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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.
AP Biology
EvolutionHonors Biology Ch 15-17
Students Will Be Able To: Give examples of early influences on the
evolutionary model. Define Lamarckian evolution, and explain
why it has been refuted. Give examples of questions that arose
from Darwin’s time on the Galapagos.
What is Evolution? Evolution
changes in living organisms over time explains how modern organisms have
descended from ancient organisms, and continue to change today
DNA
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 there so many
different kinds of living things?
Early Ideas on Evolution Influences on the discipline:
Lyell and Hutton: geologists who said that the Earth was very old and described uniformitarianism: changes and laws of nature observed today were also active in the past
Malthus: if the human population continued to grow unchecked, sooner or later there would be insufficient living space and food for everyone.
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”
Earlier ideas on Evolution
What is the problem you notice with Lamarckian evolution?
Earlier ideas on Evolution
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.
?
Robert FitzroyRobert Fitzroy
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
Voyage of the HMS Beagle Stopped in Galapagos Islands
500 miles off coast of Ecuador
GalapagosRecently 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
Many of Darwin’s observations made him wonder… Why?
Many of Darwin’s observations made him wonder… Why?
Why were these creatures found only on the Galapagos Islands?
Darwin found…many unique species
present day Armadillos
Darwin found fossilsDarwin found fossils
ancient Armadillo
Why should extinct Why should extinct armadillos & modern armadillos & modern armadillos be found on armadillos be found on same continent?same continent?
Darwin found…clues in the fossils
Darwin found:Different shells on tortoises on different islands
Darwin asked:Darwin asked:
Is there a relationship Is there a relationship between the environment between the environment
& what an animal & what an animal looks like?looks like?
Darwin found… birds
Finch? Sparrow?
Woodpecker? Warbler?
Darwin found:Darwin found:
Many different birds Many different birds on the Galapagos on the Galapagos Islands. Islands.
He thought he found He thought he found very different kinds…very different kinds…
Darwin was amazed to find out: All 14 species of birds were finches…
Finch? Sparrow?
Woodpecker? Warbler?
But Darwin found… a lot of finches
Large ground finch
Small ground finch
Warbler finch Tree finch
But there is only one species of finch on the mainland! Darwin asked:Darwin asked:
If the Galapagos If the Galapagos finches came from the finches came from the mainland, why are they mainland, why are they
so different now?so different now?
Darwin’s view of Evolution
Darwin giraffes that
already have longer necks than other giraffes survive better than their competitors
leave more offspring who inherit their long necks
Students Will Be Able To:
Describe the “ingredients” of natural selection.
Explain changes in allele frequency as the outcome of differential reproduction - natural selection.
Analyze and make predictions about natural selection scenarios.
The equation, or the “ingredients” of evolution:
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
Individuals vary from each other in millions of ways Variables are physical, behavioral, and/or biochemical
Mean beak depth of parents (mm)
Medium ground finch8
8 9 10 11
9
10
11
1977 1980 1982 1984
Dry yearDry year
Dry year
Wet year
Bea
k d
epth
Bea
k d
epth
of
off
spri
ng
(m
m)
Where does Variation come from? Mutation
random changes to DNA errors in mitosis & meiosis environmental damage (rare)
Sexual Reproduction mixing of alleles
recombination of alleles due to Crossing-over and Independent assortment new arrangements in every offspring
Gene Flow is a source of diversity, but not of new phenotypes
Variation of Traits in a Population Measuring the frequency of most traits in a
population will produce a “bell curve” or “normal distribution” graph. This shows that few individuals have the extreme
phenotype Most individuals have the average or medium
phenotype
Darwin’s “Ingredients”: Inheritance
Traits subject to evolutionary pressure 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 1988?
Year # of Adult Swallows
1983 2
1984
1985
1986
1987
1988
1989
1990
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 just one pair of elephants would have 19 million descendents 750 years later. (By contrast, there are only ~500,000 elephants alive in the whole world today.)
Living things “over-reproduce.” There Living things “over-reproduce.” There are more offspring born than can be are more offspring born than can be sustained.sustained.
Somebody has got to die Somebody has got to die Therefore, there is Therefore, there is competitioncompetition for for
resources to survive and reproduce.resources to survive and reproduce. “Nature, red in tooth and claw” -
Tennyson
Darwin’s “Ingredients”: Struggle for Life
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 – Not everyone will have the
maximum number of perfectly healthy 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
Students Will Be Able To: Differentiate between stabilizing, directional,
disruptive selection. Define sexual selection. Distinguish between change due to selection
and change due to genetic drift. Identify the types of populations most
significantly affected by genetic drift. Distinguish between the two kinds of genetic
drift. Explain the conditions under which a trait will
not undergo evolution (Hardy-Weinberg Equilibrium)
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 Smallest unit in which evolution occurs Presence of lactate
dehydrogenase
Mummichog
Fitness Fitness: A measure of
reproductive success Equal to average
contribution to the gene pool by individuals bearing the trait
Body size & egg laying in water striders
Expanding upon Darwin’s ingredients
Mutation Gene Flow
Genetic Drift Selection
Non-random mating
1. Mutation & Variation Mutation creates variation
Mutation is “rare, regular, and random” Rare = Happens every few million base pairs Regular = Genome – billions of base pairs, so
mutation always happens Random = Where it occurs, what kind of
mutation – random
2. Gene Flow Gene Flow: the process of genes
moving from one population to another Immigration = movement in,
emigration = movement out sub-populations may have
different allele frequencies causes genetic mixing
across regions + variety within population - differences between populations
3. Non-random mating Sexual selection = selection based
upon ability to successfully mate, rather than survive
Warbler
finch
Tree
finc
hes
Ground finches
4. Genetic drift Change due to random chance
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 AfricaLikely migration paths of humans out of AfricaLikely migration paths of humans out of Africa
Many patterns of human traits reflect this migrationMany patterns of human traits reflect this migration
50,000ya
10-20,000ya
10-20,000ya
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% intraspecies
diversity as if all cheetahs are
siblings
2 bottlenecks 10,000 years ago - Ice Age last 100 years
poaching & loss of habitat
Conservation issues Bottlenecking is an important
concept in conservation biology of endangered species loss of alleles from gene pool reduces variation which
reduces adaptability
Breeding programs must consciously outcrossBreeding programs must consciously outcross
Peregrine Falcon
Golden Lion Tamarin
5. Natural selection When analyzing a natural selection
scenario, explain it in terms of the ingredients!
Natural Variation and InheritanceElephants within the population have different alleles causing variation in trunk length. Trunk
length is genetic.
Very Long
Long
Very Short Short
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 SuccessElephants 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 SelectionOver many generations short trunk alleles will be reduced then 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 appear or 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! ->
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
Patterns of Natural Selection on Phenotype Frequencies
Witness to Evolution Peppered Moth
2 types: dark vs. light
Peppered moth
light
Peppered moth: Evolution in actionYear % dark % light1848 5 951895 98 21995 19 81
clean air, light-colored bark
pollution, dark-colored bark
Clean Air Act, light-colored bark
industrial melanism
But… …is it possible that a species could
stop evolving? Every generation identical to the one before it, no traits become more or less common?
…what about just one trait within that species? Could it stay equally common in every generation?
What do YOU think?
Hardy-Weinberg Equilibrium