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Dr. Carol Eunmi Lee University of Wisconsin, Madison. History of Evolutionary Thought: The Grand Evolutionary Synthesis. Today’s OUTLINE:. (1) The Sources of Confusion (2) Reconciling Mendel and Darwin (3) The Main Tenets of the Evolutionary Synthesis - PowerPoint PPT Presentation
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Dr. Carol Eunmi LeeUniversity of Wisconsin, Madison
HistoryHistory of of Evolutionary Thought:Evolutionary Thought:
The Grand Evolutionary The Grand Evolutionary SynthesisSynthesis
(1) The Sources of Confusion
(2) Reconciling Mendel and Darwin
(3) The Main Tenets of the Evolutionary
Synthesis
(4) Key Developments since the Synthesis
(5) Gaps in our Understanding Today
Today’s OUTLINE:
Charles Darwin (1809-1882)
Last time we discussed Darwin’s contributions to evolutionary thinking
Darwin’s contribution:
“Population Speciation as a result of Natural Selection”
More offspring are produced than can survive
Limited resources and competition for resources
There is heritable variation in a population
Individuals better adapted to environment survive
Survivors leave more offspring (“Survival of the
Fittest”)
Thus, average character of population is altered
But, Darwin’s theory was not complete
Because Darwin knew nothing about mutation, he had no idea how variation was generated in populations
Because Darwin knew nothing about genetics or genes, he had no idea how variation was passed on to offspring (Mendel)
Darwin did not know about nonadaptive evolutionary forces, such as Genetic Drift
Mendel published in 1865… was ignored until 1900
Presented a mechanism for how traits got passed on
“Individuals pass alleles on to their offspring intact”
(the idea of particulate (genes) inheritance)
Mendel’s work held part of the key to what was missing in Darwin’s Theory
Rediscovery of Mendel’s laws of inheritance
In 1900, Mendel’s laws of inheritance were “rediscovered” Dutch biologist Hugo de Vries, German plant
geneticist Carl Correns, and Austrian plant breeder Erich von Tschermak-Seysenegg
Worked out laws of inheritance independently Discovered Mendel’s work as they were
publishing their own
Formed the beginning of the foundation of Genetics: Mendel is considered the “Father of Genetics”
Hardy-Weinburg
Equilibrium (Lecture 3)
Wilhem Weinberg January 13, 1908
G. H. Hardy July 10, 1908 in Science
Could mathematically show expectations of Mendelian inheritance and whether expectations are realized in nature
BUT… Mendel and Darwin’s ideas seemed Incompatible
Mendel: dealt with particulate traits Darwin: observed continuous traits
Q: How would continuous traits get passed on?
Selection vs Mutations
Mutations discovered after 1900
Q: If mutations are arising, why need selection... … if things are just mutating?
Controversy between Mutationists vs Darwinists
Mutationists (+ Mendelianism) They thought that evolution required
only mutations and passing on of discrete traits
Darwinists They thought that evolution required
only Natural Selection on continuous variation
Discrete vs Quantitative Traits Darwin was unable to clearly see the pattern of
inheritance because he studied quantitative variation
•Discrete trait: a trait that has distinct values, rather than a range of phenotypes, usually encoded by one or a few genes. Examples: number of fingers, color of Mendel’s peas, sickle cell anemia, ABO blood type, number of eggs in a bird clutch, presence/absence of human widow’s peak, presence/absence of dimples, etc.
•Quantitative (continuous) trait: a trait that has a continuum of phenotypes and is encoded by multiple genes.
Examples: body size, height, weight, intelligence (IQ), Running speed, beak shape, hair color, skin color, milk yield of cows, lifespan, etc.
