Ch. 16 Darwin’s Theory of Evolution

1. Ch. 16Darwin’s Theory of Evolution

2. 16.1, Darwin’s voyage of discovery • Darwin’s epic journey • evolution: process of change over time • Charles Darwin grew up @time when scientific view of nature was shifting dramatically • geologists 1st suggested Earth was ancient & had changed over time, & biologists suggested life had too • Darwin developed theory of biological evolution that explains how modern organisms evolved over long periods of time through descent from common ancestors

3. 16.1, Darwin’s voyage, cont • journey, cont • Darwin was invited to sail on HMS Beagle’s5-yr voyage mapping coastline of South America as ship’s naturalist • collected specimens of plants & animals on voyage • observations aboard HMS Beagle • 3 major observations • different, yet ecologically similar, species inhabited separated, but ecologically similar, habitats around globe • ex.: rheas, emus, & ostriches are large, flightless birds living in similar grasslands on different continents

4. 16.1, Darwin’s voyage, cont • observations, cont • 3 major observations, cont • different, related, species often occupied different habitats w/in local area • ex.: tortoises on Galápagos Islands have different shells, depending on habitat type of their home island • some fossils of extinct species were similar to living species • fossils: preserved remains or traces of ancient organisms • ex.: glyptodont – extinct, giant, armored animal similar to armadillo

5. 16.2, ideas that shaped Darwin’s thinking • ancient, changing Earth • based on work of other researchers & evidence they uncovered, Geologists James Hutton & Charles Lyell concluded Earth is extremely old & processes that changed Earth’s past also operate now • Hutton & geological change • connections between geological processes & geological features (layered rock built by sediment, mountains pushed up & then worn down, etc.) • slow processes, so our planet must be much older than few thousand years • called this deep time – our planet’s history stretches back so long that it’s difficult to imagine

6. 16.2, ideas, cont • ancient, cont • Lyell’s Principles of Geology • presented uniformitarianism – idea that geological processes we see today must be same ones that shaped Earth millions of years ago(volcanism, erosion by rivers, etc.) • both Hutton’s & Lyell’s theories depend on ancient Earth (> few 1000 yrs. of recorded history) • Darwin saw evidence that Lyell was correct • witnessed earthquake that lifted shoreline > 3 m up, w/ sea animals clinging to it • later, observed fossils of marine animals in mountains 1000s of feet above sea level

7. 16.2, ideas, cont • Lamarck’s evolutionary hypotheses • in 1809, Jean-Baptiste Lamarck proposed that organisms could change during their lifetimes by selectively using or not using various parts of their bodies • thought all organisms w/ inborn urge to become more complex &perfect • acquired characteristics:traits altered by individual organism during its life • Lamarck also thought organisms could pass on alterations made during its lifetime to its offspring (inheritance of acquired characteristics)

8. 16.2, ideas, cont • Lamarck, cont • evaluating Lamarck’s ideas • no inborn drive to become more perfect • evolution does not mean that species becomes “better” somehow • traits acquired by individuals during their lifetime cannot be passed on to offspring • important because Lamarck was one of 1st naturalists to suggest that species are not fixed • tried to explain evolution scientifically using natural processes • recognized link between organism’s environment &its body structures

9. 16.2, ideas, cont • population growth • Thomas Malthus & populations • in 1798, Malthus noted that humans were being born faster than people were dying, causing overcrowding • suggested that forces against population growth include war, famine, & disease • reasoned that if human population grew unchecked, there wouldn’t be enough living space & food for everyone • Darwin realized that Malthus’s reasoning applied to other organisms

10. 16.2, ideas, cont • artificial selection • plant & animal breeders knew individual organisms vary, & that some of this variation could be passed from parents to offspring & used to improve crops/livestock • artificial selection: process in which nature provides variations, & humans select those they find useful • Darwin tested artificial selection by raising & breeding plants &fancy pigeon varieties • Darwin had no idea how heredity worked or what caused variation, but he knew that variation occurs in wild species too

11. 16.3, Darwin presents his case • evolution by natural selection • natural selection occurs in any situation in which more individuals are born than can survive, there is natural heritable variation, & there is variable fitness among individuals • struggle for existence • after reading Malthus, Darwin realized that if more individuals are produced than can survive, they must compete for food, living space, etc. • variation & adaptation • Darwin knew that individuals have natural variations among their heritable traits • he hypothesized that some variants are better suited to life in their environment than others

12. 16.3, case, cont • natural selection, cont • survival of the fittest • fitness: how well organism can survive & reproduce in its environment • individuals w/ adaptations that are well-suited to their environment can survive & reproduce, so are said to have high fitness • individuals w/ characteristics that are not well-suited to their environment either die w/o reproducing or leave few offspring, so are said to have low fitness • survival of the fittest:difference in rates of survival & reproduction

