Illustrated Example of Evolution by Natural Selection

Natural VariationElephants within the population have different

alleles causing variation in trunk length.

Very Long

Long

Very Short Short

Overpopulation and Struggle for Existence

Overpopulation and scarce resources cause elephants to compete for food and water.

Differential Survival & Reproduction

Elephants with longer trunks can reach food and water easier so are more likely to survive and reproduce passing on their genes to the

next generation.

X

Natural SelectionOver many generations short trunk genes will

be eliminated from the population and all of the elephants will have long trunks.

Important Notes1. Individuals don’t evolve, only groups do.

2. Long trunks didn’t spread “because elephants needed them.”

3. The elephants didn’t choose this evolutionary path.

4. They didn’t evolve for “the good of the species.”

Long trunks became more common simply because long-trunked elephants were having more

babies!

Not how it works! ->

Flowchart

• Natural selection changes the genetic composition of a population over time = evolution.

• Stabilizing Selection: individuals with the average phenotoype have the greatest fitness– Bell curve stays or becomes higher in the

center.– Example: Human birth weight

• Directional Selection: individuals with one phenotype extreme have the greatest fitness– Bell curve shifts toward the extreme with

better fitness.– Example: Giraffe neck length

• Disruptive Selection: individuals with either phenotype extreme have the greatest fitness– Bell curve splits into two.– Example: Finch beak size

Natural Selection on Polygenic TraitsNatural selection acts on polygenic traits in 3 ways:

There’s also Evolution NOT by Natural Selection

• Genetic drift – random change in allele frequencies that occurs in small populations (i.e. pure chance)

• Founder Effect – small subgroup of a population (founders) migrate to a new environment carrying a different allele frequency than the original group

Section 15-1: The Puzzle of Life’s Diversity

Voyage of the Beagle• In 1831, Darwin set off on a sea voyage around

the world making observations of nature.

• In 1859, Darwin published the results of his work: On the Origin of Species

The Galapagos The Galapagos Islands were Islands were

amazing!amazing!

• Put forth his ideas on how organisms change over time by means of “Natural Selection”

• Presented evidence that evolution has been occurring over millions of years and still continues

Darwin’s Observations

Mr. Darwin, how did you form your ideas?Mr. Darwin, how did

you form your ideas?

I made many I made many observations and observations and collected many collected many

samples along the samples along the way, especially on way, especially on

the Galapagos!the Galapagos!

This made him wonder… Why? How did they become different?

This made him wonder… Why? How did they become different?

Why were these creatures found only on the Galapagos Islands?

Darwin found…many species that are unique to the Galapagos Islands

Darwin noticed that the Galapagos species were similar to mainland

species but had distinct differences.

Darwin noticed that the Galapagos species were similar to mainland

species but had distinct differences.

present day Armadillos

Darwin found Evidence that creatures have changed over time

ancient Armadillo

Darwin asked:Darwin asked:

Why should extinct Why should extinct armadillos & modern armadillos & modern armadillos be found on armadillos be found on same continent?same continent?

Darwin observed…clues in the fossils

Darwin observed … on different islands the tortoises had different neck lengths and shells.

Food on Hood Island is sparse and high off the

ground.

Food on Hood Island is sparse and high off the

ground.Food on Isabella Island is lush,

plentiful, and low on the ground.

Food on Isabella Island is lush,

plentiful, and low on the ground.

Good thing Good thing my neck is so my neck is so

long!long!

Darwin thought Darwin thought the tortoises were the tortoises were very well suited to very well suited to their environment.their environment.

Darwin asked: Darwin asked: Is there a Is there a

relationship an relationship an animal’s traits and its animal’s traits and its

environment?environment?

I’m sure glad I I’m sure glad I don’t have that don’t have that great big neck!great big neck!

Darwin observed… many different birds.

Finch?

Sparrow?Woodpecker?

Warbler?

Darwin thought he found Darwin thought he found all kinds of different birds all kinds of different birds on the Galapagos.on the Galapagos.

