Chapter 16

Darwin’s Theory of Evolution

Chapter 16 Mystery

• SUCH VARIED HONEYCREEPERS

• Page 449…

• Hypothesis: How did all these birds get to Hawaii? How did some of them come to have such specialized diets?

Section 16.1Darwin’s Voyage of Discovery

• Objectives:– What was Charles Darwin’s contribution to

science?– What three patterns of biodiversity did Darwin

note?

• Define:– Evolution– Fossil

I. Darwin’s Epic Journey• Charles Darwin – England – 1809• Evolution – process of change over time• Darwin developed a scientific theory of

biological evolution that explains how modern organisms evolved over long periods of time through descent from common ancestors– 1831 – HMS Beagle – 5 year voyage – map

coastline of SA– Darwin planned to collect specimens of plants

and animals

• Darwin’s work = vital insights into today’s world by showing how living world is constantly changing helps us understand modern phenomena (drug-resistant bacteria & newly emerging diseases like avian flu)

II. Observations Aboard the Beagle

• 1 day – Brazilian forest = 68 species of beetles (when he wasn’t particularly looking for beetles)

• Observed characteristics & habits

• Kept eyes and mind open to larger patterns into which observations might fit

• 3 distinct patterns:1. Species vary globally2. Species vary locally3. Species vary over time

A. Species Vary Globally• SA, Australia, Africa• Darwin noticed that different,

yet ecologically similar, animal species inhabited separated, but ecologically similar habitats around the globe.– Rhea – flightless, ground-dwelling

bird SA– Ostrich – same characteristics

Africa– Emu – same characteristics

Australia

– Rabbits in Eur. grasslands, but not in SA

– Kangaroos only in Aus.

B. Species Vary Locally• Darwin noticed that different,

yet related, animal species often occupied different habitats within a local area– 2 species of rhea (SA) – 1 in

grassland & 1 in colder, harsher scrubland

– Galapagos Islands – close to each other but different ecological conditions

• Distinct forms of giant land tortoises (shells varied from one island to another)

• Different islands = different varieties of mockingbirds (all resembled mockingbirds in SA) beaks w/ different shapes (wrens, warblers, blackbirds)

Mystery Clue

• Page 452…

• Like the small brown birds on the Galapagos, Hawaiian honey creepers live on islands with slightly different habitats.

• How might these varied habitats have affected the evolution of honeycreeper species

C. Species Vary Over Time

• Darwin also collected fossils– Fossil – preserved remains or

traces of ancient organisms• Some looked like current living

organisms, but some looked nothing like current living organisms

• Darwin noticed that some fossils of extinct animals were similar to living species– Glyptodont – giant armored

animal found in area where current armadillo is found

• Armadillo = smaller version of glyptodont

D. Putting the Pieces of the Puzzle Together

– Mockingbirds = 3 species of finches found nowhere else

– Wren, warbler, blackbirds = all species of finches found nowhere else

– Tortoises, marine iguanas, plants = only on Galapogos

• Did these species evolve from SA species?

• Suggested: species are not fixed & can change by some natural process

Section 16.2Ideas that Shaped Darwin’s Theory• Objectives:

– What did Hutton and Lyell conclude about Earth’s history?

– How did Lamarck propose that species evolve?– What was Malthus’s view of population growth?– How is inherited variation used in artificial selection?

• Define:– Artificial selection

I. An Ancient, Changing Earth

• People thought: Earth was few 1000 yrs old & didn’t change much

• Hutton and Lyell concluded that the Earth is extremely old and that the processes that changed Earth in the past are the same processes that operate in the present

• 1785 – Hutton presented info• Lyell – built on Hutton –

Principles of Geology - 1830

A. Hutton and Geological Change• Recognized connections b/w

geological processes and geological features (mountains, valleys, folded layers of rock)– Some rocks form from lava & some

form from layers of sediment• Proposed force beneath surface

can push rock layers upward = mountain ranges– Worn down by rain, wind, heat &

cold• Processes = v. slow so Earth

must be much older• Deep time – idea that our

planet’s history stretches back over a period of time so long that it is difficult to imagine

B. Lyell’s Principles of Geology• Laws of nature = constant over

time• Uniformitarianism –

geological processes we see today must be same ones that shaped Earth mya– Ancient volcanoes released

gases & lava– Ancient rivers dug channels

(canyons & valleys)• Darwin uses Lyell’s insights

– Earthquake raised shoreline 3m out of sea

– Could be how Andes mountains formed

– Explains how fossils of marine animals could be found at tops of mountains

II. Lamarck’s Evolutionary Hypothesis

• Jean Baptiste Lamarck – proposed theory of evolution based on growing fossil record

• Lamarck suggested that organisms could change during their lifetimes by selectively using or not using various parts of their bodies.

