If correct, this made Darwin's pangenesis wrong and Lamarckian inheritance impossible. His experiment on mice, cutting off their tails and showing that their offspring had normal tails, was proposed as a proof of the non-existence of Lamarckian inheritance, although Peter Gauthier has argued that Weismann's experiment showed only that injury did not affect the germ plasm and neglected to test the effect of Lamarckian use and disuse.[29] Weismann argued strongly and dogmatically for Darwinism and against Lamarckism, polarising opinions among other scientists. This increased anti-Darwinian feeling, contributing to its eclipse.[30][31]

After pangenesisEdit

Darwin's pangenesis theory was widely criticised, in part for its Lamarckian premise that parents could pass on traits acquired in their lifetime.[32] Lamarckism fell from favour after August Weismann's research in the 1880s indicated that changes from use (such as lifting weights to increase muscle mass) and disuse (such as being lazy and becoming weak) were not heritable.[33][34] The rediscovery of Mendel's Laws of Inheritance in 1900 confirmed pangenesis's fallaciousness.[35]

Some of Darwin's pangenesis principles do relate to heritable aspects of phenotypic plasticity, although the status of gemmules as a distinct class of organic particles has been firmly rejected. However, starting in the 1950s and 1970s, many research groups in revisiting Galton's experiments found that heritable characteristic could arise following DNA injection or blood transfusion.[27] Further research of this heritability of acquired characteristics developed into, in part, the modern field of epigenetics. Darwin himself had noted that "the existence of free gemmules is a gratuitous assumption"; by some accounts in modern interpretation, gemmules may be considered a prescient mix of DNA, RNA, proteins, prions, and other mobile elements that are heritable in a non-Mendelian manner at the molecular level.[15][36][37]

