1. Ch. 16 Darwin’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 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’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

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 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 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

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