Transcript Darwin found…

Genetics and Environment
 We’ve learned about DNA, a genotype
gives you a phenotype…






But why do we all have different DNA?
Why did you have a postcaudal tail as
an embryo?
Why don’t most of us have the allele for
Huntington’s disease?
Why are some alleles more common in
people from one continent?
Why is more than 99.9% of your DNA
the SAME as everybody else in the
world?
Why is 80% of your DNA the same as a
cow’s? Why is 50% of your DNA the
same as a banana tree’s?
Genetics and Environment
 Certainly the single
most foundational
idea in all of biology,
and perhaps the
greatest biological
discovery…

All life is connected.
What is Evolution?
 Evolution
changes in living organisms over time
 explains how modern organisms have
descended from ancient organisms, and
continue to change today

Evolution explains two significant
questions

unity of life
 Why do all living things share fundamental
similarities?

diversity of life
 Why, in spite of fundamental similarities, are living
things so different?
DNA
Early Ideas on Evolution
 Darwin was influenced by:
Lyell and Hutton: geologists who said that
the Earth was very old and described
uniformitarianism: explaining that the
geological structures changed continuously
over time
 Malthus: if the human population
continued to grow unchecked, sooner or
later there would be insufficient living
space and food for everyone.

Earlier ideas on Evolution
 LaMarck

“acquired
characteristics”
 creatures developed traits
during their lifetime in
response to need and
activity
 give those traits to their
offspring

example
 “in reaching higher
leaves giraffes stretch
their necks & give the
acquired longer neck to
offspring”
Refuting Lamarck
 Experiment:
Cut off a mouse’s tail
 Allow mouse to breed, see if offspring are
born tailless

 …They are not.
 Conclusion: Acquired traits are not
responsible for change.
?
Voyage of the HMS Beagle
 Charles Darwin invited to travel
around the world
1831-1836 (22 years old!)
 makes many observations of nature

 main mission of the Beagle was to chart
South American coastline
Robert Fitzroy
Voyage of the HMS Beagle
 Stopped in Galapagos Islands

500 miles off coast of Ecuador
Galapagos
Recently formed volcanic
islands. Most of animals on
the Galápagos live nowhere
else in world, but they look
like species living on South
American mainland.
800 km west of Ecuador
Darwin found…many unique species
Many of Darwin’s observations made
him wonder… Why?
Darwin asked:
Why were these creatures found only
on the Galapagos Islands?
Darwin found…clues in the fossils
Darwin found:
Evidence that creatures
have changed over time
present day Armadillos
Darwin asked:
ancient Armadillo
Why should extinct
armadillos & modern
armadillos be found on
same continent?
Darwin found:
Different shells on tortoises on different islands
Darwin asked:
Is there a relationship
between the environment
& what an animal
looks like?
Darwin found… birds
Darwin found:
Many different birds
on the Galapagos
Islands.
He thought he found Finch?
very different kinds…
Woodpecker?
Sparrow?
Warbler?
But Darwin found… a lot of finches
Darwin was amazed to
find out:
All 14 species of birds
were finches…
But there is only one
species of finch on the
mainland!
Large
ground
Finch?
finch
Small
ground
Sparrow?
finch
Darwin asked:
If the Galapagos
finches came from the
mainland, why are they
so different now?
Warbler finch
Woodpecker?
Tree
finch
Warbler?
Darwin’s view of Evolution
 Darwin
giraffes that
already
have long necks
survive better
 leave more
offspring who
inherit their long
necks

“Equation”
Variation
+Inheritance
+Struggle for life
Differential reproductive success
+Time
Evolution – Change over time
Darwin’s “Ingredients”: Variation
 Variation is the raw material for natural
selection

there have to be differences within population

some individuals must be more fit than others
Where does Variation come from?

random changes to DNA
Wet year
Beak depth
 Mutation
Dry year
Dry year
Dry year
 errors in mitosis & meiosis
1977
 environmental damage (rare)

mixing of alleles
 Independent assortment
 recombination of alleles
 new arrangements in every
offspring
1982
1984
11
Beak depth of
offspring (mm)
 Sexual Reproduction
1980
10
9
8
Medium ground finch
8
9
10
11
Mean beak depth of parents (mm)
Variation of Traits in a Population
 Within a population individuals vary in traits
 Measuring the frequency of most traits will produce a
bell curve graph.


