From Darwin to the Modern Synthesis

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Organic Evolution

• From Darwin to the Modern Synthesis

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Predecessors

• The idea of evolution had been around since the
  times of the ancient Greeks
• The problem was the explanatory mechanism, in
  other words, how evolution works?

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Erasmus Darwin (1731-1802)

• One of the leading intellectuals of eighteenth
  century England. Member of The Lunar Society
• Remarkable array of interests and pursuits
• A respected physician, a well known poet,
  philosopher, botanist, and naturalist

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• As a naturalist, he formulated one of the first
  formal theories on evolution in Zoonomia, or, The
  Laws of Organic Life (1794-1796)
• This was a very popular book translated into
  several languages, published 4 years before
  Malthus' Essay on Population and 9 years before
  Lamarck's published explanation on the theory of
  use and disuse

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• He also presented his evolutionary ideas in
  verse, in particular in the posthumously
  published poem The Temple of Nature (1802)
• Organic life beneath the shoreless waves
• Was born and nurs'd in ocean's pearly caves
• First forms minute, unseen by spheric glass,
• Move on the mud, or pierce the watery mass
• These, as successive generations bloom,
• New powers acquire and larger limbs assume
• Whence countless groups of vegetation spring,
• And breathing realms of fin and feet and wing

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• Believed the Earth was millions of years old
• Rejected the theory of special creation
• Discussed how life evolved from a single common
  ancestor, forming one living filament
• Believed in the inheritance of acquired
  characters
• Differences among animals were driven by three
  different forces lust, hunger, and security

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• These forces, when interacting among themselves
  and acting upon the Natural Variation of
  individual in species improving, thus, the
  different races (natural selection?)
• Believed in spontaneous generation
• Some of his ideas on how evolution might occur
  sound like Lamarckian, but actually it is the
  other way around
• Erasmus Darwin also wrote about how competition
  and sexual selection could cause changes in
  species The final course of this contest among
  males seems to be, that the strongest and most
  active animal should propagate the species which
  should thus be improved

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• He used his observations of domesticated animals,
  the behavior of wildlife, and he integrated his
  vast knowledge of many different fields, such as
  paleontology, biogeography, systematics,
  embryology, and comparative anatomy

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• Did he anticipate the erroneous ideas of Lamarck?
• Did Erasmus Influence Charles Darwin? CD said no!
  
• CDs first edition of The Origin initially called
  Zoonomia

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• Others like William Lawrence (1783-1867), James
  C. Prichard (1786-1848), and William C. Wells
  (1857-1917) did not believe in the inheritance of
  acquired characters
• They were physicians and could not find support
  for it
• They believed in selection, mutation, segregation

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• Others Patrick Matthew (1790-1874), Edward
  Blythe (1810-1873), Charles Naudin (1815-1899),
  believed in natural selection

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Charles Darwin (1809-1882)

• Born in Shrewsbury, Shropshire, England, on 12
  February 1809
• Grandson of Erasmus Darwin
• His father was a physician, wealthy
• Family high intellectuality, professional
  ability, industriousness
• Mother died when he was eight years old

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Education

• Brought up by his sister
• Graduated from the elite school at Shrewsbury in
  1825 (Classics)
• Sent to Edinburgh to study medicine. Dropped out
  in 1827 (hated it)
• Sent to University of Cambridge to study theology
  in preparation for becoming a clergyman of the
  Church of England
• Loved to collect plants, insects, and geological
  specimens

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• Guided by his cousin William Darwin Fox, an
  entomologist
• Scientific inclinations encouraged by his botany
  professor, John Stevens Henslow
• Henslow helped build Darwin's self-confidence
  taught him to be a meticulous and painstaking
  observer of natural phenomena and collector of
  specimens
• Also influenced by Adam Sedgwick, read Humboldt,
  graduated from Cambridge in 1831

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The Voyage of the Beagle

• Henslow was instrumental in securing a place for
  Darwin as an unpaid naturalist on the surveying
  expedition of HMS Beagle to Patagonia (1831-6)
• Heavy paternal opposition
• Under Captain Robert Fitzroy (an illegitimate
  descendent of King Charles II) visited Tenerife,
  the Cape Verde Is., Brazil, Montevideo, Tierra
  del Fuego, Buenos Aires, Valparaiso, Chile, the
  Galapagos, Tahiti, New Zealand, and Tasmania

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• Opportunity to observe the various geological
  formations found on different continents and
  islands along the way, as well as a huge variety
  of fossils and living organisms
• In the Keelings he devised his theory of coral
  reefs
• At the time, most geologists adhered to the
  catastrophist theory