Then he realized he Then he realized he actually found 14 actually found 14 different species of the different species of the same bird - finches.same bird - finches.

Large Ground Finch

Small Ground Finch

Tree Finch

Warbler Finch

Darwin noticed thatdifferences between

the finches were associated with the

type of food they ate.

Darwin noticed thatdifferences between

the finches were associated with the

type of food they ate.

Big seed eater

Leaf & bud eater

Insect eater Small seed eater

Darwin observed Darwin observed that we were well that we were well

suited to our suited to our environment.environment.

There’s only 1 species of finch on the mainland.

There’s only 1 species of finch on the mainland.

Darwin asked:Darwin asked: If If the Galapagos finches the Galapagos finches

came from the came from the mainland, why are they mainland, why are they

so different now?so different now?

Did all of these Did all of these different finches different finches descend from a descend from a

common common ancestor?ancestor?

Why are there so Why are there so many different many different

species of finches species of finches on the Galapagos?on the Galapagos?

Summary of Darwin’s Observations

• Many plants and animals seem well (but not perfectly) suited to the environment they inhabit.

• Similar climates in different parts of the world have different species.

• Many fossils resemble organisms that are still alive yet others are completely different.

• Animals and plants on the Galápagos Islands look similar to species on South America yet are different from mainland species and each other.

Section 15-2: Ideas that shaped Darwin’s Thinking

• Darwin was influenced by many scientists:

• Hutton – found that Earth is millions of years old, not thousands as was previously thought

• Lyell – processes shaping Earth now have been for long periods of time

• Lamarck – posited inheritance of acquired characteristics. Even though his idea was wrong, he suggests change over time

• Malthus – predicted that the human population would grow faster than the space and food needed to sustain it, points out competition for resources

Lamarckian Evolution• By selective use or disuse of body parts, organisms

acquired or lost certain traits during their lifetime. • Acquired traits could then be passed on to their offspring. • Over time, this process led to change in a species.

• Examples:

Of course, Lamarck’s Idea is WRONG!

?

?

XStart with a mouse.Now chop off her tail.If she has babies, will they have tails?

Insect eaters

Bud eater

Seed eaters

Cactuseater

Warbler

finch

Tree

finc

hes G

round finches

Section 15-3: Darwin Presents His Case – Evolution by

Natural Selection

Darwin figures it out!Aha! It started when Aha! It started when

some South some South American finches got American finches got

stranded on the stranded on the Galapagos…Galapagos…

different beaks are inherited variations adaptations that help birds compete for food these birds survive & reproduce pass on the genes for those more fit beaks over time nature selected for different species

with different beaks

Warbler finch

Woodpecker finch

Small insectivorous

tree finchLarge

insectivoroustree finch

Vegetariantree finch

Cactus finch

Sharp-beaked finch

Small groundfinch

Mediumground finch

Large groundfinch

Insect eaters

Bud eater

Seed eaters

Cactuseater

Warbler

finch

Tree

finc

hes G

round finches

variationnatural selection for best survival & reproduction

From 1 species to 14 species…

Darwin’s finches• Darwin’s conclusions about finches

– variations in beaks • natural variety in beaks in the original flock• different foods available on different islands

– natural selection• the finches with certain beaks were more likely to reproduce• different beaks are advantageous on different islands

– offspring inherit successful traits• accumulation of “winning” beaks on different islands

– separate into different species

Section 16-3:The Process of Speciation

Speciation• Species – a group of organisms that can

breed with one another and produce fertile offspring

• Speciation – the process in which a new species emerges

Speciation• Speciation is caused by Reproductive Isolation –

members of two populations cannot interbreed or produce fertile offspring– Separate gene pools are formed, if the isolation lasts

long enough the gene pools become too different from each other

Three Isolating Mechanisms• Geographic Isolation – physical

separation of members of a population resulting in different environmental pressures

– Depending on the organism they can be separated by small distances (puddle to puddle) or by large distances (Grand Canyon, mountain range, etc.)