13. 16.3, case, cont • natural selection, cont • natural selection:process by which organisms w/ variations most suited to their local environment survive &leave more offspring • similar to artificial selection, but environment determines fitness, instead of breeder • generation to generation, populations continue to change as they become better adapted, or environment changes • acts only on inherited traits because those are only characteristics parents can pass on to offspring • adaptations don’t have to be perfect — just good enough to let organism pass its genes to next generation

14. 16.3, case, cont • common descent • according to principle of common descent, all species — living & extinct — are descended from ancient common ancestors • natural selection depends on ability of organisms to reproduce & leave descendants, so organisms today descended from parents who survived & reproduced • Darwin proposed that, over many generations, adaptation could cause successful species to evolve into new species • descent w/ modification:proposal that living species are descended, w/ some changes, from common ancestors • for evidence of descent w/ modification, Darwin pointed @ fossil record • single “tree of life” links all living things

15. 16.4, evidence of evolution • biogeography • patterns in distribution of living & fossil species tell us how modern organisms evolved from their ancestors • biogeography:study of where organisms live now & where they & their ancestors lived in past • 2 biogeographical patterns significant to Darwin’s theory • closely related species differentiate in slightly different climates • ex.: biogeography of Galápagos species suggested that island populations (tortoises, finches, etc.) evolved from mainland species • over time, natural selection on islands produced variations

16. 16.4, evidence, cont • biogeography, cont • 2 patterns, cont • very distantly related species develop similarities in similar environments • ground-dwelling birds (rheas, ostriches, and emus) inhabit similar grasslands in Europe, Australia, and Africa • differences in body structures provide evidence that they evolved from different ancestors • similarities among provide evidence that similar selection pressures caused distantly-related species to develop similar adaptations

17. 16.4, evidence, cont • age of Earth & fossils • many recently discovered fossils form series that trace evolution of modern species from extinct ancestors • age of Earth • evolution takes long time • Hutton & Lyell argued Earth was very old, but technology in their day couldn’t tell how old • radioactive dating indicates that Earth is about 4.5 billion years old • recent fossil finds • Darwin’s fossil studies convinced scientists life evolved, but hadn’t found enough fossils by 1859 to show evolution of modern species from their ancestors

18. 16.4, evidence, cont • age, cont • recent finds, cont • since Darwin, paleontologists have discovered 100s of fossils that document intermediate stages in evolution of many different groups of modern species • all historical records are incomplete, & history of life is no exception

19. 16.4, evidence, cont • comparing anatomy & embryology • evolutionary theory explains existence of homologous structures adapted to different purposes as result of descent w/ modification from common ancestor • homologous structures: structures shared by related species & inherited from common ancestor • test if structures are homologous by studying anatomical details, how structures develop in embryos, & pattern of appearance over evolutionary history

20. 16.4, evidence, cont • comparing, cont • homologous, cont • homologies (a.k.a. homologous structures) may have different functions • clue to common descent is common structure, not common function • analogous structures: body parts w/ common function, but not structure (doesn’t indicate any relationship) • vestigial structures: inherited from ancestors, but have lost much or all of original function due to different selection pressures • if presence of vestigial structure does not affect organism’s fitness, natural selection would not eliminate it

21. 16.4, evidence, cont • comparing, cont • embryology: study of development of unborn organisms (in egg or mother’s uterus) • early developmental stages of many animals w/ backbones (called vertebrates) look very similar • similarities in embryological development provide evidence that organisms have common ancestor • genetics & molecular biology • @ molecular level, universal genetic code & homologous molecules provide evidence of common descent • all cells use DNA & RNA to carry info from one generation to next & direct ptn synthesis

22. 16.4, evidence, cont • genetics, cont • DNA, cont • genetic similarities are major evidence that all organisms evolved from common ancestors • homologous molecules • homologous ptnsshare extensive structural & chemical similarities • homologous ptns in different species have nearly identical DNA sequences • testing natural selection • gather evidence for evolutionary change by observing natural selection in action • ex.: Galápagos finches; Darwin’s proposed that natural selection shaped beaks of bird populations to eat different foods

23. 16.4, evidence, cont • testing, cont • evidence, cont • ex.: finches, cont • Peter & Rosemary Grant tested this by catch & release studies, documenting changes in food supply each year • found that natural selection takes place in wild finch populations frequently, & sometimes rapidly

24. 16.4, evidence, cont • evaluating evolutionary theory • evolutionary theory offers insights that are vital to all branches of biology, from research on infectious disease to ecology • evolution is often called the grand unifying theory of the life sciences • evolutionary theory is constantly reviewed w/ new data • still debate important questions, like details of how new species arise &become extinct • significant uncertainty about exactly how life 1st began • remaining questions about how evolution works — not if it occurs