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• The catastrophist viewpoint was challenged by Sir
  Charles Lyell in Principles of Geology (1830-33).
  Lyell maintained that the Earth's surface is
  undergoing constant change, the result of natural
  forces operating uniformly over long periods
• Read Charles Lyell
• Most impressed with the effect that natural
  forces had on shaping the Earth's surface
• Fossils in South America

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• Noted that certain fossils of supposedly extinct
  species closely resembled living species in the
  same geographical area
• The Galápagos
• Observed that each island supported its own form
  of tortoise, mockingbird, and finch closely
  related but differed in structure and eating
  habits from island to island

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Natural Selection

• Returned to England in 1836
• Began recording his ideas about changeability of
  species in his Notebooks on the Transmutation of
  Species
• In 1838 read An Essay on Population (1798), by
  the British economist Thomas Robert Malthus
• Explained how human populations remain in
  balance. Increase in the availability of food for
  basic human survival could not match the
  geometrical rate of population growth. The
  latter, therefore, had to be checked by natural
  limitations such as famine and disease, or by
  social actions such as war

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• Darwin applied Thomas Malthus's argument to
  animals and plants
• By 1838 he had arrived at a sketch of a theory of
  evolution through natural selection
• In 1842 he drew up his observations in some short
  notes
• Expanded in 1844 into a sketch of conclusions for
  his own use
• These embodied the idea of natural selection

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• Typically cautious, he delayed publication of his
  hypothesis
• For the next two decades he worked on his theory
  and other natural history projects
• In 1839 he married his first cousin,
  Emma Wedgwood (1808-96)
• In 1842 moved to a small estate,
  Down House, outside London
• They had ten children, three of whom died in
  infancy
• Independently wealthy and never had to earn an
  income

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• A country gentleman among his gardens,
  conservatories, pigeons, and fowls
• Practical knowledge in variation and
  interbreeding
• He developed a friendship with
  Sir Charles Lyell, became secretary
  of the Geological Society (1838-41)
• Continuous ill-health (suffered from
  Chagas's disease?, Panic disorder?)
• By 1846 he had published several works on the
  geological and zoological discoveries of his
  voyage

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• Placed him at once among the front rank of
  scientists
• Variability among barnacles
• Artificial selection
• Giraffe example
• Vestigial organs
• After five years of collecting the evidence, he
  began to speculate on the problem of the origin
  of species

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Alfred Russell Wallace (1823-1913)

• Natural history collector and explorer
• Also read Malthus. Wrote to Lyell
• Wrote to Darwin
• Wallace sent him a memoir on the
  Malay Archipelago
• Contained in essence the main ideas of theory of
  natural selection
• Lyell and Joseph Hooker persuaded Darwin to
  submit a paper of his own read simultaneously
  with Wallace's before the Linnean Society in 1858

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• Darwin then set to work to condense his vast mass
  of notes, and put into shape his great work, The
  Origin of Species by Means of Natural Selection,
  published in 1859. 6 editions
• Extremely readable book. First edition sold out
  the first day!
• Because of the food-supply the young born to any
  species intensely compete for survival
• The young that survive embody favorable natural
  variations the process of natural selection and
  these variations are passed on by heredity

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• Therefore, each generation will improve
  adaptively over the preceding generations, and
  this gradual and continuous process is the source
  of the evolution of species
• Also introduced the concept that all related
  organisms are descended from common ancestors
• Provided additional support for the older concept
  that the earth itself is not static but evolving

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Reaction

• Some biologists criticized Darwin's concept of
  variation, arguing that he could explain neither
  the origin of variations nor how they were passed
  to succeeding generations
• The most publicized attacks on Darwin's ideas
  came from religious opponents
• Seemed to place humanity on a plane with the
  animals
• Darwin spent the rest of his life expanding on
  different aspects of problems raised in the
  Origin
• 10 years after its publication most learned
  people had accepted it

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• Neither Darwin nor Wallace were present when the
  first public confrontation took place

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Other Books by Charles Darwin

• The Fertilization of Orchids (1862)
• The Variation of Plants and Animals under
  Domestication (1867)
• The Descent of Man and Selection in Relation to
  Sex (1871)
• The Expression of Emotions in Man and Animals
  (1872)
• Insectivorous Plants (1875)
• The Effects of Cross and Self Fertilization in
  the Vegetable Kingdom (1876)
• Different Forms of Flowers in Plants of the Same
  Species (1877)
• The Formations of Vegetable Mould through the
  Action of Worms (1881)