Frequency
Frequency
Type
Type
Proponents of Darwinism were correct about mechanisms of Natural Selection, but they did not understand what Selection was acting on, as they were unaware of the unit of inheritance (genes) or how the variation was passed on to the next generation
They came up with the idea of “Blending inheritance” where offspring gain characteristics of both parents, like mixing colors of paint… but, this was a vague idea that was incorrect
Many of them were Biometricians (statistical types) that thought that evolution was gradually acting on continuous traits
Proponents of the Darwinist Theory
Many Prominent Geneticists at the time supported the Mutationist/Mendelist theory
Proponents of the mutationist theory included Hugo de Vries, among those who “discovered” Mendel’s 1900 paper and Thomas Hunt Morgan, founder of Drosophila genetics
Thought that evolution arose through genetic changes (mutations) that were discrete and sudden
New species originated when they mutated from pre-existing species, but this process was independent of natural selection
Proponents of the Mutationist/Mendelist Theory
Controversy between Mutationists vs Darwinists
The controversy persisted for ~30 years up until the 1930s, during which relatively little progress was made
Problems to Resolve:
At the heart was the question of whether Mendelian genetics and Mutation could be reconciled with mechanisms of Natural Selection.
A second issue was whether the broad-scale changes (macroevolution) seen by palaeontologists could be explained by changes seen in local populations (microevolution).
Problem caused by: Binary thinking (Black or White thinking): it’s
this or that… “if I’m right, you must be wrong”
When in fact the two or more factors might interact
Inability to see overarching mechanism that could explain a wide range of phenomena: “How could your Hardy-Weinberg (Mendel) explain the inheritance of 5.1 cm, 5.5 cm beak length (continuous characters)?”
When in fact, one principle might govern and explain the different patterns
There were other competing ideas of evolution, including Neo-Lamarckianism
Genetic Drift
A concept as important as Natural Selection But, not as prominent on people’s minds
1872 Gulick: Neutral theory (Genetic Drift) 1921 A.C. Hagedoorn produced data to support
Neutral Theory Genetic Drift
The Modern SynthesisThe Modern Synthesis1930s ~ 1940s
Also called the “Synthesis of Evolution and Genetics”The synthesis of population genetics(role of mutation, selection, genetic drift),paleontology, systematics
Darwin and Mendel Reconciled
The Modern SynthesisThe Modern Synthesis1930s ~ 1940s
Also called the “Synthesis of Evolution and Genetics”
Among the Greatest Scientific Revolutions of the Century
Three of the "architects" of the evolutionary synthesis:G. Ledyard Stebbins, Jr., George Gaylord Simpson, Theodosius Dobzhansky
Photograph from Smocovitis, V. B. 1997. G. Ledyard Stebbins, Jr. and the evolutionary synthesis (1924-1950). American Journal of Botany 84: 1625-1637.
The Evolutionary Synthesis was important because many scientists from different fields convened to discuss the evolutionary mechanisms and clear up confusion and inconsistencies
Some Key Tenets of the Modern Synthesis
Populations are the units of Evolution
Mendel vs Darwin: continuous traits are also coded by particulate genes, but many genes
Mutation vs Selection: Mutations are sources of genetic variation upon which Selection acts
Natural Selection and Mutation are not the only evolutionary forces. Examples: Genetic Drift, Recombination
Microevolutionary processes, such as Drift, Selection, Mutation, lead to Macroevolutionary changes (such as speciation)
Some Tenets of the Evolutionary Synthesis The phenotype is different from the genotype Acquired characters (phenotypic plasticity) are not inherited Traits are inherited via genes, and they do not “blend” with other genes
(Darwin was wrong about this one) Genes mutate, resulting in different alleles Evolution occurs at the population level, due to a change in proportions of
individuals with different genotypes Changes in proportion in a population could occur via random genetic drift
(Sewall Wright) or Natural Selection… the rate of mutation is usually too low to cause large changes in proportions
Even very weak natural selection could cause substantial changes over a longer time scale
Mutations generate the genetic variation upon which natural selection acts Microevolutionary processes lead to Macroevolutionary changes (speciation) All organisms on the planet are related to one another in a great “tree of life”,
and have diverged by branching from common ancestors Gaps in the fossil record are likely due to incompleteness of the fossil record.
