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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