Three Isolating Mechanisms

• Temporal Isolation – populations reproduce at different times of the day or year

• Behavioral Isolation –populations are capable of interbreeding, but don’t because of courtship rituals or behaviors

Example of Speciation:Galapagos Finches

1. Founder Effect: Few birds came from mainland South America

2. Some birds crossed to another island, but rarely flew back—isolated

3. Gene pool on each island specialized and adapted to specific environment

4. Reproductive Isolation—if birds flew back to original island, would have changed too much to mate with original birds

5. Competition—2 species live together and compete for food—become specialized

6. Continue to adapt, move, and change

Evidence for Evolutionby

Natural Selection

Hunting for evolution clues!

Common Descent• Common or Shared Ancestor - An

ancient population that, through speciation, eventually gave rise to two modern ones– All living things have shared ancestry, it’s just

a matter of how far back it was in history

The Basic IdeaMore Similar = More Recent Shared Ancestor

• Who do you share more DNA with… your brother or your fourth cousin? Why?

• Who do you probably share more traits with… your brother or your fourth cousin? Why?

• The more recent your shared ancestor, the less time you’ve been growing apart from each other, so the more similar you’ll be– Manifests in many ways, letting us study evolutionary

history from many angles!

Fossils tell a story…

The Earth is old.The Earth is old.

Life is old.Life is old.

Life on Earth has changed.Life on Earth has changed.

• Fossils create a record over time as new layers of rock form over old ones.

• Fossils show how characteristics of organisms change over time.

• Fossils show that organisms alive today haven’t always been around.

The Fossil Record

Land Mammal

?

???

Ocean Mammal

Complete seriesof transitional

fossils

The fossils have been found!

Evolution from sea to land2006 fossil discovery of early 4 legged animal2006 fossil discovery of early 4 legged animal

Like all transitions, sea to land was gradualLike all transitions, sea to land was gradual

Geographic Distribution of Species

• Species in connected areas are similar even though their environments are different.

• Evidence of a common ancestorHad a common ancestorHad a common ancestor

Adapted or changed after moving to different types

of environments

Adapted or changed after moving to different types

of environments

Homologous Body Structures• Homologous structures – originate from the

same embryonic tissue

Animals with different structures on the outside that are used for different purposes,

Animals with different structures on the outside that are used for different purposes,

But under the skin…But under the skin…

They have the same bones!They have the same bones!

Homologous Body Structures• Homologous structures – originate from the

same embryonic tissue• Inherited from common ancestor

Vestigial organs• Vestigial structures –body structures that have lost

their original function and diminished over time

• Help illuminate what the ancestors were like

Why would whales have pelvis and leg

bones if they are sea creatures?

Why would whales have pelvis and leg

bones if they are sea creatures?

Why would blind fish that live in dark

caves have eyes?

Why would blind fish that live in dark

caves have eyes?

Because my ancestors walked

on land!

Because my ancestors could

see!

Similarities in Early Development• Embryos of many vertebrates develop in similar

patterns.• Patterns inherited from common ancestor

All vertebrate embryos have a “gill pouch” at one stage of development.

All vertebrate embryos have a “gill pouch” at one stage of development.

Similarities in DNA

• Probably most powerful tool in studying evolution today.

• DNA sequences and protein structures can be compared to determine how closely related organisms are.

• Evidence of common ancestors

100 20 30 40 50 60 70 80 90 100 110 120

LampreyFrogBirdDogMacaqueHuman

328 45 67 125

# of different amino acids in hemoglobin between

humans and other species

# of different amino acids in hemoglobin between

humans and other species

Chromosome Numbers in the great apes:

human (Homo) 4646chimpanzee (Pan) 48gorilla (Gorilla) 48orangutan (Pogo) 48

Chromosome Numbers in the great apes:

human (Homo) 4646chimpanzee (Pan) 48gorilla (Gorilla) 48orangutan (Pogo) 48

If these organisms share a common ancestor, then why is

the chromosome number different in humans?

If these organisms share a common ancestor, then why is

the chromosome number different in humans?

Human Chromosome #2 shows the exact point at which this fusion took place.

