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Ch. 17
Part 2
Evolution
• Long-term change in the characteristics of a species
• Frequency of a particular allele within a population
• When a group of organisms change over time
• Changes in allele frequency within a population over many
generations
• Occurs because natural selection gives some alleles a better chance at
surviving than other
• Over many generations, populations gradually changed
• Get better adapted to environment
4 Main Principles of Natural
Selection
1. Variation exists within a
population
2. Organisms compete for
limited resources
3. Organisms produce more
offspring than can actually
survive
4. Individuals with variations
suitable for their habitat are
the ones that SURVIVE and
REPRODUCE
Three Major Types of Selection
1.
2.
3.
Stabilizing Selection
• Selection pressures are on the two extreme
phenotypes
• Center range/mean phenotype (average) is favored
• Variation centered around mean value
• Environment is stable
• If organisms are well adapted to the environment,
most common alleles in the population will lead to an
advantage on the organisms and alleles wil continue to
be passed on
Directional Selection
• Aka EVOLUTIONARY SELECTION
• Occurs when new environmental factor or new allele
appears within a population
• Results in change of characteristic in ONE particular
direction
• ONE specific phenotype is favored
Disruptive Selection
• Occurs when conditions favor both extreme
phenotypes of a population
• Maintains different phenotypes with the population
• Polymorphism
1. Stabilizing Selection
Favors the more common
traits in a population as
opposed to the more
EXTREME or unusual traits
• Eliminates individuals that
have EXTREME or unusual
traits
• Maintains the existing
population frequencies of
common traits while selecting
against all other trait variations
• Example: Infant birth weight
2. Directional selection
Favors traits at one EXTREME
of the range of possible traits
• Result:
– traits at the opposite end
disappear
• If this continues for many
generations, favored traits
become more and more
extreme
– Result: distinct changes in allele
frequencies of the population
• Examples:
– Insecticide resistance
– Peppered moth
3. Disruptive Selection
• Environment favors more
EXTREME or UNUSUAL
traits over common traits
• Extreme or unusual traits at
both ends of the spectrum are
favored…common traits
disappear over time
• Leads to BALANCED
POLYMORPHISM
– Population divided into two
phenotypes
Mutations
• Introduces new alleles into a gene pool
that NEVER existed before
• VITAL b/c provides source for new variation
• What is a mutation:
• Change to an organisms genetic material
(DNA)
• Change the NUCLEIC ACIDS that make up one
or more genes
• Changes can produce new traits that can either
HELP or HURT the survival of an organism
• BENEFICIAL Mutations help organism
• NEUTRAL Mutations have no effect on organism
• NEGATIVE Mutations hurt organisms chances
for survival
CAUSES:
• Spontaneous
• Natural factors
• Ultraviolet
radiation from
the sun
• Exposure to
chemicals
• Exposure to
radiation
New Environmental Factors
• Environmental changes can make certain phenotypes that were once
NOT beneficial suddenly become beneficial
• Ex. Ice age
•
•
•
•
Rabbit fur
Agouti brown originally favored
New environment makes white fur more beneficial
Frequency of the allele for white fur increases at the expense of the allele for
agouti brown fur
• Result after many generations?
• Many white furred rabbits
• Changes in environmental factors ONLY affect the likelihood of an
allele surviving in a population
• Environmental factors do NOT affect the likelihood of an allele
arising by mutations
Antibiotic
Resistance
• Antibiotics chemicals produced by living organisms that
inhibit or kill bacteria but do NOT harm human tissue
• Most produced by fungi
• Penicillin  1st antibiotic
• Produced by fungus Penicillium
• Treats wide range of bacteria
• Stops cell wall formation  prevents bacterial cell
reproduction
• Kills all bacteria sensitive to penicillin
• Inhibitor of enzymes called glycopeptidases
•
Glycopeptidases used to form cross-links between peptidoglycan
molecules in cell walls of bacteria; makes cell wall rigid so cell do not
burst when taking up water
• Hopefully the ENTIRE population
• What happens when entire population is NOT killed by
antibiotic?
