What is Speciation?
'''Speciation''' is about how [[species]] form. It is a major part of [[evolutionary biology]]. [[Charles Darwin|Darwin]] thought most [[species]] came directly from pre-existing species. This is called '''anagenesis''': species by changing, or 'phyletic evolution'. For much of the 20th century, scientists thought most species came when earlier species split. This is called '''cladogenesis'''.<ref>{{cite journal | doi = 10.1126/science.23.587.506 | author = Cook O.F. | year = 1906 | title = Factors of species-formation | url = https://zenodo.org/record/1447954| journal = Science | volume = 23 | issue = 587| pages = 506–507 | pmid = 17789700 | bibcode = 1906Sci....23..506C }}</ref><ref>{{cite journal | doi = 10.1086/279001 | author = Cook O.F. | year = 1908 | title = Evolution without isolation | url =https://archive.org/details/sim_american-naturalist_1908-11_42_503/page/727| journal = American Naturalist | volume = 42 | issue = 503| pages = 727–731 | s2cid = 84565616 }}</ref> The general view was that most species splitting is caused or helped on its way by '''isolating mechanisms'''.<ref>{{cite web| last =King| first =David| title =The concept of species| work =Zoology 304, Evolution| publisher =| date =2003| url =http://www.science.siu.edu/zoology/King/304/species.htm#15.2.3| accessdate =2007-07-18| archive-date =2007-05-29| archive-url =https://web.archive.org/web/20070529014449/http://www.science.siu.edu/zoology/king/304/species.htm#15.2.3| url-status =dead}}</ref><ref>{{cite web| last =Mallet| first =J.L.B. | title =Isolating mechanisms| date =1998| url =http://abacus.gene.ucl.ac.uk/jim/Sp/isolmech.html| accessdate = 2007-07-18 }}</ref> No doubt the physical separation of species which once lived together is a main factor. It is illustrated by so many examples, some of which are discussed below. However, work in the last 20 years has shown some other causes. Analysing the [[DNA sequence]] of living things has shown that there is often some [[hybrid]]isation between related species. That means [[genes]] h