Gradual changes seen in many parts of the fossil record suggest gradual changes over time
Mutation vs Selection
and Reconciling Mendel and Darwin
Mutation vs Selection And Reconciling Mendel and Darwin
continuous and discrete traits could follow the same principles of inheritance (Mendel), just that continuous traits are coded by many genes (loci)
If there are many genes (loci) coding for a trait, rather than one, the offspring look intermediate between the parents (looks like “blending inheritance”)
BUT, the SAME Mendelian patterns of inheritance apply, it’s just that you don’t see the particulate inheritance of each gene in the offspring, but the average effect across all the genes affecting the trait
How do you deal with quantitative traits? (2nd point below)
Hardy Weinberg: multiple alleles: 3 alleles: (p + q + r)2 which expands to...
p2 + 2pq + q2 + 2pr + 2qr + r2 =1.0 4 alleles: (p + q + r + s)2
Hardy Weinberg: multiple loci HW principle still applies to each locus
independently Need to use principles of Quantitative
Genetics to examine effects of multiple loci
JBS Haldane
Sewall Wright
RA Fisher
The Population Geneticists
Mathematical theory of population genetics showed that mutation and selection TOGETHER cause adaptive evolution:
Mutation is NOT an alternative to Natural Selection, but the raw material upon which natural selection acts.
Fisher vs WrightBoth appreciated the importance of Natural Selection AND Genetic DriftBut they argued about the relative importance
Background in math, physics, astronomy, and genetics
Made key contributions to the field of Statistics
Ronald Aylmer Fisher (1890-1962)
Other Contributions:
• Adding mathematical rigor into the theory of selection
• Elegant synthesis of Mendelian inheritance into the Theory of Selection
• Important developments in Statistics (ANOVA)
Ronald Aylmer Fisher (1890-1962)• Natural selection occurs in large populations
• Many genes are involved
• Mutations are the main substrate for selection
Heavily influenced by examples from agriculture
Worked for the US Dept of Agriculture: breeding in guinea pigs and cattle
Became a professor at UW-Madison in Genetics
Sewall Wright (1889-1988)
• Inbreeding and Genetic Drift are important for creating new gene interactions
• These new gene interactions (epistasis caused by new recombinations) are the main substrate for selection
Sewall Wright (1889-1988)
The Population Geneticists
If you want to read more about this topic, this book is a good read
Reconciling Microevolutionary Mechanisms and Macroevolutionary
processes
Microevolution Macroevolution
Microevolutionary processes within species account for macroevolution among species (Lecture #18)
That is, mutation, recombination, natural selection, and other processes that act within species (microevolution) are the SAME mechanisms that account for the origin of new species and major long term evolution (macroevolution) (Lecture #18)
Ernst Mayr
Ernst MayrGeorge Gaylord SimpsonG. Ledyard StebbinsBernhard Renschand others
George Gaylord Simpson
http://www.genetics.wisc.edu/CATG/crow/index.html
James F. Crow (1916-2012)University of Wisconsin, Madison
His work has touched on nearly every area of evolutionary genetics-will discuss some of his contributions in Lecture #7
The Importance of Natural Selection vs Genetic Drift
Ongoing debate after the Evolutionary Synthesis
Even after the synthesis the relative importance of Natural Selection and Genetic Drift was debated
• During the Evolutionary Synthesis, Sewall Wright focused more on importance of Genetic Drift, whereas Fisher focused on Natural Selection
• Shortly after the Evolutionary Synthesis many focused on selection to the point of assuming that most phenotypes were the result of Natural Selection
• Emphasis on Genetic Drift resurged in the 1970s, 80s with Kimura’s “Neutral Theory”
• Then in the 2000s and 2010s interest in Selection increased with the ability to detect signatures of Natural Selection in genome sequence data
Motoo Kimura (1924-1994)
The Neutral Theory of Molecular Evolution
Classic Paper: Kimura, Motoo. 1968. Evolutionary rate at the molecular level. Nature. 217: 624–626.
Classic Book: Kimura, Motoo (1983). The neutral theory of molecular evolution. Cambridge University Press.
The Neutral Theory of Molecular Evolution (Lecture #6)
• Neutral theory is not incompatible with Darwin's theory of evolution by natural selection: adaptive changes are acknowledged as present and important, but hypothesized to be a small minority evolutionary change.
• Recent tests of selection have found that in many cases evolution is not neutral, even in non-coding regions of the genome.
