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Descent with Modification Darwinian View of Life Chapter 22 Why is Evolution So Controversial? Often evolution (and science in general) is pitted against religion in “winner take all” stance Modern debate mostly involves the teaching of evolution rather than the theory itself And that means mixing science, religion and politics, etc.--Never an easy combination! Plenty of misconceptions and confusion surround evolution, adding to controversy 3 Key Observations About Life 1. Organisms are suited for life in their environments. 2. Organisms share many characteristics of life (unity). 3. There is a rich diversity of life. These observations led Charles Darwin to develop a scientific explanation for these observations. Endless Forms Most Beautiful • • Study of biology upended in 1859 when Darwin published The Origin of Species Darwin proposed: Descent with modification • • Species are descendants of ancestral species which were different from present day species Evolution: change in genetic composition of population from generation to generation Summary of Other Ideas • • Many scientists before and during the time of Darwin theorized on what was the cause of the unity and diversity of living organisms Some ideas supported Darwin’s theories while some did not Intellectual Context of Darwin’s Ideas 1809 Lamarck publishes his hypothesis of evolution. 1798 Malthus publishes “Essay on the Principle of Population.” 1812 Cuvier publishes his extensive studies of vertebrate fossils. 1795 Hutton proposes his principle of gradualism. 1830 Lyell publishes Principles of Geology. 1858 While studying species in the Malay Archipelago, Wallace (shown in 1848) sends Darwin his hypothesis of natural selection. 1790 1870 1809 Charles Darwin is born. 183136 Darwin travels around the world on HMS Beagle. The Galápagos Islands 1859 On the Origin of Species is published. 1844 Darwin writes his essay on descent with modification. Not in Darwin’s “Camp” • Aristotle viewed species as unchanging • • Linneaus: Developed taxonomy and the binomial classification system • • Arranged them on a scala naturae, ladder of permanent “rungs” Believed in a divine placement of organisms in this classification, not necessarily changing either Cuvier (geologist): Founded of modern paleontology (study of fossils) • • Saw that fossils are deposited in strata (layers) of rocks with older layers containing fossils of species that may be extinct now Believed in catastrophism, the sudden loss of local species due to disasters Lamarck’s Hypothesis of Evolution • Other scientists were thinking about life forms evolving over time • Lamarck hypothesized: • Use and disuse of body parts could change organisms over their lifespan • These changes could be passed to offspring by Inheritance of Acquired Characteristics Lamarck’s Hypothesis of Evolution • Lamarckism unsupported by much of genetic evidence • BUT emerging field of epigenetics may prove to support some of Larmarck’s ideas! Scientists in Darwin’s “Camp” • Hutton and Lyell believed: • • • Earth’s surface changes from cumulative, SLOW continuous actions still operating today (uniformitarianism) Strongly influenced Darwin’s thinking • He reasoned the Earth must be much older than previously believed • Also reasoned biological organisms may also have slow, continuous change Malthus (economist): • Limited resources shape population sizes Voyage of the HMS Beagle Darwin in 1840, after his return from the voyage HMS Beagle in port Great Britain EUROPE NORTH AMERICA ATLANTIC OCEAN The Galápagos Islands AFRICA PACIFIC OCEAN Pinta Genovesa Santiago Fernandina Isabela 0 20 40 Kilometers SOUTH AMERICA Equator Daphne Islands Pinzón Santa Santa Cruz Fe Florenza Equator Chile PACIFIC OCEAN San Cristobal Española Andes Mtns. Marchena PACIFIC OCEAN Brazil Argentina Cape Horn Malay Archipelago AUSTRALIA Cape of Good Hope Tasmania New Zealand Darwin’s Focus on Adaptation • During the voyage, Darwin observed many adaptations: • Inherited characteristics of organisms which increase their survival and reproduction in different environments • Darwin saw how adaptation to environment and the origin of species is closely related • Theorized: Could new species arise from gradual accumulation of adaptations to different environments? Darwin’s Focus on Adaptation How do adaptations arise? Largely dependent on: Natural selection: Process in which individuals with certain inherited traits tend to survive and reproduce at higher rates than other individuals because of those traits Example of Speciation in Galapagos Finches (a) Cactus-eater (b) Insect-eater (c) Seed-eater Artificial Selection • Humans have modified other species by selective breeding for 1000’s of years, a process called artificial selection The Reason Why You Shouldn’t Procrastinate • In 1844, Darwin wrote an essay on natural selection--but did not publicize it • In June 1858, Wallace sent Darwin a paper to review with an almost identical theory of natural selection • Darwin quickly finished The Origin of Species by Means of Natural Selection and published it • Wallace graciously stepped away so now we call it “Darwinism” and not “Wallacism”! Observations of Darwin • Observation #1: Members of a population often vary in their inherited traits Observations of Darwin Observation #2: All species can produce more offspring than the environment can support • Many offspring fail to survive and reproduce Important Points About Natural Selection • Note that individuals do not evolve; but populations evolve over time • Natural selection can only increase or decrease heritable traits that vary in a population • If everyone is genetically identical, no basis for selection • Adaptations vary with different environments • Single adaptation not preferred for all environments Direct Observations of Evolutionary Change • The bacterium Staphylococcus aureus is commonly found on people • One strain, methicillin-resistant S. aureus (MRSA) is a dangerous pathogen • S. aureus became resistant to penicillin in 1945 and resistant to methicillin in 1961 Direct Observations of Evolutionary Change • Methicillin works by inhibiting a protein used by bacteria in their cell walls • But MRSA bacteria use a different protein in their cell walls • When exposed to methicillin, MRSA strains are more likely to survive and reproduce than nonresistant S. aureus strains • MRSA strains are now resistant to many antibiotics Homology • Homology is similarity resulting from common ancestry • Homologous structures are anatomical resemblances that represent variations on a structural theme present in a common ancestor Mammalian Forelimbs: Homologous Structures Humerus Radius Ulna Carpals Metacarpals Phalanges Human Cat Whale Bat Embryonic Homologies Pharyngeal pouches Post-anal tail Chick embryo (LM) Human embryo • Comparative embryology reveals anatomical homologies not visible in adult organisms Vestigial Structures • Vestigial structures are remnants of features that served important functions in the organism’s ancestors • Common human vestigial structures: • Appendix • Male nipples • Wisdom teeth • Body hair and goosebumps • Ear muscles Evolutionary Tree Branch point Lungfishes Digitbearing limbs Amnion Amniotes Mammals 2 Lizards and snakes 3 4 Homologous characteristic Crocodiles Ostriches 6 Feathers Hawks and other birds Birds 5 Tetrapods Amphibians 1 Different Cause of Resemblance: Convergent Evolution • Convergent evolution is the evolution of similar, or analogous, features in distantly related groups • Analogous traits arise when groups independently adapt to similar environments in similar ways • Convergent evolution does not provide information about ancestry Examples of Convergent Evolution Homology vs. Convergent Evolution • Both are evidence of evolution • Analogous features (convergent evolution) share similar function but not common ancestry • Homologous features (homology) share common ancestry but not necessarily similar function