Download BIOE 103

BIOE 103
Evolution
Review
Galapagos Islands:
birds similar to S.
America
Cape Verde
Islands: birds
similar to Africa
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Our question for today:
“How do species change?”
Darwin figured out how.
Charles Darwin
1809 - 1882
Lecture outline
1. Selective breeding
2. Natural selection
3. Requirements for natural selection
4. Definitions
5. An example
6. Origin of variation
7. Avoiding misunderstanding
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1. Darwin argued
evolution was
caused by a
natural process
similar to plant
and animal
breeding.
Charles Darwin
1809 - 1882
Selective Breeding example #1
All dog are descended from wolves* and have
been bred by people into existing breeds
Thousands of
years later
*Or an extinct wolf-like animal
Selective breeding example #2
Modern corn is much larger than corn raised by
Native Americans
Hundreds of
years later
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Evolution in nature is similar to this
So we are going to examine how this occurs in some detail
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Basic biology information you need to
understand natural selection
The physical characteristics of any living organism are affected
by its genes and by its environment.
Examples: Height
-Height affected by genes
- This is why tall parents tend to have tall children
- Height is also affected by nutrition
-This is why people in well-fed countries tend to be taller
than people in less well-fed countries
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Examples of genes and environment
(in humans)
Trait
Skin color
Native language
Number of fingers
Athletic performance
Academic performance
Genes or environment?
Both genes and environment
All environment
All genes (excepting accidents)
Both genes and environment
Both genes and environment
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Wolves are larger, have
thicker fur, different
shape, bigger teeth, etc.
than Chihuahuas
because they have
different genes.
Modern corn is
genetically different
than native corn
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These are genetic transformations
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How would you do this?
Please give me step-by-step instructions for doing this that
you conceivably do yourself.
(E.g., Don’t tell me you are going to use genetic engineering)
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How to turn a wolf into a Chihuahua
1. Start with a population of
wolves that have varying
sizes and shapes.
2. Select the wolves that are
smallest, or look at least a
little like a Chiuahua.
3. Breed these “Chihuahua-like”
dogs.
4. These dogs have Chihuahualike” puppies
5. Continue until you have a
Chihuahua.
When you select short wolves to breed, you
are selecting individuals with certain genes
Wolves
Small dogs
TTCTGTTCGCCAGCCGGGCCCTGGCAAGCTGAGAC
...............A...................
For example, chihuahuas and other miniature dog breeds
have a different version of the gene insulin-like growth
factor I receptor.
Selectively breeding small dogs will increase frequency of
the gene sequence having an “A” (above) in population.
Remove “G’s”.
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Examples of selective breeding
Wild jungle fowl
Lay 30 eggs / year
Domestic hen
Lays 300 eggs / year
Examples of selective breeding
Broccoli, cabbage, kale, cauliflower, kohirabi all bred
from wild mustard
Selective
breeding can
change almost
any trait in any
organism
E.g. Colors in
carrots
See “Selective breeding”
2. Natural selection causes evolution in
a similar way
(except nature selects who reproduces)
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Alaskan winter “breeds” musk ox to
have warm wool
Some oxen have warmer wool than others.
Musk ox without warm wool do not survive to reproduce.
Offspring of surviving musk ox inherit warm wool.
This changes the population of musk ox.
Mountains “breed” sheep to be surefooted
Sheep that are not surefooted do not survive to reproduce
Cheetahs “breed” dik diks to be fast
Cheetah’s don’t want dik diks to be fast. But cheetahs eat
the slow dik diks, so only the fast dik diks survive to
reproduce.
Desert climate “breeds” cactus to conserve water
Desert animals “breed” cacti to have spines
3. Requirements for natural selection
Hypothetical case study:
Giraffes feeding from acacia trees
During a drought, there is a food shortage
The shortest giraffes die
Surviving (tall) giraffes have tall calves
And the average height of the
population has increased
Requirements for natural selection
1. Variable trait
2. Trait is heritable
(genetically based)
3. Differential
survival/reproduction
(“selection”)
If these three conditions are met,
a population WILL evolve
If these conditions are NOT met, a population will NOT evolve.
Natural selection practice problem
The giant anteater of South America eats only ants and
termites. Anteaters eat ants by sticking their tongue into
anthills and slurping up ants. Anteaters have an elongated
snout and their tongue is two feet long. According to the theory
of evolution, anteaters evolved from animals that did not have
such a long tongue. How would an evolutionary biologist
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explain how this change occurred?
