darwin’s)two)objec0ves:)adamson/biol446/postdarwin.pdf · darwin’s)two)objec0ves:)...
TRANSCRIPT
Darwin’s two objec0ves:
1. Show that organisms are related through common descent with modifica0on.
2. Show that natural selec0on was the main cause of this transforma0on.
Darwin’s problem
• Why don’t new variants simply regress back into the original popula0on?
• Large areas more important for Darwin even than isola0on.
Heredity
• Current sense: traits which result from passage of gene0c material (DNA) from one genera0on to next.
• Opposed to everything caused by physiological response to the environment.
• Nature vs nurture.
The problem of reversion and inheritance
Francis Galton 1822 to 1911. Bri0sh polymath, Darwin’s cousin.
Cowan, R.S. 1972.
• Galton’s contribu0on to gene0cs. Journal of the History of Biology 5 (2), pp. 389-‐412
Problem for Darwin’s theory
• Small progressive evolu0onary change will be ineffec0ve because new variants will inevitably breed with old types and regress to the mean.
• Suggested that change requires “sports”; true breeding discon0nuous changes. But these were a problem for Darwin who rejected them as contributors to evolu0on.
Darwin’s “pre-‐potent” characters
• If the organism is stressed, this compromises the forces of inheritance and allows the appearance of pre-‐potent characters which then dominate. E.g. cross two differently colored pigeons…can result in appearance of the slate-‐blue variety;
Theories on varia0on
• Varia0on: tendency of offspring to differ slightly from their parents.
• Inheritance: tendency of parents to produce like offspring.
• Are there two different forces? Or one force which fails to work perfectly?
• P. Lucas 1847. Traite philosophique et physiologique de l’heredite. 2 Volumes. – Varia0on occurs aeer birth (Environmental factors)
– Varia0on occurs aeer concep0on before birth (maternal factors)
– Varia0on occurs at concep0on (maternal/paternal inheritance.
– Varia0on occurs at concep0on (past genera0onal inheritance)
Darwin’s interpreta0on
• Reproduc0ve system unusually sensi0ve to environmental change.
• Extreme condi0ons provoke it to develop plas0city in offspring.
Agacks on Darwin’s posi0on on varia0on
• Varia0on random. New species arise by chance.
• If environmental effects are so crucial, then adaptability is not so important.
• If con0nuous varia0on are the “stuff” of evolu0on (and these are minor and con0nuous) then change can never surpass the “species” line.
Pangenesis
• “As Darwin explained it, pangenesis was the highly abstract no0on that every 0ssue, cell and living part of an organism produced minute, unseen gemmules (or what he some0mes called granules or germs) which carried inheritable characteris0cs and were transmiged to the offspring via the reproduc0ve process. “
Galton’s ancestral law
• Parents each contribute 1/4; grandparent 1/8, great grandparent 1/16, etc. of heritage to their offspring.
• Small con0nuous change doesn’t contribute to evolu0on but produces varia0on about a mean.
• Raw materials of evolu0on are discon0nuous varia0ons or “sports”.
Galton’s polyhedron
Galton on fingerprints
Orthogene0c theories
William Bateson 1861 to 1926. English gene0cist.
Hugo de Vries. 1848 to 1935. Dutch gene0cist
Oenothera spp.
De Vries muta0on theory
• Muta0ons give direc0on to evolu0on, not the natural selec0on for variants represen0ng con0nuous change.
• Saw himself with Darwin and Lamarck as a prime contributor to evolu0on and the originator of an experimental approach to the subject.
August Weismann 1834 to 1914. German evolu0onary biologist
Rudolph Leuckart 1822 to 1898. German Parasitologist/microscopist
One of Leuckart’s posters
Leuckart’s posters…
Weismann at Albert Ludwig University of Frieberg, Breisgau
• Cell theory: Schleiden and Schwann in early 1800s; completed by Virchow
• Earlier German workers (Van Beneden, Strasburger) established difference between cytoplasm and nucleoplasm in cells
• Mitosis discovered by Walther Flemming
Chroma0c aberra0on and its correc0on with double lens design
Aniline dyes and other stains
• Allowed more differen0a0on of internal cons0tuents of cells.
• Metachroma0c effects
Meiosis
• Described by Hertwig 1876 in seas urchin eggs.
• Later van Beneden described the process in Ascaris
• 1890 Weissman proposes the biologcal significance of meiosis-‐-‐-‐reduc0on of chromosome number in prepara0on for fer0liza0on.
Weismann recognizes significance of meiosis
Parascaris and chroma0n diminu0on
Weismann 1883. “On inheritance”
• Proposed that germ plasm which would be used to create further genera0ons of individuals, separated from the rest of the embryo early in development.
• Protected from any further modifica0ons during the lifespan of the organism.
Experiments in Lamarckian inheritance
• Cuts tails off mice over many genera0ons; was unable to change the tail’s poten0al in any way.
• Experiment was not seen as cri0cal because Lamarckians saw it as invalid; no use/disuse was involved.
• But Weismann’s interpreta0on of meiosis gave the physical basis for Mendels theory.
