Editing Modern synthesis (20th century) (section)

===Dobzhansky's evolutionary genetics, 1937===

{{further|Genetics and the Origin of Species}}

[[File:Drosophila pseudoobscura-Male.png|thumb|left|''[[Drosophila pseudoobscura]]'', the fruit fly which served as [[Theodosius Dobzhansky]]'s [[model organism]]]]

[[Theodosius Dobzhansky]], an immigrant from the [[Soviet Union]] to the [[United States]], who had been a postdoctoral worker in Morgan's fruit fly lab, was one of the first to apply genetics to natural populations. He worked mostly with ''[[Drosophila pseudoobscura]]''. He says pointedly: "Russia has a variety of climates from the Arctic to sub-tropical... Exclusively laboratory workers who neither possess nor wish to have any knowledge of living beings in nature were and are in a minority."<ref>{{harvnb|Mayr|Provine|1998|p=231}}</ref> Not surprisingly, there were other [[Russia]]n geneticists with similar ideas, though for some time their work was known to only a few in the [[Western world|West]]. His 1937 work ''[[Genetics and the Origin of Species]]''<ref>{{harvnb|Dobzhansky|1937}}</ref> was a key step in bridging the gap between population geneticists and field naturalists. It presented the conclusions reached by Fisher, Haldane, and especially Wright in their highly mathematical papers in a form that was easily accessible to others.<ref name="Larson221-243"/><ref name="Bowler325-339"/> Further, Dobzhansky asserted the physicality, and hence the biological reality, of the mechanisms of inheritance: that evolution was based on material genes, arranged in a string on physical hereditary structures, the [[chromosome]]s, and [[genetic linkage|linked]] more or less strongly to each other according to their actual physical distances on the chromosomes. As with Haldane and Fisher, Dobzhansky's "evolutionary genetics"<ref>{{harvnb|Smocovitis|1996|p=127}}</ref> was a genuine science, now unifying cell biology, genetics, and both micro and macroevolution.<ref name=Smocovitis122/> His work emphasized that real-world populations had far more genetic variability than the early population geneticists had assumed in their models and that genetically distinct sub-populations were important. Dobzhansky argued that natural selection worked to maintain genetic diversity as well as by driving change. He was influenced by his exposure in the 1920s to the work of [[Sergei Chetverikov]], who had looked at the role of recessive genes in maintaining a reservoir of genetic variability in a population, before his work was shut down by the rise of [[Lysenkoism]] in the [[Soviet Union]].<ref name="Larson221-243"/><ref name="Bowler325-339"/> By 1937, Dobzhansky was able to argue that mutations were the main source of evolutionary changes and variability, along with chromosome rearrangements, effects of genes on their neighbours during development, and polyploidy. Next, genetic drift (he used the term in 1941), selection, migration, and geographical isolation could change gene frequencies. Thirdly, mechanisms like ecological or sexual isolation and hybrid sterility could fix the results of the earlier processes.<ref>{{cite book |last=Eldredge |first=Niles |title=Unfinished Synthesis: Biological Hierarchies and Modern Evolutionary Thought |url=https://books.google.com/books?id=fEYdRMjhPC4C&pg=PR9 |year=1985 |publisher=[[Oxford University Press]] |isbn=978-0-19-536513-9 |page=17}}</ref>