Editing Modern synthesis (20th century) (section)

==Developments leading up to the synthesis==

[[File:Darwin's Pangenesis.svg|thumb|upright=1.35|[[Charles Darwin|Darwin]]'s [[pangenesis]] theory. Every part of the body emits tiny gemmules which migrate to the [[gonad]]s and contribute to the next generation via the fertilised egg. Changes to the body during an organism's life would be inherited, as in [[Lamarckism]].]]

{{further|History of evolutionary thought}}

===Darwin's evolution by natural selection, 1859===

{{main|Evolution|Natural selection}}

[[Charles Darwin]]'s 1859 book, ''[[On the Origin of Species]],'' convinced most biologists that [[evolution]] had occurred, but not that [[natural selection]] was its primary mechanism. In the 19th and early 20th centuries, variations of [[Lamarckism]] (inheritance of acquired characteristics), [[orthogenesis]] (progressive evolution), [[saltationism]] (evolution by jumps) and [[mutationism]] (evolution driven by mutations) were discussed as alternatives.<ref>{{harvnb|Bowler|2003|pp=236–256}}</ref> Darwin himself had sympathy for Lamarckism, but [[Alfred Russel Wallace]] advocated natural selection and totally rejected Lamarckism.<ref>{{cite journal |last=Kutschera |first=Ulrich |author-link=Ulrich Kutschera |date=December 2003 |title=A comparative analysis of the Darwin–Wallace papers and the development of the concept of natural selection |journal=Theory in Biosciences |volume=122 |issue=4 |pages=343–359 |doi=10.1007/s12064-003-0063-6|s2cid=24297627 }}</ref> In 1880, Samuel Butler labelled Wallace's view [[neo-Darwinism]].<ref>{{cite book |last1=Butler |first1=Samuel |author-link1=Samuel Butler (novelist)|title=Unconscious Memory |date=1880 |publisher=David Bogue |page=280 |url=https://archive.org/details/unconsciousmemo00hartgoog |quote=I may predict with some certainty that before long we shall find the original Darwinism of Dr. [[Erasmus Darwin]] … generally accepted instead of the neo-Darwinism of to-day, and that the variations whose accumulation results in species will be recognised as due to the wants and endeavours of the living forms in which they appear, instead of being ascribed to chance, or, in other words, to unknown causes, as by Mr. Charles Darwin's system}}</ref><ref>{{cite web |url=http://wallacefund.info/terms-darwinism-and-neo-darwinism |title=On the Terms "Darwinism" and "Neo-Darwinism" |last=Beccaloni |first=George |date=2013 |website=A. R. Wallace Website |access-date=2017-09-14 |archive-date=2019-04-11 |archive-url=https://web.archive.org/web/20190411231439/http://wallacefund.info/terms-darwinism-and-neo-darwinism |url-status=dead }}</ref>

[[File:Blending Inheritance.svg|thumb|upright=1.1|[[Blending inheritance]], implied by pangenesis, causes the [[mean|averaging out]] of every characteristic, which as the engineer [[Fleeming Jenkin]] pointed out, would make evolution by [[natural selection]] impossible.]]

===The eclipse of Darwinism, 1880s onwards===

{{main|The eclipse of Darwinism}}

From the 1880s onwards, biologists grew skeptical of Darwinian evolution. This [[eclipse of Darwinism]] (in [[Julian Huxley]]'s words) grew out of the weaknesses in Darwin's account, with respect to his view of inheritance. Darwin believed in [[blending inheritance]], which implied that any new variation, even if beneficial, would be weakened by 50% at each generation, as the engineer [[Fleeming Jenkin]] noted in 1868.<ref name="Bowler196-250">{{harvnb|Bowler|2003|pp=196–253}}</ref><ref name="Larson105-129">{{harvnb|Larson|2004|pp=105–129}}</ref> This in turn meant that small variations would not survive long enough to be selected. Blending would therefore directly oppose natural selection. In addition, Darwin and others considered Lamarckian inheritance of acquired characteristics entirely possible, and Darwin's 1868 theory of [[pangenesis]], with contributions to the next generation (gemmules) flowing from all parts of the body, actually implied Lamarckism as well as blending.<ref>{{cite book |author=Gayon, Jean |date=1998 |title=Darwinism's Struggle for Survival: Heredity and the Hypothesis of Natural Selection |publisher=Cambridge University Press |pages=2–3 |isbn=978-0-521-56250-8}}</ref><ref name="Darwin68">{{cite book | last=Darwin | first=Charles | author-link=Charles Darwin | year=1868 | title=The variation of animals and plants under domestication | publisher=John Murray | url=http://darwin-online.org.uk/content/frameset?itemID=F880.1&viewtype=text&pageseq=1 | isbn=978-1-4191-8660-8}}</ref><ref>{{cite journal |author=Holterhoff, Kate |date=2014 |title=The History and Reception of Charles Darwin's Hypothesis of Pangenesis |journal=Journal of the History of Biology |volume=47 |issue=4 |pages=661–695 |doi=10.1007/s10739-014-9377-0|pmid=24570302 |s2cid=207150548 }}</ref>

[[File:Weismann's Germ Plasm.svg|thumb|upright=1.35|[[August Weismann]]'s [[germ plasm]] theory. The hereditary material, the germ plasm, is confined to the [[gonad]]s and the [[gamete]]s. Somatic cells (of the body) [[embryology|develop]] afresh in each generation from the germ plasm.]]

===Weismann's germ plasm, 1892===

{{main|Germ plasm}}

[[August Weismann]]'s idea, set out in his 1892 book ''Das Keimplasma: eine Theorie der Vererbung'' ("The Germ Plasm: a Theory of Inheritance"),<ref>{{cite book |author=Weismann, August |date=1892 |title=Das Keimplasma: eine Theorie der Vererbung |trans-title=The Germ Plasm: A theory of inheritance |publisher=Fischer |location=Jena |url=http://www.deutschestextarchiv.de/book/show/weismann_keimplasma_1892}}</ref> was that the hereditary material, which he called the [[germ plasm]], and the rest of the body (the [[Somatic (biology)|soma]]) had a one-way relationship: the germ-plasm formed the body, but the body did not influence the germ-plasm, except indirectly in its participation in a population subject to natural selection. If correct, this made Darwin's pangenesis wrong, and Lamarckian inheritance impossible. His experiment on mice, cutting off their tails and showing that their offspring had normal tails, demonstrated that inheritance was 'hard'.{{efn|Peter Gauthier has however argued that [[Weismann's experiment]] showed only that injury did not affect the germplasm. It did not test the effect of Lamarckian use and disuse.<ref>{{cite journal |last=Gauthier |first=Peter |date=March–May 1990 |title=Does Weismann's Experiment Constitute a Refutation of the Lamarckian Hypothesis? |journal=BIOS |volume=61 |number=1/2 |pages=6–8 |jstor=4608123}}</ref>}} He argued strongly and dogmatically{{sfn|Bowler|1989|p=248}} for Darwinism and against Lamarckism, polarising opinions among other scientists. This increased anti-Darwinian feeling, contributing to its eclipse.<ref>{{harvnb|Bowler|2003|pp=253–256}}</ref>{{sfn|Bowler|1989|pp=247–253, 257}}