Human Chromosome #2 shows the exact point at which this fusion took place.

Hillier et al (2005) “Generation and Annotation of the DNA sequences of human chromosomes 2 and 4,” Nature 434: 724 – 731.

Hillier et al (2005) “Generation and Annotation of the DNA sequences of human chromosomes 2 and 4,” Nature 434: 724 – 731.

“Chromosome 2 is unique to the human lineage of evolution, having emerged as a result of head-to-head fusion of two acrocentric chromosomes that remained separate in other primates. The precise fusion site has been located in 2q13–2q14.1 (ref. 2; hg 16:114455823 – 114455838), where our analysis confirmed the presence of multiple subtelomeric duplications to chromosomes 1, 5, 8, 9, 10, 12, 19, 21 and 22 (Fig. 3; Supplementary Fig. 3a, region A). During the formation of human chromosome 2, one of the two centromeres became inactivated (2q21, which corresponds to the centromere from chimp chromosome 13) and the centromeric structure quickly deterioriated (42).”

“Chromosome 2 is unique to the human lineage of evolution, having emerged as a result of head-to-head fusion of two acrocentric chromosomes that remained separate in other primates. The precise fusion site has been located in 2q13–2q14.1 (ref. 2; hg 16:114455823 – 114455838), where our analysis confirmed the presence of multiple subtelomeric duplications to chromosomes 1, 5, 8, 9, 10, 12, 19, 21 and 22 (Fig. 3; Supplementary Fig. 3a, region A). During the formation of human chromosome 2, one of the two centromeres became inactivated (2q21, which corresponds to the centromere from chimp chromosome 13) and the centromeric structure quickly deterioriated (42).”

Two of the ancestral chromosomes fused together in humans.

Two of the ancestral chromosomes fused together in humans.

More Similar = More Recent Shared Ancestor

*Similarities and differences illuminate relationships between species, and paint a

picture of what ancestral species were like too

And let’s not forget the fun of watching evolution in action today!

-There is no such thing as “done evolving,” or being “more highly evolved” than another

species.

Evolution in Action!

Pesticide resistancePesticide resistance

Antibiotic resistanceAntibiotic resistance

Observed Natural Selection• Spray the field, but…

– insecticide didn’t kill all individuals• variation

– resistant survivors reproduce

– resistance is inherited

– insecticide becomes less & less effective

Artificial Selection

• Artificial Selection = Natural selection intentionally “performed” by humans

Artificial Selection

• Teosinte + Artificial Selection over 9000 years -> Corn!– Nearly every human crop was created by

wielding the power of natural selection

Artificial Selection

• So were some of our best friends…

WolfKalahari Wild Cat

Artificial Selection

• So were some of our best friends…

Modern dogsModern cats

Artificial Selection

• Sometimes, it’s less than totally intentional.– Normal crabs before the Battle of Dan-no-Ura

in 1185, but now…

Artificial Selection• Artificial selection is also how we made

measles, smallpox, and polio vaccines.– Smallpox = Completely eradicated! Polio =

Close to eradicated! Measles = Was getting close to eradication, unfortunately vaccination rates have dropped and so now it’s coming back.

Observed Speciation Speciation is slow, but

nevertheless, we’ve seen a few new ones since 1905: Evening Primrose Kew Primrose Tragopogon Raphanobrassica Hemp nettle Madia citrigracillis Maidenhair fern Woodsia fern

Stephanomeira malheurensis Yellow Monkey Flower Apple Maggot Fly Gall Former Fly Flour Beetle Nereis acuminata Fireweed (Epilobium

angustifolium) Faroe Island House Mouse Goatsbeards Drosophila paulistorum

Observed Speciation

• And now, speciation events… because of us!– Originally, London harbored the mosquito Culex pipiens. It

tolerates cold well, hibernates in winter, and feeds on birds.– Then they built the Underground. Mosquitoes that moved into

the tunnels adapted to this different environment, and evolved into a new species, molestus. They do not tolerate cold, do not hibernate, and feed on mammals, mainly rats.

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