evolution/population genetics

EVOLUTION/POPULATION GENETICS

CH. 15-16

SOME INFLUENCES ON DARWIN’S THOUGHT

• James Hutton • Charles Lyell• Jean-Baptiste Lamarck• Thomas Malthus• Alfred Russell Wallace

:

Contributor’s to Darwin’s thinking included:

• Charles Lyell –uniformatarianism

• Georges Cuvier – species extinction (Catastrophism)

• Thomas Malthus – struggle for existence (resources)

:

Contributor’s to Darwin’s thinking included:

• James Hutton - Gradualism• Jean Baptiste Lamarck –

Inheritance of acquired Characteristics and Law of Use and Disuse

• Alfred Russell Wallace – organisms evolved from common ancestors

Hutton’s Theory of Geological Change

•James Hutton, 1795, Scottish geologist•Studied invertebrate fossils in Paris Museum•Described The Geological Forces That Have Changed Life on Earth Over Millions of Years (erosion, earthquakes, volcanoes…)

Hutton’s Theory of Geological Change

• Changes in Earth’s crust is due to slow continuous processes

• Idea Known as

Gradualism

Charles Lyell (1797-1875)• Proposed theory of

Uniformitarianism• Geological processes

occur at uniform rates building & wearing down Earth’s crust

• Proposed that the Earth was millions of years instead of a few thousand years old

Principles of Geology• Published by Lyell Just Before The

Beagle Set Sail & read by Darwin

• Explained Geological Processes That Shaped The Earth

• Helped Darwin Understand Sea Shells In The Andes Mountains At 12,000+ Feet

– Expanded Earth’s Age

Lamarck’s Theory of Evolution

• Jean-Baptiste Lamarck, 1809

• One Of First Scientists To Understand That Change Occurs Over Time

• Stated that Changes Are Adaptations To Environment acquired in an organism’s lifetime

• Said acquired changes were passed to offspring


• Idea called Law of Use and Disuse

• If a body part were used, it got stronger

• If body part NOT used, it deteriorated


• Inheritance Of Acquired Traits– Traits Acquired During One’s Lifetime

Would Be Passed To Offspring

Clipped ears of dogs could be passed to offspring!

Lamarck’s Mistakes

• Lamarck Did NOT Know how traits were inherited (Traits are passed through genes)

• Genes Are NOT Changed By Activities In Life

• Change Through Mutation Occurs Before An Organism Is Born

Population Growth

• Thomas Malthus, 1798

• Economist

• Observed Babies Being Born Faster Than People Were Dying

• Population size limited by resources such as the Food Supply

The Struggle for Existence

• Malthus’ Influence:– High Birth Rates & Limited Resources

Would Force Life & Death Competition

• Each Species Struggles For:– Food– Living Space– Mates

Population Growth

• Malthus Reasoned That If The Human Population Continued To Grow Unchecked, Sooner or Later There Would Be Insufficient Living Space & Food For Everyone

• Death Rate Will Increase To Balance Population size & Food Supply

Population Growth• Darwin Realized

Malthus’s Principles Were Visible In Nature

• Plants & Animals Produce Far More Offspring Than Can Be Supported– Most Die– If They Didn’t – Earth

Would Be Overrun

Wallace’s Contribution

• Alfred Russell Wallace Independently came to same Conclusion as Darwin that species changed over time because of their struggle for existence

• When Darwin read Wallace’s essay, he knew he had to publish his findings

Darwin's Theory

1. Individual Organisms In Nature Differ From One Another.

Some Of This Variation Is Inherited

2. Organisms In Nature Produce More Offspring Than Can Survive, And Many Of These Offspring Do not Reproduce

Darwin's Theory

3. Because More Organisms Are Produced Than Can Survive, Members Of Each Species Must Compete For Limited Resources

4. Because Each Organism Is Unique, Each Has Different Advantages & Disadvantages In The Struggle For Existence

Darwin's Theory

5. Individuals Best Suited To Their Environment Survive & Reproduce Successfully – Passing Their Traits To Their Offspring.