• He also suggested that individuals could pass these acquired traits onto offspring, enabling species to change over time– Published ideas in 1809 – year

Darwin was born

A. Lamarck’s Ideas• Proposed all organisms have

inborn urge to become more complex and perfect

• As a result organisms change and acquire features that help them live more successfully in environments– Orgs. can change size/shape of

organs by using bodies in new ways

• Example: waterbird = long legs– Structures of orgs. could change if

not used• Example: bird stopped using wings

= smaller wings• Acquired characteristics – traits

altered by an org during its life• Inheritance of acquired

characteristics – traits acquired during lifetime can be passed on to offspring

B. Evaluating Lamarck’s Hypothesis

• Lamarck was Incorrect b/c:1. Orgs do not have inborn drive to be

perfect2. Evolution does not mean that over time

species becomes “better” somehow3. Evolution does not progress in

predetermined direction4. traits acquired by orgs during lifetime

cannot be passed on to offspring

• Lamarck was one of 1st to try to explain evolution scientifically using natural processes

• Recognized link b/w org’s environment and body structures

III. Population Growth• 1798 – Thomas Malthus –

humans were being born faster than they were dying = overcrowding

• Malthus reasoned that if human population grew unchecked, there wouldn’t be enough living space and food for everyone– Forces that work against pop

growth = war, famine, disease• Darwin applied to other orgs

(maple tree, oyster)• Darwin realized most

organisms don’t survive and reproduce… wondered which individuals survived and why

IV. Artificial Selection• Darwin studied change produced by

plant and animal breeders– Breeders knew that indiv. orgs. vary:

some bear larger/smaller fruit than other species & some cows bear more/less milk than others in same herd

– Told Darwin that some of this variation could be passed from parent to offspring (used to improve crops/livestock)

• Artificial selection – nature provides the variation and humans select those they find useful & allow them to reproduce to produce offspring with similar traits

• Darwin recognized natural variation was v. impt b/c provided raw material for evolution

Section 16.3Darwin Presents His Case

• Objectives:– Under what conditions does natural selection

occur?– What does Darwin’s mechanism for evolution

suggest about living and extinct species?

• Define:– Adaptation– Fitness– Natural selection

I. Evolution by Natural Selection

• Evolution is a Scientific mechanism

• On the Origin of Species

• Combined ideas of his own with Lamarck and Malthus

A. Struggle for Existence• If more individuals are

produced than can survive members of a population must compete to obtain food, living space, and other limited necessities of life

• Called: Struggle for Existence

• Which individuals come out on top of struggle?

B. Variation and Adaptation• Individuals have natural variations

among heritable traits• Some variants are better suited to

life in their environment than others– Predatory species that are faster, longer

claws, sharper teeth catch more prey– Prey species that are faster, better

camouflaged avoid being caught

• Adaptation – any heritable characteristic that increases an org’s ability to survive and reproduce in its environment– Body parts or structures (tiger’s claws,

camouflage)– Physiological functions (photosynthesis)– Behaviors (complex avoidance

strategies of prey)

C. Survival of the Fittest

• Differences in adaptations affect an indiv’s fitness

• Fitness – how well an org can survive and reproduce in its environment– High fitness – well suited to env; can

survive & reproduce– Low fitness – not well suited to env die w/o

reproducing or leave few offspring

• Survival of the fittest – difference in rates of survival and reproduction– Survival – means more than living; must

also reproduce and pass on adaptations to next generation

D. Natural Selection• Natural selection – mechanism for evolution

- process by which orgs w/ variations most suited to local env survive and leave more offspring

– Only certain indivs in population produce new indivs • Fitness determined by environment• Natural selection occurs in any situation

in which more individuals are born than can survive (struggle for existence), there is natural heritable variation (variation and adaptation), and there is variable fitness among indivs (survival of fittest).

– Well adapted indivs survive and reproduce– Populations continue to change as they

become better adapted or as env changes– Natural selection acts only on inherited

traits b/c those are the only chars that parents can pass onto offspring

• Natural selection does not make orgs “better”

• Adaptations don’t have to be perfect – just good enough to enable org to pass its genes to next generation

• doesn’t have to move in fixed direction – no one perfect way of doing something

• If local env changes traits that were once adaptive may no longer be useful and different traits may become adaptive– If env changes faster than species

can adapt = extinct

Mystery Clue

• Page 463…

• How might natural selection explain the history of the Hawaiian honeycreepers?