“Eclipse of Darwinism” 1880 to 1916
Asa Gray 1810 to 1888. American Botanist
Edward Drinker Cope. 1840 to 1897. American Paleontologist, Ichthyologist,,
Herpetologist
Walter Sugon 1877 to 1916, American gene0cist
Meanwhile…back in Gene0cs
• Developed from de Vries original experiments.
• Thomas Hunt Morgan. 1866 to 1945. American evolu0onary biologist, gene0cist.
Sex linked inheritance
A Cri&que of the Theory of Evolu&on (1916)
• Elaborated his theory of how genes are Mendel’s factors.
• Selec0on acts by incorpora0ng beneficial muta0ons into the popula0on (selec0on)
• Established the grounding for the Darwinian synthesis organisms
Gene0cs and Eugenics
• Instrumental ra0onality: we can solve our problems by using science.
• Dangerous when used in conjunc0on with a “promethean” outlook wherein man dominates (rather than is part of) nature.
Plato’s Republic
• In his republic argued for careful screening of mates to produce the best children.
Francis Galton
Francis Galton and Biometrics
• Studied genius; saw that genius was clustered in families.
• Ini0ated twin studies b/c he recognized that nurture and nature had to be separated to understand cause.
Galton, cont’d
• Socie0es that shelter their weak were degrading human stock
• Advised against helping the poor • Also advised careful marriages among the most gieed (ie Europeans).
• Coined “eugenics” in 1883.
Davenport 1866 to 1944. biologist and gene0cist
Madison Grant. 1865 to 1937. lawyer and eugenicist
Franz Boas. 1858 Germany–1942 US. Anthropologist and opponent of
eugenics
Policies emboldened by eugenics
• Family planning: Margaret Sanger et alia. Targeted black popula0ons with birth control.
• Forced steriliza0on programs for racial minori0es and the poor.
• Immigra0on controls; some0mes immigrants were hunted and returned to their na0ve country.
Problems with eugenics
• Is there a problem to solve? Overemphasied determinsim of genes; ignored environment
• Misrepresented role of genes; treated a strictly determinis0c. But depends on interac0on with environment
• Misrepresented the power of ar0ficial selec0on. E.g., can a controlled breeding program of a minority of humans swamp the rest of the popula0on? Does selec0ve breeding for a few genes improve the lineage?
Overall effect of eugenics
• Prevent poor (including racial minori0es) from reproducing or from accessing resources for their families.
• Policies, ostensibly based in science, were notoriously secre0ve and coercive rather than informa0ve and permissive.
God and Science in the 20th century
• Since the enlightenment, religion had seen nature as illustra0ve of Gods works
• Darwin explained the remarkable fit of organisms to their environment without using the concept of God.
• This lee ligle for religion to explain
Frederich Nietzsche
• “God is dead” • “superman” is the expression of God in every man
Scien0fic atheism
• William Draper 1871. The History of the Conflict between Religion and Science
• Contrasted religion as based on myth and belief without evidence
• Science based on evidence and experiment
Andrew Dixon White 1832 to 1911. American educator
Among Scien0sts
• Religion is a more private affair • Scien0st less likely to cri0cize religion; more likely to see religion as pertaining to different areas of their life.
• E.g. Thomas Huxley coined the term agnosicism to describe his posi0on
e.g. Huxley versus Wilberforce 1860
• The great debate. • Lucas, J. R. (1979). "Wilberforce and Huxley: a legendary encounter". The Historical Journal 22 (2): 313–330.
The Scopes Monkey Trial
Sir Ronald Fisher, Bri0sh Sta0s0cian, gene0cist 1890 to 1962
R.A. Fisher 1918. The correla0on between rela0ves on the supposi0on of mendelian inheritance. Trans. Roy Soc Edinburgh 52: 399–433.
Richard Dawkins. 1941. English Biologist
• E.g. The God delusion
Sir Ronald Fisher, Bri0sh Sta0s0cian, gene0cist 1890 to 1962
Fisher, Ronald A. (1921). "On the "Probable Error" of a Coefficient of Correla0on Deduced from a Small Sample". Metron. 1: 3–32. Fisher, R. A. (1930). The Gene0cal Theory of Natural Selec0on. Clarendon Press.
Sewell Wright, 1889 to 1988 American gene0cist
Provine, William (1986). Sewall Wright and Evolu0onary Biology. University of Chicago Press.
Wright, S. (1921). "Correla0on and causa0on". J. Agricultural Research. 20: 557–585
Cavia porcellus
F-‐sta0s0cs
Shieing balance theory
• Popula0on structure is key to understanding how evolu0on will work.
• Muta0on and migra0on have comparable effects (though their sizes are oeen disparate)
• Selec0on and drie can work together to produce change (most powerful effect).
Fitness landscapes
Wright and Fisher
• Very different approaches to their science • Very different outlooks on how evolu0on worked
• Complemented each others work greatly • Along with J.B.S. Haldane created the new synthesis which combined gene0cs with ecology in evolu0onary theory.
Elements lee out of the new synthesis
• Development appreciated by Wright as a complica0ng factor, but not well integrated into theory.
• This occurred (and is occurring) only recently. See for example “evo-‐devo” and “epigene0cs”