See alsoEdit

ReferencesEdit

  1. ^ Holterhoff, Kate (2014). "The History and Reception of Charles Darwin's Hypothesis of Pangenesis". Journal of the History of Biology. 47: 661–695. doi:10.1007/s10739-014-9377-0. 
  2. ^ a b c de Beer, Gavin (1965). Charles Darwin: A Scientific Biography. Garden City, New York: Doubleday & Company. p. 203. 
  3. ^ a b c Zirkle, Conway (1935). "The Inheritance of Acquired Characters and the Provisional Hypothesis of Pangenesis". The American Naturalist. 69: 417–445. doi:10.1086/280617. 
  4. ^ a b Deichmann, Ute. (2010). Darwinism, Philosophy, and Experimental Biology. Springer. pp. 41-42. ISBN 978-90-481-9901-3
  5. ^ Mayr, Ernst (1981). The Growth of Biological Thought. Harvard University Press. pp. 328, 646. ISBN 978-0674364462. 
  6. ^ a b de Beer, Gavin (1965). Charles Darwin: A Scientific Biography. Garden City, New York: Doubleday & Company. p. 205. 
  7. ^ Hull, David L. (1988). Science as a Process: An Evolutionary Account of the Social and Conceptual Development of Science. University of Chicago Press. p. 86. ISBN 0-226-36051-2 "As Darwin was to discover many years later, Buffon had devised a system of heredity not all that different from his own theory of pangenesis."
  8. ^ Deichmann, Ute. (2010). Darwinism, Philosophy, and Experimental Biology. Springer. p. 42. ISBN 978-90-481-9901-3 "Among the other authors were Buffon, who proposes "organic molecules" with affinities to various organs, and, in particular, Erasmus Darwin, who in 1801 anticipated his grandson's concept of pangenesis, suggesting that small particles were given off by parts of the bodies of both parents; and that they are circulated in the blood, ending in the sexual organs from where they could be combined during reproduction in order to form the nucleus of an offspring."
  9. ^ Browne, E. Janet (1995). Charles Darwin: Vol. 1, Voyaging. London: Jonathan Cape. pp. 72–88. ISBN 1-84413-314-1. 
  10. ^ The Cambridge companion to Darwin. Hodge, M. J. S. (Michael Jonathan Sessions), 1940-, Radick, Gregory. Cambridge, UK: Cambridge University Press. 2003. pp. 40–41. ISBN 0511077696. OCLC 57383252. 
  11. ^ The Cambridge companion to Darwin. Hodge, M. J. S. (Michael Jonathan Sessions), 1940-, Radick, Gregory. Cambridge, UK: Cambridge University Press. 2003. p. 63. ISBN 0511077696. OCLC 57383252. 
  12. ^ Macalister, Alexander. (1870). Reviews and Bibliographical Notices. In Dublin Quarterly Journal of Medical Science, Volume 50. Fannin and Company. p. 131
  13. ^ Freke, Henry (1861). On the origin of species by means of organic affinity. Longman and Co. 
  14. ^ Letter 4875 – Huxley, T. H. to Darwin, C. R., 16 July (1865), Darwin Correspondence Project 
  15. ^ a b Geison, G. L. (1969). "Darwin and heredity: The evolution of his hypothesis of pangenesis". J Hist Med Allied Sci. XXIV (4): 375–411. doi:10.1093/jhmas/XXIV.4.375. 
  16. ^ Jablonka, E.; Lamb, M. (2005). Evolution in four dimensions: Genetic, epigenetic, behavioural and symbolic. MIT Press. ISBN 0-262-10107-6. 
  17. ^ Allaby, Michael. Animals: From Mythology to Zoology "Plastitude"
  18. ^ a b c d Browne, Janet (2002). Charles Darwin--The Power of Place. London: Jonathon Cape. p. 275. 
  19. ^ Darwin, Charles (1868). The variation of animals and plants under domestication. London: John Murray. ISBN 1-4191-8660-4. 
  20. ^ Browne 2002, p. 276.
  21. ^ Browne 2002, p. 283.
  22. ^ de Vries, Hugo (1910) [1889]. Intracellular Pangenesis. p. 63. Retrieved May 2, 2015. 
  23. ^ a b Browne 2002, p. 281.
  24. ^ (Bulmer 2003, pp. 116–118)
  25. ^ Browne 2002, p. 291–292.
  26. ^ a b Darwin, Charles R. (27 April 1871). "Pangenesis". Nature. A Weekly Illustrated Journal of Science. 3: 502–503. 
  27. ^ a b Liu, Yongsheng (2008). "A new perspective on Darwin's Pangenesis". Biological Reviews. 83 (2): 141–149. doi:10.1111/j.1469-185x.2008.00036.x. 
  28. ^ Weismann, August (1892). Das Keimplasma: eine Theorie der Vererbung [The Germ Plasm: A theory of inheritance]. Jena: Fischer. 
  29. ^ Gauthier, Peter (March–May 1990). "Does Weismann's Experiment Constitute a Refutation of the Lamarckian Hypothesis?". BIOS. 61 (1/2): 6–8. JSTOR 4608123. 
  30. ^ Bowler, Peter J. (2003). Evolution: The History of an Idea (3rd completely rev. and expanded ed.). Berkeley, CA: University of California Press. pp. 253–256. ISBN 978-0-520-23693-6. 
  31. ^ Bowler, Peter J. (1989). Evolution: The History of an Idea (2nd revised ed.). Berkeley, CA: University of California Press. pp. 247–253, 257. ISBN 978-0-520-06386-0. 
  32. ^ Liu, Yongsheng; Li, Xiuju (2014-09-23). "Has Darwin's Pangenesis Been Rediscovered?". BioScience. 64 (11): 1037–1041. doi:10.1093/biosci/biu151. ISSN 0006-3568. 
  33. ^ Ghiselin, Michael T. (September–October 1994). "Nonsense in schoolbooks: 'The Imaginary Lamarck'". The Textbook Letter. The Textbook League. Retrieved 2008-01-23. 
  34. ^ Magner, Lois N. (2002). A History of the Life Sciences (Third ed.). Marcel Dekker, CRC Press. ISBN 978-0-203-91100-6. 
  35. ^ de Beer, Gavin (1965). Charles Darwin: A Scientific Biography. Garden City, NY: Doubleday & Company. p. 206. 
  36. ^ West-Eberhard, M. J. (2008). "Toward a modern revival of Darwin's theory of evolutionary novelty" (PDF). Philosophy of Science. 75 (5): 899–908. doi:10.1086/594533. JSTOR 10. 
  37. ^ Liu, Y. S.; Zhou, X. M.; Zhi, M. X.; Li, X. J.; Wan, Q. L. (2009). "Darwin's contributions to genetics" (PDF). J Appl Genetics. 50 (3): 177–184. doi:10.1007/BF03195671. PMID 19638672. Archived from the original (PDF) on 2012-03-30. 

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