II. Common Descent• Natural selection depends on ability

of orgs to reproduce– Every org alive today is descended from

parents who survived and reproduced

• Well-adapted indivs in species survived and reproduce well adapted species survive over time

• Living species are descended with modification from common ancestors (descent with modification)– Implies life has been on Earth for v. long

time– Hutton & Lyell – deep time gave enough

time for natural selection to occur

• Used fossil record for evidence

• Darwin based explanation for diversity of life on idea that species change over time

• “Tree Thinking” – implies that all organisms are related– Common ancestor shared by

tigers, panthers, cheetahs– Common ancestor shared by

these felines and horses, then bats

– Farther back all mammals share ancestor with birds, alligators, fish

• According to the principle of common descent, all species – living and extinct – are descended from common ancestors

Section 16.4Evidence of Evolution

• Objectives:– How does the geographic distribution of species today

relate to their evolutionary history?– How do fossils help to document the descent of

modern species from ancient ancestors?– What do homologous structures and similarities

• Define:– Biogeography– Homologous structure– Analogous structure– Vestigal structure

I. Biogeography• Biogeography – study of

where orgs live now and where they and their ancestors lived in the past

• Patterns in the distribution of living and fossil species tell us how modern orgs evolved from their ancestors

1. Pattern in which closely related species differentiate in slightly different climates

2. Pattern in which very distantly related species develop similarities in similar environments

A. Closely Related but Different• Biogeography of

Galapagos species suggested that population on island evolved from mainland species

• Over time natural selection on islands produced variations among populations– Resulted in different but

closely related island species

Weevils

Wasps

Snakes

Cotton

Beetle

Scorpion

Finches

B. Distantly Related but Similar• Similar habitats around world

often home to animals and plants that are only distantly related

• Similar ground-dwelling birds inhabit similar grasslands in Europe, Aus, Africa

– Differences in body structures among those animals provide evidence that they evolved from different ancestors

– Similarities among those animals provide evidence that similar selection pressures caused distantly related species to develop similar adaptations

Mystery Clue

• Page 465…

• How can biogeography help explain why some species of honeycreepers are found only on the Hawaiian Islands?

II. The Age of Earth and Fossils• 2 potential difficulties

– age of Earth– Gaps in fossil record

A. The Age of Earth• 50 yrs after Darwin physicists discovered

radioactivity• Geologists use radioactivity to establish age of

certain rocks and fossils– Radioactive dating indicates that Earth is ~4.5

billion years old– This is plenty of time for natural selection to take

place

B. Recent Fossil Finds• “imperfection of geological record” – 1859 –

paleontologists had not found enough fossils of intermediate forms of life to document evolution of modern species from their ancestors

• Many recently discovered fossils form series that trace evolution of modern species from extinct ancestors

• Since Darwin paleontologists have discovered hundreds of fossils that document intermediate stages in evolution of many different groups of modern species– Whales from ancient land mammals– Dinosaurs & birds– Fish & four-legged land animals

II. Comparing Anatomy and Embryology

• All vertebrate limbs have same basic bone structure– Some were used for crawling, some for

climbing, some for running, others for flying

A. Homologous Structures• Darwin proposed animals w/ similar structures evolved from

common ancestor w/ basic version of that structure– Homologous structures – structures shared by related species and have been

inherited from common ancestor• Evolutionary theory explains existence of homologous

structures adapted to different purposes as result of descent with modification from a common ancestor

– Biologists test whether structures are homologous by studying anatomical details, way structures develop in embryos, pattern in which they appeared over evolutionary history

• Similarities and differences among homologous structures help determine how recently species shared a common ancestor– Common ancestor to all four-limbed animals = ancient lobe-finned fish

(380 mya)– Also some plants have homologous roots, stems, flowers

1. Analogous Structures

• Clue to common descent = common structure

• Not common function

• Analogous structures – body parts that share common function but not structure– Example: wing of bee and wing of bird

2. Vestigial Structures• Vestigial structures – inherited from

ancestors but have lost much or all of their original function due to different selection pressures acting on descendant– Example: hipbones of bottlenose dolphin

• Presence does not affect organism’s fitness so natural selection does not act to eliminate it