6. Species Change Over Time. Over Long Periods, Natural Selection Causes Changes That May Eventually Lead To New Species

Darwin's Theory

7. Species Alive Today Have Descended With Modifications From Species That Lived In The Past

8. All Organisms On Earth Are United Into A Single Tree Of Life By Common Descent

Concept MapSection 15-3

includes

Evidence of Evolution

Physical remains of organisms

Common ancestral species

Similar genes Similar genes

which is composed of which indicates which implies which implies

The fossil recordGeographic

distribution of living species

Homologous body structures

Similaritiesin early

development

Fossil Record

• Old remains of the past

• Earth is Billions of Years Old

• Fossils In Different Layers of Rock (sedimentary Rock Strata) Showed Evidence Of Gradual Change Over Time

Homologous StructuresHomologous Structures

Beaver

NORTH AMERICA

Muskrat

Capybara SOUTH AMERICA

Coypu

Geographic Distribution of Living SpeciesSection 15-3

Beaver

Muskrat

Beaver andMuskrat

Coypu

Capybara

Coypu andCapybara

Turtle Alligator Bird Mammal

Ancient lobe-finned fish

HOMOLOGOUS STRUCTURESSection 15-3

In evolutionary biology, homology refers to any similarity between characteristics of organisms that is due to their shared ancestry. The word homologous derives from the ancient Greek ομολογειν, 'to agree'.

Evidence for Evolution - Comparative Embryology

Similarities In Embryonic Development

Homologous Body Structures

• Not All Serve Important Functions– Vestigial Organs- no

longer serve a function• Appendix In Man• Legs On Skinks

Similarities in DNA Similarities in DNA SequenceSequence

Evolutionary Time Scales

Macroevolution: Long time Long time scale events scale events

that that create and create and destroydestroy species.species.

Microevolution:

Short time scale events

(generation-to-generation) that

change the genotypes and phenotypes of

populations

Evolutionary Time Scales

Sample Population

48% heterozygous

black

36% homozygous

brown

16% homozygous

black

Frequency of Alleles

allele for brown fur

allele for black fur

VARIATION & GENE POOLSSection 16-1

Gene pool

Relative frequency # of times an allele occurs in a gene pool, compared w/ the # of times other alleles for the same gene occur

MUTATION

• THE ULTIMATE SOURCE OF GENETIC VARIATION!!!!!!

Fre

qu

ency

of

Ph

eno

typ

e(%

)100

80

60

40

20

0 Widow’s peak No widow’s peak

Phenotype

Phenotypes for Single-Gene Trait- # of phenotypes a given trait has is determined by how

many genes control the traitSection 16-1

* Controlled by a single gene that has 2 alleles leads to 2 distinct phenotypes

Fre

qu

enc

y o

f P

hen

oty

pe

Phenotype (height)

Generic Bell Curve for Polygenic Trait: many possible genotypes and phenotypesSection 16-1

*Controlled by 2/ more genes (2/ more alleles)

Directional Selection

Food becomes scarce.

Key

Low mortality, high fitness

High mortality, low fitness

NATURAL SELECTION ON POLYGENIC TRAITS: 3 MODELS

Section 16-2

Favors traits at 1 extreme of a range of traits

Key

Per

cen

tag

e o

f P

op

ula

tio

n

Birth Weight

Selection against both

extremes keep curve narrow and in same

place.

Graph of Stabilizing SelectionSection 16-2



Stabilizing SelectionIndividuals with the most common trait are most adapted, while individuals who differ from the norm are poorly adapted.

Stabilizing SelectionNatural selection acts in opposite directions

*** “Heterozygote Advantage”- ind who is heterozygous for a particular gene has a greater fitness than a homozygous ind

EX: Distribution of sickle-cell allele coincides with the occurrence of malaria

SS Normal hemoglobin

ss Sicke-cell disease

Ss- codominance (protects against malaria)

http://en.wikipedia.org/wiki/Fitness_(biology)

Disruptive Selection

Largest and smallest seeds become more common.

Nu

mb

er o

f B

ird

sin

Po

pu

lati

on

Beak Size

Population splits into two subgroups specializing in different seeds.

Beak Size

Graph of Disruptive Selection

Nu

mb

er o

f B

ird

sin

Po

pu

lati

onKey



Section 16-2

When both extreme phenotypes are favored by natural selectionActs in opposite directions

GENTIC DRIFTA RANDOM CHANGE IN ALLELE

FREQUENCY• IN SMALL POPULATIONS, INDIVIDUALS

THAT CARRY A PARTICULAR ALLELE MAY LEAVE MORE DESCENDANTS THAN OTHER INDIVIDUALS, JUST BY CHANCE.

• OVER TIME, A SERIES OF CHANCE OCCURRENCES OF THIS TYPE CAN CAUSE AN ALLELE TO BECOME COMMON IN A POPULATION.

• FOUNDER EFFECT: A SITUATION IN WHICH ALLELE FREQUENCIES CHANGE AS A RESULT OF THE MIGRATION OF A SMALL SUBGROUP OF A POPULATION.