This shows that few individuals have the extreme
phenotype
Most individuals have the average or medium
phenotype
Darwin’s “Ingredients”: Inheritance
 Evolution-vulnerable traits have a genetic basis
 Gene Pool: Total genetic information available in a
population
 Allele Frequency: how common an allele is within a
gene pool of a
population

Determined by dividing
the number of a certain
allele by the total number
of alleles of all types
Darwin’s “Ingredients”:
Struggle for Life
 A pair of barn swallows arrived at my

house in summer 1983. Barn swallows
have an average of 10 chicks a year,
and chicks return to their birthplace to
start their own nests the next year.
Adults live for 10 years, and siblings
will breed. How many barn swallows
were at my house by 1990?
Year
1983
1984
1985
1986
1987
1988
1989
1990
# of Adult Swallows
2
Year
1983
1984
1985
1986
1987
1988
1989
1990
# of Adult Swallows
2
12
Year
1983
1984
1985
1986
1987
1988
1989
1990
# of Adult Swallows
2
12
72
Year
1983
1984
1985
1986
1987
1988
1989
1990
# of Adult Swallows
2
12
72
432
Year
1983
1984
1985
1986
1987
1988
1989
1990
# of Adult Swallows
2
12
72
432
2592
Year
1983
1984
1985
1986
1987
1988
1989
1990
# of Adult Swallows
2
12
72
432
2,592
15,552
93,312
559,872
Overreproduction
 A single bacterium dividing every 20


minutes would have
40,000,000,000,000,000,000,000 offspring by
the end of one day. After two days, it would
have enough offspring to cover the entire
Earth in a 2 meter layer of bacteria.
In one year, a single pair of fruit flies would
have so many offspring, their descents
would weigh more than the planet.
The elephant is one of the slowest breeding
species. But one pair of elephants would
have 19 million descendents 750 years later.
By contrast, there are only 700,000
elephants alive in the whole world today.
Darwin’s “Ingredients”: Struggle for
Life
 Living things “overreproduce.” There
are more offspring born than can be
sustained.
 Therefore, there is competition for
resources to survive and reproduce.

“Nature, red in tooth and claw” Tennyson
Competition
 Why aren’t we buried in barn swallows,
bacteria, fruit flies, elephants, and everything
else?
 Limiting factors = things that limit equal
survival and reproduction for all individuals
include…
Limited resources (examples: oxygen, nitrogen,
water, food, space)
Predation (being eaten, disease)
Dangerous environments (climate, disasters)
Available mates
Darwin’s “Ingredients”
 Variation – Everyone’s different
 Inheritance – Differences are passed down to
offspring
 Struggle for Life – Everyone’s in competition
to have offspring
 Result: Differential reproductive success

Individuals with an ADVANTAGE in the
competition will leave more offspring on
average
Darwin’s “Ingredients”
 Repeated over generations, differential
reproductive success results in natural
selection
= Traits that confer a reproductive
advantage become more common, traits
that confer a disadvantage become less
common
Populations evolve
 Natural selection pressures act
on individuals…

differential survival
 “survival of the fittest” (not a great
term, but famous now )

differential reproductive success
 “someone has more babies”
 …but it’s populations that evolve


genetic makeup of
population changes
over time
Population: collection of individuals
of the same species that inhabit the
same area and interbreed
Mummichog
 Smallest unit in which evolution occurs
Presence of lactate dehydrogenase
Body size & egg laying in water striders
Fitness
 Fitness: A measure of
reproductive success

Equal to average
contribution to the gene
pool by individuals
bearing the trait
Expanding Upon Darwin’s Ingredients:
Agents
of
evolutionary
change
Mutation
Gene Flow
Non-random mating
Genetic Drift
Selection
1. Mutation & Variation
 Mutation creates variation

