Editing Modern synthesis (20th century) (section)

===Mayr's allopatric speciation, 1942===

{{main|Systematics and the Origin of Species|Allopatric speciation}}

[[File:Ernst Mayr PLoS.jpg|thumb|left|[[Ernst Mayr]] argued that [[allopatric speciation|geographic isolation]] was needed to provide sufficient [[reproductive isolation]] for [[speciation|new species to form]].]]

[[Ernst Mayr]]'s key contribution to the synthesis was ''[[Systematics and the Origin of Species]]'', published in 1942.<ref>{{harvnb|Mayr|1999}}</ref> It asserted the importance of and set out to explain population variation in evolutionary processes including speciation. He analysed in particular the effects of [[Taxon|polytypic]] species, geographic variation, and isolation by geographic and other means.<ref>{{cite journal |last1=Hey |first1=Jody |last2=Fitch |first2=Walter M. |last3=Ayala |first3=Francisco J. |title=Systematics and the origin of species: An introduction |journal=[[PNAS]] |date=2005 |volume=102 |issue=supplement 1 |pages=6515–6519 |doi=10.1073/pnas.0501939102 |pmid=15851660|bibcode=2005PNAS..102.6515H |pmc=1131868 |doi-access=free }}</ref> Mayr emphasized the importance of [[allopatric speciation]], where geographically isolated sub-populations diverge so far that [[reproductive isolation]] occurs. He was skeptical of the reality of [[sympatric speciation]] believing that geographical isolation was a prerequisite for building up intrinsic (reproductive) isolating mechanisms. Mayr also introduced the [[Species problem#Biological species concept|biological species concept]] that defined a species as a group of interbreeding or potentially interbreeding populations that were reproductively isolated from all other populations.<ref name="Larson221-243" /><ref name="Bowler325-339" /><ref>{{harvnb|Mayr|Provine|1998|pp=33–34}}</ref>{{sfn|Mayr|1982}} Before he left [[Germany]] for the United States in 1930, Mayr had been influenced by the work of the German biologist [[Bernhard Rensch]], who in the 1920s had analyzed the geographic distribution of polytypic species, paying particular attention to how variations between populations correlated with factors such as differences in climate.<ref>{{harvnb|Rensch|1947}}; {{harvnb|Rensch|1959}}</ref><ref>{{cite web |url=http://people.wku.edu/charles.smith/chronob/RENS1900.htm |title=Rensch, Bernhard (Carl Emmanuel) (Germany 1900-1990) |last=Smith |first=Charles H. |author-link=Charles H. Smith (historian of science) |work=Some Biogeographers, Evolutionists and Ecologists: Chrono-Biographical Sketches |publisher=[[Western Kentucky University]]  |access-date=14 December 2017}}</ref><ref>{{harvnb|Mayr|Provine|1998|pp=298–299, 416}}</ref>

[[File:Equine evolution.jpg|thumb|upright=1.2|[[George Gaylord Simpson]] argued against the naive view that [[evolution of the horse|evolution such as of the horse]] took place in a "straight-line". He noted that any chosen line is one path in a complex branching tree, [[natural selection]] having no [[orthogenesis|imposed direction]].]]