B. Embryology

• Early developmental stages of many animals with backbones (vertebrates) look very similar

• Same groups of embryonic cells develop in same order and in similar patterns to produce many homologous tissues and organs in vertebrates

• Similar patterns of embryological development provide further evidence that organisms have descended from a common ancestor

III. Genetics and Molecular Biology• Darwin had no idea how heredity worked• Some of most impt evidence for theory comes

from genetics• At molecular level, universal genetic code

and homologous molecules provide evidence of common descent

• Also: we now understand how mutation and reshuffling of genes during sexual reproduction produce heritable variation on which natural selection operates

A. Life’s Common Genetic Code• All living cells use info coded in DNA and

RNA to carry info from one generation to next and to direct protein synthesis

• Genetic code is nearly identical in almost all orgs (bacteria, yeasts, plants, fungi, animals)

• ALL ORGS EVOLVED FROM COMMON ANCESTORS THAT SHARED THIS CODE

B. Homologous Molecules• Homology not limited to physical

structures– Homologous proteins – share

extensive structural and chemical similarities

• Example: cytochrome c – cellular respiration – found in almost all living cells

– Homologous genes – makes sense b/c all animals share same code

• Example: Hox genes – determine head-to-tail axis in embryonic development – found in almost all multicellular animals

• LIVING ORGS EVLOVED THROUGH DESCENT WITH MODIFICATION FROM A COMMON ANCESTOR

IV. Testing Natural Selection• To gather evidence observe natural

selection in progress• Evolutionary change takes place over

millions of years tough to see change actually happening

• Some have been observed and studied in labs and controlled outdoor environments

• Bacteria, guppies– Results support Darwin’s basic ideas

• Best examples of natural selection in action comes from observations on animals living in natural environment: Galapagos finches

A. A Testable Hypothesis• Galapagos finches – darwin

thought they were wrens, warblers, and blackbireds b/c looked so different

• Once he learned all finches, he hypothesized: descended from common ancestor– Beaks: different size and shapes –

used to pick up and eat different foods natural selection shaped beaks as became adapted to eat different foods

• Peter and Rosemary Grant – Princeton• Darwin’s theory rested on 2 testable

assumptions:1. For beak size and shape to evolve, there must

be enough hertiable variation in those traits to provide raw material for natural selection

2. Differences in beak size and shape must produce differences in fitness

• Caught and tagged each of one species on one island recaught to see which lived and reproduced

– Each individual bird recorded anatomical characteristics (wing length, leg length, total mass, beak length, beak depth, beak color, feather color)

• Showed great variation among finches

B. Natural Selection• Individual finches w/ different-size beaks have

better or worse chances of surviving both seasonal droughts and longer dry spells– food scarce birds w/ largest beaks – more

likely to survive• Result: average beak size in finch population

has increased dramatically• The Grants have documented that natural

selection takes place in wild finch populations frequently, and sometimes rapidly

• Changes in food supply created selection pressure that caused finch populations to evolve w/in decades– Occurred much faster than many researchers

thought possible

• Grants documented natural selection in nature

• Their data also confirm that competition and environmental change drive natural selection– Traits that don’t matter much under one set of

environmental conditions became adaptive as environment changes during a drought

• The Grants’ work shows that variation within a species increases the likelihood of the species’ adapting to and surviving environmental change– Without heritable variation in beak sizes, the

medium ground finch would not be able to adapt to feeding on larger tougher seeds during a drought

C. Evaluating Evolutionary Theory

• Advances in fields of biology & other sciences confirmed and expanded most of Darwin’s hypotheses

• Evolutionary theory – includes natural selection – offers insights that are vital to all branches of biology

• Evolution often called unifying theory of the life sciences• Constantly reviewed as new data is gathered• Debate questions (how new species arise and why

species become extinct)• Uncertainty about exactly how life began• Questions remain about how evolution works; not

whether evolution occurs• To scientists, evolution is key to understanding natural

world

Solve the Mystery• Page 477….1. Suppose a small group of birds, not unlike the modern

honeycreepers, landed on one of Hawaii’s islands millions of years ago and then reproduced. Do you think all the descendants would have stayed on that one island? Explain.

2. Do you think that the climate and other environmental conditions are exactly the same everywhere on the Hawaiian Islands? How might environmental conditions have affected the evolution of honeycreeper species?

3. Explain how the different species of honeycreepers in Hawaii today might have evolved from one ancestral species.

4. Why are islands often home to species that exist nowhere else on Earth?