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Chapter 17
Population
Genetics
Genes & Variation
What were the two big gaps
in Darwin’s theory?
1. He had no idea how
heritable traits were passed
on to the next generation.
2. He had no idea how the
variation within a population
appeared.
The connection between
Mendel & Darwin wasn’t
made until the 1930s.
Charles Darwin
Genes & Variation
Key Ideas:
1. Natural selection acts on phenotypes, not
genotypes.
- Natural selection acts on the whole
organism, not a single a gene.
2. A single gene pair could affect the
phenotype to the point that the organism is
no longer fit.
Genes & Variation
Population of wild pigs
Gene Pool
What is a population?
A group of individuals of
the same species that
live in the same area &
interbreed.
What is a gene pool?
All of the genes (alleles)
in a population.
Genes & Variation
What is relative frequency
(allele frequency)?
1. The number of times an
allele occurs in the gene pool
2. Often expressed as a
percentage or a decimal.
3. Example: 100 alleles in the
pool. 75 dominant alleles has
a frequency of .75 (75/100).
Allele frequency has nothing
to do with whether the allele
is dominant or recessive.
Genes & Variation
What is evolution in genetic terms?
Any change in the relative frequencies of
alleles within a population over time.
Genes & Variation
What are the sources of genetic/heritable variation?
1.
2.
3.
Mutation – any change in DNA base sequence.
Mutations within the germ line (eggs & sperm) are the
ones that can be passed on to the next generation.
Genetic Recombination – This is the gene shuffling that
occurs during meiosis.
- independent assortment
- crossing-over
- some alleles from Dad & some from Mom
Lateral Gene Transfer – Transformation in bacteria is a
form of this. Many bacteria pick up antibiotic resistance
from lateral gene transfer.
Genes & Variation
What is the relationship between
genotype & phenotype?
The genotype determines the phenotype.
Genes & Variation
What is a single-gene trait?
1. A trait determined by a single gene that
has two alleles.
2. There can only be two or three different
phenotypes.
2 = complete
dominance
3 = incomplete
or codominance
Genes & Variation
What is a polygenic trait?
1. A trait controlled by two or more genes.
2. Produces many possible phenotypes.
3. The range of phenotypes typically
creates a bell-shaped curve (normal
distribution).
4. Human height & skin color are two
examples of a polygenic trait.
Evolution as Genetic Change
Does natural selection act directly on
genes?
No
Why?
Natural selection works
directly on the entire
organism.
What is an adaptation?
Genetically controlled
trait that increases fitness.
Evolution as Genetic Change
Natural Selection on single-gene traits
can lead to changes in allele
frequencies, and thus evolution.
Evolution as Genetic Change
Natural Selection on Polygenic Traits:
1. Phenotype range creates a bell-shaped
curve.
2. Fitness can vary from one end of the
curve to the other end.
Evolution as Genetic Change
There are three ways in which natural
selection can affect phenotype
distribution:
1. Directional Selection
2. Stabilizing Selection
3. Disruptive Selection
Evolution as Genetic Change
What is directional selection?
1. One end of the distribution curve has
higher fitness.
2. Selection against one of the extremes.
3. The range of phenotypes will shift.
Evolution as Genetic Change
What is stabilizing
selection?
1. The individuals in the
center of the curve has
higher fitness.
2. Selection against the
extreme phenotypes at
both ends of the curve.
Evolution as Genetic Change
What is disruptive (diversifying)
selection?
1. Both of the extreme phenotypes have
higher fitness.
2. The average phenotype is selected
against.
Evolution as Genetic Change
What is genetic drift?
The random change in allele frequency
within a small population.
Evolution as Genetic Change
What is the founder effect?
A change in allele frequencies as a result
of migration.
Evolution as Genetic Change
What is the Hardy-Weinberg Principle?
1. It is a model in which no evolution
occurs.
2. No evolution = genetic equilibrium
- no change in allele frequencies
3. This never really occurs in nature, but it
helps science understand how evolution
occurs.
Evolution as Genetic Change
1.
2.
3.
4.
5.
What are the five conditions for
genetic equilibrium?
Large Population (no genetic drift)
Random Mating (no sexual selection)
No Immigration or Emigration
(no gene flow)
No Mutations (no new alleles)
No Natural Selection (all traits aid
fitness)
Evolution as Genetic Change
How is the Hardy-Weinberg Principle expressed mathematically?
p2 + 2pq + q2 = 1
p = frequency of one allele
q = frequency of the other allele
p2 = frequency of homozygous dominant
2pq = frequency of heterozygous
q2 = frequency of homozygous recessive
p+q=1
This formula can be used to calculate changes in allele
frequencies.
Demo Question
Speciation
What is a species?
A group of organisms that breed with
each other and produce fertile offspring.
What is speciation?
1. The process that creates new species.
2. The key part of the process is
reproductive isolation.
Speciation
What are the three types of
reproductive isolation?
1. Behavioral Isolation
2. Geographical Isolation
3. Temporal Isolation
Speciation
What is behavioral isolation?
Mating rituals and/or other strategies keep
populations from interbreeding.
Speciation
What is geographic
isolation?
A geographic barrier
keeps populations from
interbreeding.
Speciation
What is temporal isolation?
