CH. 16DARWIN’S THEORY

OF EVOLUTION

Darwin’s epic journeyevolution: process of change over timeCharles 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 of discovery

journey, contDarwin was invited to sail on HMS

Beagle’s 5-yr voyage mapping coastline of South America as ship’s naturalistcollected 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 globeex.: rheas, emus, & ostriches are large, flightless birds living in similar grasslands on different continents

16.1, Darwin’s voyage, cont

observations, cont3 major observations, cont

different, related, species often occupied different habitats w/in local areaex.: tortoises on Galápagos Islands have different shells, depending on habitat type of their home island

some fossils of extinct species were similar to living speciesfossils: preserved remains or traces of ancient organisms

ex.: glyptodont – extinct, giant, armored animal similar to armadillo

16.1, Darwin’s voyage, cont

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 traitshe 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 environmentindividuals 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 theoryclosely related species differentiate in slightly different climatesex.: 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 environmentsground-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 Earthevolution takes long timeHutton & 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 findsDarwin’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, contsince 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, conthomologies (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 pressuresif 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, contgenetic similarities are major evidence that all organisms evolved from common ancestors

homologous moleculeshomologous ptns share extensive structural & chemical similarities

homologous ptns in different species have nearly identical DNA sequences

testing natural selectiongather evidence for evolutionary change by

observing natural selection in actionex.: Galápagos finches; Darwin’s proposed that natural selection shaped beaks of bird populations to eat different foods

16.4, EVIDENCE, CONT testing, cont

evidence, contex.: 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 ecologyevolution is often called the grand unifying theory of the life sciences

evolutionary theory is constantly reviewed w/ new datastill 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