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Genetic Drift Question History of Evolutionary Thought: The Grand Evolutionary Synthesis Considered one of the most important Biological Revolutions of the Century Dr. Carol Eunmi Lee University of Wisconsin, Madison Copyright ©2020; do not upload without permission Today’s OUTLINE: (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 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 ■ Darwin did not know about nonadaptive evolutionary forces, such as Genetic Drift Question: ■ What body of knowledge provided Darwinian theory with the concept of “genes” and inheritance? Mendel’s work held part of the key to what was missing in Darwin’s Theory ■ 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) 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 ■ These rediscoveries formed the beginning of the foundation of Genetics: Mendel is considered the “Father of Genetics” Hardy-Weinburg Equilibrium (Lecture 4) ◆ Wilhem Weinberg ✦ January 13, 1908 ◆ G. H. Hardy ✦ July 10, 1908 in Science ■ Mathematical description of Mendelian Inheritance: Could mathematically show expectations of Mendelian inheritance and whether Mendelian expectations are realized in nature ■ Did the Darwinists accept Mendelian Genetics? ■ Why or Why Not? PROBLEMS! BUT… Mendel and Darwin’s ideas seemed Incompatible ■ Mendel’s principles: dealt with particulate (discrete) traits (e.g. yellow vs. green, wrinkled vs. smooth) ■ BUT, Darwin observed continuous traits (e.g. beak size, body length) Q: So, 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 Problem ■ Darwin did not understand the unit of inheritance (what is being inherited, genes) ■ Mendelism provide that (particulate inheritance), but Darwinian traits did not seem to be inherited in a Mendelian fashion ■ Discrete vs. Quantitative Traits: Darwin was unable to clearly see the pattern of inheritance because he studied quantitative 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 Frequency 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. Type • Quantitative (continuous) trait: a trait that has Frequency 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. Type Discrete vs Quantitative Traits ■ Darwin was unable to clearly see the pattern of inheritance because he studied quantitative variation Frequency • 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. Type Example: human height encoded by ~423 genes: http://www.nature.com/ng/journal/v46/n11/full/ng.3097.html Proponents of the Darwinist Theory ■ 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 Mutationist/Mendelist 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 Controversy between Mutationists vs Darwinists Controversy persisted for ~30 years up till the 1930s, during which 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” à 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)?” à When in fact, one principle might govern and explain the different patterns 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 Synthesis 1930s ~ 1940s Also called the “Synthesis of Evolution and Genetics” The synthesis of population genetics (integrate the roles of mutation, selection, genetic drift, paleontology, systematics) Darwin and Mendel Reconciled The Modern Synthesis 1930s ~ 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, Migration ■ Microevolutionary processes, such as Drift, Selection, Mutation, lead to Macroevolutionary changes 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 ■ With which of these tenets do you agree? ■ With which do you not agree? Mutation
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