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Download Lecture 1: Introduction to Evolution
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Lecture 1: Introduction to Evolution Evolution = change in allele frequencies in a population Great, is class over now? No (sigh), because the process & results are interesting & important to our understanding of biology Questions in evolution WHY is life on earth diverse? (we’ll get to this later) HOW is life adapted ? (questions leading to more questions) Adaptation Questions • Are all traits adaptive? • Are all adaptations perfect? • Who/what benefits from adaptation? Where did all these questions come from? BD : Before Darwin • • • • • Orthodoxy – species as fixed, designed by God Bible = literal truth variation = imperfection BUT, even before Darwin this was questioned Idea of changeable/old Universe, allowed idea of changeable life on earth (geology & paleontology) • Enlightenment – fossils: spontaneous generation, transmutation Lamarck • spp. change into new spp. over time Transformism: -no branching -no extinction Acquired Characters: e.g. Giraffe’s neck So, Darwin had somewhere to start… Darwin (1809-1882) • Noticed: geographic variation in very similar spp. • Rejected orthodoxy – b/c did not explain adaptation • Theory : why spp. change & why they are well designed for their lives • Influences: – Malthus (population principle) – Lyell (continuous change vs. catastrophists) – Social influences (materialism) Darwin’s conclusion • ‘Struggle for existence’ only some survive • Expect favourable variations to survive = Natural Selection • Mechanism = most important aspect of theory • Poor Alfred Russel Wallace – same conclusions on a much smaller budget – Co-presented the idea but Darwin is remembered because of The Origin of Species Darwin’s Dangerous Idea The Origin of Species (1859) Theses: 1. Descent with Modification from Common Ancestors (Evolution) 2. Natural Selection is main agent of Evolution Origin in a nutshell 1. Observed overproduction of offspring Many born – few survive to reproduce WHY? Limited resources – pop’ns limited in size Therefore STRUGGLE FOR EXISTENCE 2. Individuals vary If competition for resources then only favourable variants survive to reproduce 3. Most of the variation is inherited (e.g. domestic species – sexual repro:diversity) Favourable variants have more offspring & increase in frequency = Change in the population over time Plus lots about pigeons… Deduction Descent with modification via natural selection Testable??? Comparative Anatomy Fossil Record Predictions Comparative Embryology Behaviour Geographic Distribution Why Dangerous? • Static dynamic view of nature • Creationism = implausible • Platonic essentialism – variation = basic/neutral feature • Refuted teleology & anthropocentrism • Natural Selection – No goal – No consistent direction • EVOLUTION PROGRESS Evolution is a bush not a ladder Humans Mammals Birds Amphibians Fish Reception “Evolution” generally accepted but: 1. Viewed as progressive (towards a goal) 2. Natural selection rejected • • • No theory of heredity (how characteristics passed on) Problem of uncrossable “gaps” in evolution Back to transmutation The Modern Synthesis Mendelian genetics “rediscovered” in 1920s By 30s/40s widely accepted : 1. acquired characters not inherited 2. Continuous variation explained by Mendelian genetics (Fisher) 3. Theoretical works show N.S. can work with what is available in nature, nothing else required Speciation only requires N.S. not macromutation /acquired characters 4. Species are not morphotypes – dynamic concept Tenets of Modern Synthesis • Populations have variation from random, not adaptively directed, mutation & recombination • Populations evolve through changes in gene frequency by drift, gene flow & N.S. • Change is gradual because most genetic variants have slight effects on phenotype • Diversification (speciation) is due to gradual reproductive isolation among populations • Overtime, changes give rise to new taxa Genetics…more important than Darwin? • After Darwin, many accepted the idea of evolution as change in species over time • BUT, much argument against NS • Genetics forced the rejection of evolution as goal-oriented & the widespread acceptance of NS Modern Evolutionary Biology Two principle goals: 1. Inferring history of evolution 2. Elucidating the mechanisms Modern evolutionary theory: Provide explanation for patterns of life in space & time & the processes by which these patterns arose