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• Darwin was elected to the Royal Society (1839)
  and the French Academy of Sciences (1878)
• He was also honored by burial in Westminster
  Abbey after he died in Down, Kent, on 19 April
  1882

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• Raised the idea of evolution from a hypothesis to
  a verifiable theory
• Thus, after Darwin, the following major cultural
  revolutions had taken place in the human thought
• 1. The Expansion of the Time-Scale
• 2. The Concept of a Changing Universe
• 3. The Elimination of Design
• 4. The Elimination of Miracles
• 5. The Inclusion of Humans within Nature

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How Original was Charles Darwin?
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The Eclipse of Darwinism

• Until 1875 Darwinism was hotly debated
• From then on, Darwinism fell out of favor among
  many biologists
• This was the result of many difficulties

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Difficulties with Darwinism

• Absence of transitional forms
• Organs of extreme perfection
• Evolution of instinct (behavior)
• Hybrid sterility and interfertility of
  varieties
• Nature of variation
• Orthogenesis
• Lamarckism

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Impact of Mendels Work

• Mendel himself rejected Darwins ideas
• Blending Inheritance (Mendels work was published
  in 1865 but Darwin was unaware of it, as was the
  rest of the world, until 1900)
• Macro-Mutationism

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Further

• Mutation not necessarily adaptive
• Small variants among individuals and populations
  not the same as differences among species
• Selection may act on those variants but it takes
  a different kind of mutation to generate new
  species
• Natural selection was speculation, not science

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Goldschmidt and hopeful monsters

• Richard Goldschmidt (German-American, 1878-1958)
• Understanding hereditary changes in
  development crucial to understanding evolution of
  morphology
• Argued for important role of macromutation
• hopeful monsters
• Modern genetics refuted Goldschmidts arguments
  but two key elements remain
• Role of mutations of large effect
• Sudden transformations
• Genomic reorganization

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Orthogenesis

• The belief that evolution was directed by some
  kind of undescribed force
• That evolution had some sort of directionality
• Therefore, evolution of a species was influenced
  most strongly by internal factors and was not
  subject to the external forces of natural
  selection

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Modern Synthesis

• Began in the 1930s
• Was the recognition that evolution as a synthetic
  theory able to explain patterns of current and
  past biological diversity and must propose a
  mechanism (process) or series of processes, by
  which these patterns can arise

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The Major Architects

• 1930 R.A. Fisher The Genetical Theory of
  Natural Selection
• 1931 S. Wright Evolution in Mendelian
  populations
• 1932 J.B.S. Haldane The Causes of Evolution
• 1937 T. Dobzhansky Genetics and the Origin of
  Species
• 1942 J.S. Huxley The Modern Synthesis
• 1942 E. Mayr Systematics and the Origin of
  Species
• 1944 G.G. Simpson Tempo and Mode in Evolution

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Ronald Aymer Fisher (British, 1890-1962)

• Mathematician by training
• He could not find a job, worked as
  a farmer
• His writing was difficult to read
• 1933 Chair of Eugenics at University College,
  London
• 1943 Professor of Genetics, Cambridge University

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• Fisher was a student at a time when there was
  still controversy about Darwin's theories and
  when Mendel's work on genes had just been
  rediscovered
• Made important discoveries in statistics (e.g.,
  maximum likelihood), genetics, selection and
  (genetic) dominance
• It could be said that he invented a large part of
  modern statistics

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• Fisher's models emphasized mass selection in
  panmictic populations and tended to assume that
  phenotypes results from many genes of small,
  additive effect
• This put him in direct conflict with the American
  neo-Darwinian theorists such as Wright leading to
  a famous public debate that lasted three decades

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Sewall Wright (1889-1988)

• American mathematician and
  biologist
• First publication at 7 yo.
• Interested in inbreeding (his parents
  were first cousins)
• Specialized in small populations
• For him genetic drift was crucial

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• As a statistician, Wright worked with covariant
  analyses to determine the importance of various
  factors in defining traits
• This expertise was extended into animal breeding
• Mammalian genetics was one of his main interests
• Wright published on color inheritance in 1917 and
  1918, the year in which he pioneered path
  analyses in his study of body characteristics in
  animals

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• He developed a theory that attributed a
  substantial amount of genetic variance or
  creativity to small genetic fluctuations among
  small population groups
• This was an innovative idea and caused the
  well-documented debate with Fisher, who insisted
  that variance could be analyzed only in relation
  to large populations
• Wright specialized in this field of population
  genetics while a professor at the
  University of Chicago

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John Burdon Sanderson Haldane(British, 1892-1964)