Mutation is “rare, regular, and random”
 Rare = Happens few million base pairs
 Regular = Genome – billions of base pairs, so
mutation always happens
 Random = Where it occurs, what kind of
mutation – random
 Mutation changes DNA sequence

changes in protein may
change phenotype &
therefore change fitness
2. Gene Flow
 Gene Flow: the process of genes
moving from one population to another


seed & pollen distribution by
wind & insect
migration of animals
 sub-populations may have
different allele frequencies
 Immigration = movement in, emigration
= movement out
 causes genetic mixing
across regions
 + variety within pop
 - differences between pops
3. Non-random mating
 Sexual selection
4. Genetic drift
 Effect of chance, including
Founder effect
 Bottleneck effect

Founder effect
 When a new population is started
by only a few individuals
some rare alleles may be at high
frequency; others may
be missing
 skew the gene pool of
new population

 human populations that
started from small group
of colonists
 example:
colonization of New World
Distribution of blood types
 Distribution of the O type blood allele in native
populations of the world reflects original settlement
Distribution of blood types
 Distribution of the B type blood allele in native
populations of the world reflects original migration
Out of Africa
Likely migration paths of humans out of Africa
10-20,000ya
10-20,000ya
50,000ya
Many patterns of human traits reflect this migration
Bottleneck effect
 When large population is
drastically reduced by a
disaster
famine, natural disaster,
loss of habitat…
 loss of variation by chance event

 alleles lost from gene pool
 not due to fitness
 narrows the gene pool
Cheetahs
 All cheetahs share a small number of alleles
less than 1% diversity
 as if all cheetahs are
identical twins

 2 bottlenecks

10,000 years ago
 Ice Age

last 100 years
 poaching & loss of habitat
Conservation issues
 Bottlenecking is an important
Peregrine Falcon
concept in conservation
biology of endangered
species
loss of alleles from gene pool
 reduces variation which
reduces adaptability

Breeding programs must
consciously outcross
Golden Lion
Tamarin
5. Natural selection
 Illustrated example of evolution by
natural selection:
Natural Variation and Inheritance
Elephants within the population have different
alleles causing variation in trunk length. Trunk
length is genetic.
Short
Very Short
Very Long
Long
Overreproduction and
Struggle for Existence
More elephants are born than can survive and
reproduce. Food and water are limited, and
elephants are in competition with each other
for these resources.
Differential Success
Elephants with longer trunks can reach food and
water more easily, so are more likely to survive,
more likely to support healthy offspring, and
therefore more likely to pass on their genes to the
next generation. Including the long trunk allele.
X
Natural Selection
Over many generations short trunk alleles will be
reduced then eliminated from the population and
all of the elephants will have long trunks.
Important Notes
1. 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! ->
Patterns of Natural Selection
on Phenotype Frequencies
 Stabilizing Selection: individuals
with the average form have the
highest fitness
 Directional Selection: individuals
that have one extreme have a
greater fitness
 Disruptive Selection: individuals
with either extreme have the
greatest fitness
Determining Phenotype Frequency
 Phenotype Frequency: # of individuals with a
phenotype ÷ the total number of individuals in a
population
 Phenotypes can be predicted by multiplying frequencies
of alleles.


Frequency of Rr = frequency of R X frequency of r
Frequencies of phenotypes should add to 1 or 100%
Gene Pool Problems:
A total of 50 alleles,
20 alleles are Black (B),
and 30 are brown (b).
What
percentage
of
the
What
percentage
of
the
What percentage of the
alleles
individuals
are b?
would
be
individuals
would
be
alleles is B?
brown?
black?
20/50
.4===.36
40%
BB:
30/50
.4 xx=
=.6
.4
.6
.16
60%
16%
bb:
.6
== 36%
Bb: 2 (.4 x .6) = 2 (.24)
= .48 = 48%
Hardy-Weinberg Equilibrium
 H-W Law: Allele frequencies within a

population remain mathematically constant
over the generations unless acted on by
outside forces
A population will not evolve if…
1.
2.
3.
4.
No mutations occur
No immigration or emigration
Individuals randomly mate
And selection does not occur
– …which never happens in nature!
– Therefore the H-W principle is always in
effect.
Witness to Evolution
 Peppered Moth

2 types: dark vs. light
Peppered moth
Peppered moth: Evolution in action
Year % dark % light
1848
5
95 clean air, light-colored bark
1895
98
2 pollution, dark-colored bark
1995
19
81 Clean Air Act, light-colored bark
industrial melanism