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic DriftSection 16-2

In small populations, an allele can become more or less common by chance (explain how allele frequencies can fluctuate unpredictably from 1 gen to the next)




Descendants


FOUNDER EFFECT:2 small groups from a large, diverse population could produce new populations that differ from the original population




Descendants


EVOLUTION VERSUS GENETIC EQUILIBRIUM

HARDY-WEINBERG PRINCIPLE: ALLELE FREQUENCIES IN A POPULATION WILL REMAIN CONSTANT AS LONG AS FIVE CONDITIONS (FACTORS) REMAIN CONSTANT. GENETIC EQUILIBRIUM IS REACHED. (IS THE POPULATION EVOLVING?)

5 CONDITIONS REQUIRED TO MAINTAIN GENETIC EQUIIBRIUM1. RANDOM MATING

- Select mates w/o bias2. LARGE POPULATION

- Genetic drift does not affect large pops3. NO MOVEMENT INTO OR OUT OF THE POPULATION

- no intro of new allele4. NO MUTATION

- no new allele introduced5. NO NATURAL SELECTION

- No phenotype can have selective advantage

DOES THIS EVER HAPPEN?????

• In 1908, Hardy and Weinberg independently demonstrated that DOMINANT ALLELES DO NOT REPLACE RECESSIVE ALLELES IN A POPULATION!

HARDY-WEINBERG EQUATION

You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that 36%, calculate the following:

The frequency of the "aa" genotype.

The frequency of the "a" allele.

The frequency of the "A" allele.

The frequencies of the genotypes "AA" and "Aa."

The frequencies of the two possible phenotypes if "A" is completely dominant over "a."

• FREQUENCY OF aa GENOTYPE: 36% (GIVEN)

• FREQUENCY OF a ALLELE:The frequency of aa is 36%, which means that q2 = 0.36, by definition. If q2 = 0.36, then q = 0.6, again by definition. Since q equals the frequency of the a allele, then the frequency is 60%.

• The frequency of the "A" allele. Answer: Since q = 0.6, and p + q = 1, then p = 0.4; the frequency of A is by definition equal to p, so the answer is 40%.

• The frequencies of the genotypes "AA" and "Aa." Answer: The frequency of AA is equal to p2, and the frequency of Aa is equal to 2pq. So, using the information above, the frequency of AA is 16% (i.e. p2 is 0.4 x 0.4 = 0.16) and Aa is 48% (2pq = 2 x 0.4 x 0.6 = 0.48).

• The frequencies of the two possible phenotypes if "A" is completely dominant over "a." Answers: Because "A" is totally dominate over "a", the dominant phenotype will show if either the homozygous "AA" or heterozygous "Aa" genotypes occur. The recessive phenotype is controlled by the homozygous aa genotype. Therefore, the frequency of the dominant phenotype equals the sum of the frequencies of AA and Aa, and the recessive phenotype is simply the frequency of aa. Therefore, the dominant frequency is 64% and, in the first part of this question above, you have already shown that the recessive frequency is 36%.

AS NEW SPECIES EVOLVE (SPECIATION), POPULATIONS

BECOME REPRODUCTIVELY ISOLATED FROM EACH OTHER.Section 16-3

results from

which include

produced by produced byproduced by

which result in

which result in

Reproductive Isolation

Isolating mechanisms

Behavioral isolation Temporal isolationGeographic isolation

Behavioral differences Different mating timesPhysical separation

Independentlyevolving populations

Formation ofnew species

Section 17-4

Flowchart

that are

can undergo can undergo can undergo can undergo can undergo

in underunderform inin

Species

Unrelated Related

Inter-relationshiops

Similar environments

Intense environmental

pressure

Small populations

Different environments

Coevolution Convergent evolution

ExtinctionPunctuated equilibrium

Adaptive radiation

Types of Reproductive Isolation

A) Behavioral- occurs when pops have different courtship rituals.

B) Geographic- occurs when pops are separated by geographic barriers.

Ex: mountains & rivers

C) Temporal- occurs when pops reproduce at different times.

Ex: orchids in the rainforest

a. Divergent evolution when 2/ more species originate from a common ancestor

Ex: flipper of whale and limb of human

b. Convergent evolution process by which unrelated organisms come to resemble each other

Ex: torpedo shape of shark and penguin/ wing of insect and bat

c. Coevolution process by which 2 species evolve in response to changes in each other over time

evolution/population genetics

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