• One of the most influential scientists of the
  20th. century, he studied relationships among
  different disciplines and problems, including the
  consequence of Mendelian genetics on evolutionary
  theory, the relationship between enzymology and
  genetics, and the application of mathematics and
  statistics to the study of biology
• He was a science popularizer
• Disillusioned with Marxism moved to India

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• He learned Mendelian genetics while still a boy
  by breeding guinea pigs
• In The Causes of Evolution he helped to marry
  genetics to the older evolutionary theory
• He reestablished natural selection as
  the premier mechanism of evolution by
  explaining it in terms of the
  mathematical consequences of Mendelian
  genetics

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Theodosius Dobzhansky (Russian-American,
1900-1975)

• Decided to become a biologist at 12
• Influenced by The Origin
• Migrated to the U.S. in 1927
• He defined evolution in his 1937 as A
  change in the frequency of an allele in a gene
  pool
• Nothing in Biology Makes Sense Except in the
  Light of Evolution

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• Dobzhansky's studies in population genetics
  served as a basis for his explanation of how the
  evolution of races and species could have come
  about through adaptation
• He discovered that successful species tend to
  have a wide variety of genes that, while they do
  not appear to be useful to the organism in its
  present environment, do provide a species as a
  whole with genetic diversity
• This diversity enables the species to adapt
  effectively to changes in the
  surrounding environment

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Julian Huxley (British, 1887-1975)

• Humanist, atheist and science popularizer, Julian
  Huxley, a Professor of Zoology, was the brother
  of Aldous Huxley and grandson of Thomas H. Huxley
• His research on hormones, developmental
  processes, ornithology, and ethology influenced
  the modern development of embryology,
  classification, and studies of behavior
  and evolution and led him to develop a
  synthetic view of life

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Ernst Mayr (German-American, 1904-)

• Expeditions to New Guinea
• Worked at the AMNH (1932-1953)
• Harvard (from 1953 on)
• His career interests have spanned a remarkable
  five different fields, including (1)
  ornithology, (2) systematics, (3) zoogeography,
  (4) evolutionary theory, and (5) philosophy and
  history of science

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• Mayr developed the biological species concept,
  population thinking in taxonomy, and the theory
  of allopatric speciation
• LIke the other neo-Darwinians, Mayr argued
  against orthogenesis, neo-Lamarckism, and
  saltationist versions of Mendelism
• In the 60's and 70's Mayr defended evolutionary
  taxonomy (which blended phylogenetic and
  ecomorphological information to produce
  qualitatively-justified classifications)
• In the 70's Mayr's belief that the isolation of
  small, peripheral populations led to rapid
  speciation and morphological change
  (i.e., the founder principle and
  peripheral isolates speciation)

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George Gaylord Simpson (American, 1902-1984)

• Most influential paleontologist of the 20th
  century the only major participant in the
  Modern Synthesis to come from that field
• Wrote hundreds of papers and books
• Leading expert on Mesozoic, Paleocene, and South
  American mammals, plus penguins
• Arguably the first paleontologist to make
  consistent use of statistics, not just in alpha
  taxonomy, but in paleoecology and in
  macroevolution

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• In his 1944 book, Tempo and Mode in Evolution,
  Simpson divided evolutionary change into tempo,
  the rate of change, and mode, the manner or
  pattern of change, with tempo being a basic
  factor of mode
• Simpson saw paleontology revealing the long
  history of life on earth, as a unique field
  through which to study the history of evolution

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Major Contributions of the Modern Synthesis

• Differentiation between Phenotype and Genotype
• That there is not inheritance of acquired
  characteristics
• Genes are the basis of inheritance and there is
  not blending inheritance
• Genes mutate and that is the source of variation
• Mutation is undirected
• Evolutionary change is a populational process
• Change in gene frequencies within populations can
  occur via Drift or Selection

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• Even slight intensity of selection can result in
  substantial evolutionary change over a short time
• Selection can alter populations beyond the
  original range of variation
• Natural populations are genetically variable
• Populations differ geographically and these
  differences have a genetic basis
• Natural selection occurs in current populations
• Differences among populations are often adaptive
• Species represent different gene pools are
  defined by genetically based reproductive
  isolation

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• There is continuous variation in phenotype,
  genes, and degree of reproductive isolation among
  populations
• Higher taxa arise through the accumulation of
  small differences (i.e., the processes that drive
  divergence among populations are the same as
  those that lead to higher taxa)
• Gaps in the fossil record are explained by the
  incompleteness of the fossil record
• Observations in the fossil record are consistent
  with all of the above principles of evolutionary
  change