Editing Modern synthesis (20th century) (section)

===Castle's hooded rats, 1911===

Starting in 1906, [[William Ernest Castle|William Castle]] carried out a long study of the effect of selection on coat colour in [[rat]]s. The [[piebald]] or hooded pattern was [[recessive]] to the grey wild type. He crossed hooded rats with both wild and "Irish" types, and then back-crossed the offspring with pure hooded rats. The dark stripe on the back was bigger. He then tried selecting different groups for bigger or smaller stripes for 5 generations and found that it was possible to change the characteristics considerably beyond the initial range of variation. This effectively refuted de Vries's claim that continuous variation was caused by the environment and could not be inherited. By 1911, Castle noted that the results could be explained by Darwinian selection on a heritable variation of a sufficient number of Mendelian genes.{{sfn|Provine|2001|pp=109–114}}<ref>{{cite journal|last=Castle |first=W. E. |date=7 April 1905 |title=The Mutation Theory of Organic Evolution, from the Standpoint of Animal Breeding |url=http://dx.doi.org/10.1126/science.21.536.521 |journal=[[Science (journal)|Science]] |volume=21 |issue=536 |pages=521–525 |doi=10.1126/science.21.536.521 |pmid=17770959 |bibcode=1905Sci....21..521C |issn=0036-8075}}</ref><ref>{{cite book |last=Vicedo |first=Marga |title=Castle, William Ernest (1867-1962), biologist |year=2000 |publisher=[[Oxford University Press]] |series=American National Biography Online |doi=10.1093/anb/9780198606697.article.1302308}}</ref>