genetics and environment

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

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 living things so different?

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.

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

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?

Darwin asked:

Why were these creatures found only on the Galapagos Islands?

Darwin found…many unique species

present day Armadillos

Darwin found:Darwin found:

Evidence that creatures that creatures have changed over timehave 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 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 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

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

Independent assortment recombination of alleles

new arrangements in every offspring

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 # of Adult Swallows

1983 2

1984

1985

1986

1987

1988

1989

1990

Year # of Adult Swallows

1983 2

1984 12

1985

1986

1987

1988

1989

1990

Year # of Adult Swallows

1983 2

1984 12

1985 72

1986

1987

1988

1989

1990

Year # of Adult Swallows

1983 2

1984 12

1985 72

1986 432

1987

1988

1989

1990

Year # of Adult Swallows

1983 2

1984 12

1985 72

1986 432

1987 2592

1988

1989

1990

Year # of Adult Swallows

1983 2

1984 12

1985 72

1986 432

1987 2,592

1988 15,552

1989 93,312

1990 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 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: Agents of evolutionary change

Mutation Gene Flow

Genetic Drift Selection

Non-random mating

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

Warbler

finch

Tree

finc

hes

Ground finches

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 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% 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

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 Illustrated example of evolution by

natural selection:

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 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

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 alleles is B?

20/50 = .4 = 40%

What percentage of the alleles are b?

30/50 = .6 = 60%

What percentage of the individuals would be brown?

bb: .6 x .6 = .36 = 36%

What percentage of the individuals would be black?

BB: .4 x .4 = .16 = 16%

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. No mutations occur2. No immigration or emigration3. Individuals randomly mate4. 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

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