Introduction to evolution: Difference between revisions

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== Source of variation ==

Darwin's theory of natural selection laid the groundwork for modern evolutionary theory, and his experiments and observations showed that the organisms in populations varied from each other, that some of these variations were inherited, and that these differences could be acted on by natural selection. However, he could not explain the source of these variations. Like many of his predecessors, Darwin mistakenly thought that heritable traits were a product of use and disuse, and that features acquired during an organism's lifetime could be passed on to its offspring. He looked for examples, such as large ground feeding [[bird]]s getting stronger legs through exercise, and weaker wings from not flying until, like the [[ostrich]], they could not fly at all.<ref name=toos>{{harvnb|Darwin|1872|p=[http://darwin-online.org.uk/content/frameset?itemID=F391&viewtype=text&pageseq=136 108]}}, "Effects of the increased Use and Disuse of Parts, as controlled by Natural Selection"</ref> This misunderstanding was called the [[Inheritance of acquired characteristics|inheritance of acquired characters]] and was part of the theory of [[transmutation of species]] put forward in 1809 by [[Jean-Baptiste Lamarck]]. In the late 19th century this theory became known as [[Lamarckism]]. Darwin produced an unsuccessful theory he called [[pangenesis]] to try to explain how acquired characteristics could be inherited. In the 1880s [[August Weismann]]'s experiments indicated that changes from use and disuse could not be inherited, and Lamarckism gradually fell from favour.<ref name="ImaginaryLamarck">{{cite journal |last=Ghiselin |first=Michael T. |author-link=Michael Ghiselin |date=September–October 1994 |title=The Imaginary Lamarck: A Look at Bogus 'History' in Schoolbooks |url=http://www.textbookleague.org/54marck.htm |journal=The Textbook Letter |___location=Sausalito, CA |publisher=The Textbook League |oclc=23228649 |access-date=2008-01-23 |archive-url=https://web.archive.org/web/20080212174536/http://www.textbookleague.org/54marck.htm |archive-date=2008-02-12 |url-status=deadusurped }}</ref>

The missing information needed to help explain how new features could pass from a parent to its offspring was provided by the pioneering genetics work of [[Gregor Mendel]]. Mendel's experiments with several generations of pea plants demonstrated that inheritance works by separating and reshuffling hereditary information during the formation of sex cells and recombining that information during fertilisation. This is like mixing different hands of [[playing cards]], with an organism getting a random mix of half of the cards from one parent, and half of the cards from the other. Mendel called the information ''factors''; however, they later became known as genes. Genes are the basic units of heredity in living organisms. They contain the information that directs the physical development and behaviour of organisms.