The populations don’t mate at the same
time.
Speciation
Speciation in Darwin’s Finches:
1. Founders arrive on the Galapagos
2. Separation of populations
3. Changes in the gene pools of each
population
4. Reproductive Isolation
5. Ecological Competition
6. Continued Evolution
Section 19-3: Early Earth’s
History
Earth’s Early History
Formation of the Earth
• Geologic evidence shows Earth = 4.6
Billion years old
• Not “born” in a single event; cosmic
collisions attracted & accumulated
elements; arranged by density
• Early atmosphere = hydrogen cyanide,
CO2, CO, N2, hydrogen sulfide, H2O vapor
Evolution of Life Concept Map
Evolution of Life
Early Earth was hot; atmosphere contained poisonous gases. (4.6 BYA)
Earth cooled and oceans condensed. (3.8 BYA)
Simple organic molecules may have formed in the oceans..
Small sequences of RNA may have formed and replicated.
First prokaryotes may have formed when RNA or DNA was enclosed in microspheres.
Later prokaryotes were photosynthetic and produced oxygen.
An oxygenated atmosphere capped by the ozone layer protected Earth.
First eukaryotes may have been communities of prokaryotes.
Multicellular eukaryotes evolved.
Sexual reproduction increased genetic variability, hastening evolution.
Earth’s Early History
The First Organic Molecules
• Stanley Miller & Harold Urey set up an
experiment to simulate early Earth conditions to
see how organic molecules (building blocks of
life) formed.
• Filled sterile flask with a mixture of gases found
in early atmosphere; sparked with electricity to
simulate lightning
• Results: amino acids (building blocks of protein
formed); Suggests how mixtures necessary for
life could have arisen from compounds present
on primitive Earth.
Miller-Urey Experiment
Mixture of gases
simulating
atmospheres of
early Earth
Spark simulating
lightning storms
Condensation
chamber
Cold water cools
chamber, causing
droplets to form
Water
vapor
Liquid containing
amino acids and
other organic
compounds
Earth’s Early History
The Puzzle of Life’s Origin: How might
cells have arisen?
• Under certain conditions, large organic
molecules can form tiny bubbles called
proteinoid microspheres which have
some cellular characteristics:
1. Selectively permeable membranes
2. Simple means to store/release energy
The Puzzle of Life’s Origins
Evolution of RNA & DNA (See Fig 17-10,
pg 425)
• Scientists don’t know how these molecules
evolved, but under certain conditions, RNA
can help DNA replicate
• Experiments show that small sequences of
RNA could have formed & replicated on
their own in the early Earth conditions, so
scientists think RNA evolved before DNA
Evolution of Prokaryotes & Free Oxygen
1. Microfossils, or microscopic fossils of
unicellular prokaryotes that resemble modern
bacteria have been found in rocks > 3.5 billion
years old!
• They were anaerobic since Earth’s 1st
atmosphere contained little oxygen
• These photosynthetic bacteria, called
cyanobacteria, evolved in shallow seas; they
released O2 which accumulated in the
atmosphere & removed iron from the oceans
• O2 drove some life forms to extinction, while
new ones evolved
Photosyntheis Equation
Recall: Prokaryotes
• Single-celled
• Lack membranebound organelles
• Lack nucleus but
have DNA
• Also called
“bacteria”
Recall: Eukaryotes
•Larger
•Complex internal
membranes
•DNA enclosed within a
nucleus
•Most have mitochondria
•Some have chloroplasts
Evolution Eukaryotic Cells
About 2 billion years ago, prokaryotes began
evolving internal cell membranes
(ancestor of eukaryotes)
Origin of Mitochondrion & Chloroplasts
(Endosymbiotic Theory)
Endosymbiotic Theory – American
biologist Lynn Margulis proposed this
theory that states:
1.Mitochondria are descendants of
symbiotic aerobic bacteria
2.Chloroplasts are descendents of
symbiotic, photosynthetic bacteria
Origin of Mitochondrion & Chloroplasts
(Endosymbiotic Theory)
3. Bacteria entered larger cells as
parasites/undigested prey; they began to
live inside the host where they performed
either cellular respiration
(mitochondria) or photoysnthesis
(chloroplasts)
4. Explains why mitochondria &
chloroplasts have their own DNA
Endosymbiotic Theory
Chloroplast
Aerobic
bacteria
Ancient Prokaryotes
Nuclear
envelope
evolving
Plants and
plantlike
protists
Photosynthetic
bacteria
Mitochondrion
Primitive Photosynthetic
Eukaryote
Ancient Anaerobic
Prokaryote
Primitive Aerobic
Eukaryote
Animals, fungi, and
non-plantlike protists
Observations Supporting the
Endosymbiotic Theory
1. Size & Structure – mitochondria are about the
same size as most bacteria & its membrane is
like that of aerobic bacteria
2. Genetic material- mitochondria & chloroplasts
have circular DNA similar to bacterial DNA &
genes different from nuclear DNA
3. Ribosomes in mitochondria & chloroplasts
have similar size & structure of bacterial DNA
4. Reproduction- Like bacteria, mitochondria &
chloroplasts reproduce by binary fission; Takes
place independently of cell cycle of the host
cell