Answer:
There was variation in the ancestral population of anteaters;
some of the ancestral anteaters had longer tongues than
others. The anteaters with longer tongues were more likely to
survive and reproduce than anteaters with shorter tongues.
The surviving anteaters had offspring that inherited the long
tongue.
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Natural selection practice problem
Sequoia trees live in mountains
in California where forest fires
are common. Sequoia trees have
very thick bark that protects
them from forest fires. According
to the theory of evolution,
sequoias evolved this thick bark
from ancestors that did not have
it. How would an evolutionary
biologist explain how this change
occurred?
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Natural selection practice problem
Whales are large mammals with streamlined bodies
that allow them to swim easily in the ocean. Unlike
most mammals, whales do not have hind limbs. How
would a biologist explain how whales lost their hind
limbs, assuming their ancestors had hind limbs?
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4. Definitions
selection
An environmental influence that gives
some individuals in a population a
reproductive or survival advantage over
others.
Principal cause of evolution.
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Evolution: 2 definitions
1. The theory that all species are descended
from a common ancestor, and have changed
with time
2. A change in genetic composition of a
population.
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This population has
different genes after
the drought.
Therefore it has
evolved.
“Evolving” in everyday speech means
something different than in biology
Evolve (everyday speech) : To undergo gradual change; develop
“Britney had to evolve since the music industry …”
“How did Ford evolve from one of the most admired companies…”
“…people need to evolve with the changing markets…”
“Lance Armstrong’s brand is evolving…”
5. An example of evolution in nature:
Beak depth in Darwin’s Finch in Galapagos Islands
Beak depth
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Finches with wide
beaks are better at
cracking open large
seeds.
Finches with narrow
beaks are better at
eating small seeds.
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Beak depth is a genetic trait
Parents with deep beaks have offspring with
deep beaks
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Bill depth in Darwin’s finches before and after a
drought makes small seeds scarce
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Before
drought
N
60
40
Avg = 9.59mm
20
0
7
8
9
10
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12
13
80
N
60
40
After drought
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Avg = 10.09 mm
0
7
8
9
10
11
Beak depth
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13
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This may seem like a trivial change…
…but changes like this can transform a species if they
keep occurring over a long time.
(Similar to plate tectonics).
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6. Origin of new variation
• Natural selection changes proportions of
individuals in a population.
• It won’t work unless there are some tall
individuals.
• So where do the tall giraffes come from in the first
place?
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Mutation creates individuals with new traits
This giraffe has a
mutation that makes
it taller.
• Mutations are random changes
in genes.
• Mutations are caused by
mistakes copying genes when
cells reproduce or by the effect
of radiation or chemicals.
• “Random” means mutations
happen by chance—not because
they are needed.
• Most mutations are harmful; but
some are beneficial.
7. Avoiding common misunderstandings
Evolution does not happen
through a process similar to sun
tanning in which individuals
change.
Getting a sun tan may darken
your skin, but it will not change
your genes (and therefore does
not affect your children).
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Shivering doesn’t change genes of individual musk ox.
Evolution occurs when environment changes genes in a
population by selecting who survives and who
reproduces.
Giraffes didn’t evolve long necks by
stretching to reach branches
Stretching may lengthen a giraffe’s neck, but will not change a
giraffe’s genes. This change will not be inherited.
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Species don’t evolve just because they
“need to”
All of the finches
“needed” larger
beaks, but that
didn’t cause any
finch to change.
Finches with
narrow beaks died.
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The key to understanding natural
selection is applying these requirements
Be ready for exam questions that test whether you can do this.
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Quiz Wednesday on Natural
selection
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Study guide
Be able to explain the difference between evolution and natural
selection.
Be able to define evolution in two ways.
Learn the requirements for natural selection.
Be able to use the requirements for natural selection to describe
how evolution occurs (see ‘cheetah evolution sample question’)
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Sample exam questions
Cheetahs are able to run faster than 60 miles per hour when chasing prey. How
would a biologist explain how the ability to run fast evolved in cheetahs,
assuming their ancestors could run only 20 miles per hour?
Bowhead whales are the only species of the great whales that live their entire
life in the cold water of the Arctic Ocean. Bowhead whales have a thick layer of
fat called blubber that helps keeps them warm in the cold water they live in.
According to the theory of evolution, bowhead whales evolved their thick
blubber from ancestors whose blubber was not as thick. How would an
evolutionary biologist explain how this change occurred?
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The end
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