Ch. 16 Darwin’s Theory of Evolution

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

16.1, Darwin’s voyage, cont journey, cont Darwin was invited to sail on HMS Beagle’s 5-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

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

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

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

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)

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

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

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

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

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

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

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

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

16.4, 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

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

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

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

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

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

16.4, evidence, cont genetics, cont DNA, cont genetic similarities are major evidence that all organisms evolved from common ancestors homologous molecules homologous ptns share 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

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

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