• Nevertheless, the neutral theory is useful as a null hypothesis, against which selection could be tested.
Motoo Kimura
• The Neutral theory posits that the vast majority of evolutionary change at the molecular level is caused by random genetic drift rather than natural selection.
While the Evolutionary Synthesis was a HUGE
leap in the right direction, there were a few tenets
that required modification (as a result of new
discoveries in Genetics)
Some Tenets of the Evolutionary Synthesis The phenotype is different from the genotype Acquired characters (phenotypic plasticity) are not inherited –not always true Traits are inherited via genes, and they do not “blend” with other genes (Darwin
was wrong about this one) Genes mutate, resulting in different alleles Evolution occurs at the population level, due to a change in proportions of
individuals with different genotypes Changes in proportion in a population could occur via random genetic drift (Sewall
Wright) or Natural Selection… the rate of mutation is usually too low to cause large changes in proportions
Even very weak natural selection could cause substantial changes over a longer time scale
Mutations generate the genetic variation upon which natural selection acts Microevolutionary processes lead to Macroevolutionary changes (speciation) All organisms on the planet are related to one another in a great “tree of life”, and
have diverged by branching from common ancestors Gaps in the fossil record are likely due to incompleteness of the fossil record.
Gradual changes seen in many parts of the fossil record suggest gradual changes over time –not always true
Completing the Synthesis
Epigenetic Inheritance (Lecture #9)
Some genetic changes could lead to radical changes in phenotype (Lecture #7)
Polyploidization seen often in plants Changes in developmental genes Regulatory Evolution Transposons
Advances in Genetics
Evolution at the Molecular Genetic Level
Which types of mutations predominate and contribute to adaptations more often?Structural vs Regulatory?
Is phenotypic evolution occurring predominantly at the level of gene products (e.g. proteins) or at the level of gene regulation (e.g. transcription, RNA processing, translation, etc.)?
cis-Regulation vs trans-Regulation? Is regulatory evolution occurring predominantly at the level of
cis-regulatory elements (e.g. promoter, enhancers) or at the level of trans-acting factors (e.g. transcription factors, etc.)?
More on Lectures on Molecular Evolution (Lecture #13)
The role of Epigenetics(Lecture #9)
Lamarck Revisited
Lamarck was incorrect in thinking that the inheritance of acquired characters is the main mechanism of evolution
However, we do now know that the inheritance of acquired characters does happen sometimes, through the inheritance of epigenetic modifications
Evolution of Development(Lecture #26)
How small changes in developmental genes (like Hox genes) could radically cause the evolution of body plans
Will talk more about this when I get to lecture on Animal Diversity
Role of Genomics(Lectures #15, 16)
How does the whole genome evolve?
How does selection act on networks of interacting genes?
How many and which genes are involved in the formation of new species?
Today: Genome Evolution and Systems Biology
How do multiple genes interact? How do multiple genes affect a phenotype? Systems Biology: How does selection act on
interacting gene networks? Evolution of Genome Architecture
Questions:(1) What were the sources of confusion regarding evolutionary
mechanisms prior to the Evolutionary Synthesis?
(2) What was the Modern (Evolutionary) Synthesis?
(3) What were the main tenets of the Evolutionary Synthesis?
(4) What is the relationship between natural selection, genetic drift,
mutations, and recombination?
(5) What were some of the limitations of the Evolutionary Synthesis?
(6) What were some key developments since the Evolutionary
Synthesis?
(7) What gaps remain in our understanding today?
Moral of the Story
Scientists from different fields should talk to each other
Should avoid binary thinking (this vs that, right vs wrong), as different mechanisms might work together in an integrated fashion
We are often biased by what we study (example of continuous vs discrete traits)
Sample Exam Question
Which of the following was NOT a tenet of the Evolutionary Synthesis?
(a) Evolution occurs at the level of populations, in terms of changes in allele frequencies, rather than changes at the individual level
(b) Selection could act on traits that are coded by multiple genes
(c) Selection acts on genetic variation in traits that are caused by mutations
(d) Natural Selection and Mutation are the only causes of evolutionary change
(e) Microevolutionary processes within populations lead to Macroevolutionary changes among populations
Answer: d