• Due to one or more individual bacteria carrying an allele that
makes them resistant to penicillin
• NOT GOOD
• Bacterial DNA
•
•
•
•
Single loop of DNA
Only ONE copy of each gene
Mutant alleles have IMMEDIATE effect on phenotype
Provides bacteria with selective advantage (those with
beneficial allele will survive and reproduce…those without will
die) if there are ideal conditions
• Bacteria divide rapidly by BINARY FISSION
Antibiotic Resistance
• Arises when an existing gene with the bacterial genome changes
(mutates) spontaneously to give rise to a nucleotide sequence that
codes for a slightly different protein that is NOT affected by antibiotic
• DNA MUTATION!!!!
• Incorrect dosage or stopping a cycle of antibiotic treatment increase
chances of antibiotic resistant bacteria
Penicillin Resistant Bacteria
• Produces enzymes that make penicillin
ineffective against them
• B lactamase
• Group of Enzymes that breaks apart the penicillin molecule
• Penicillinase
• Enzyme that inactivates penicillin
• Staphylococcus
• Cause Staph Infection
• may cause disease due to direct infection or due to the
production of toxins by the bacteria
• Methicillin-resistant Staphylococcus aureus
• MRSA type of Staphylococcus aureus that is resistant to
the antibiotic methicillin and other drugs in this class
(penicillin)
Human Use of Antibiotics
• Antibiotic use  change in environmental factors of
bacteria
• Exert selection pressures on bacteria
• Enable more antibiotic bacteria t proliferate
• Constantly trying to find new antibiotics that bacteria are
NOT resistant to
• More humans use antibiotics = greater selection
pressures exerted on bacteria to evolve resistance to
antibiotics
• BAD!
• Bacteria can pass off genes to other bacteria increase proliferation
of resistant bacteria
Industrial Melanism
• Example of changing environmental factor
• Peppered Moth Biston betularia in UK and Ireland
• Night-flying
• During the day found on branches of tress to camouflage from insect
eating birds (sight hunters)
• Before 1849
• Pale wings with dark markings
• In 1849
• Black (melanic) moth found
• Population of melanic moth increased in certain areas
• Difference between Speckled and Melanic Moths
• One gene
• Normal speckled coloring recessive allele c
• Black coloring  dominant allele C
• Frequency of dominant C allele increased in areas near industrial
cities
• Frequency of recessive c allele increased in non-industrial areas
• Selection pressure causing change in allele frequency in
industrial areas  predation by birds
• Unpolluted (non-industrial) areas  green, brown, and grey lichens
covered tree branches = good cover for speckled moths
• Polluted (industrial) areas  sulfur dioxide affects lichens  lichens do
NOT grow on trees = dark bark presides = good cover for melanic moths
Pollution from Sulfur Dioxide Released
into the Air = no more lichens
• 1970s decrease in pollutants = change
in melanic allele frequencies
• Mutated C alleles were always present in
moth population
• Those with dominant C allele never survived
because they were always spotted by the
predatory birds and eaten (and never passed
off their genes)
• Dominant C allele only increased its
frequency when having dark pigment gave
moths advantage in polluted areas
• Mutations to the C allele NOT caused by
pollution
• Changes in environmental factors ONLY affect
the likelihood of an allele surviving in a
population
• Environmental factors do NOT affect the
likelihood of an allele arising by mutations
Malaria Review
• Caused by protoctist parasite Plasmodium
• Insect vector  mosquito (salivary glands)
• Mosquito bites  Plasmodium enters blood
• Parasite enters RBCs  multiplies in RBCs  illness
and death
Sickle Cell Anemia and Malaria
• HBS (sickled hemoglobin) and HBA (normal
hemoglobin) alleles
• Possible genotypes:
• HBSHBS (homozygous dominant, sickled RBCs)
• Great selective disadvantage  less likely to
survive and reproduce
• HBSHBA (heterozygous dominant, some sickled
RBCs, carriers)
• Less likely to suffer from serious attack of malaria
• Contain about 1/3rd the number of Plasmodium
parasites is their blood compared to homozygous
normal HBAHBA
• HBAHBA (homozygous dominant  normal)
• More likely to suffered from serious attack of
malaria
• Parts of the world with increased cases of
MALARIA have higher frequency of HBS
(sickled hemoglobin) allele
Two Strong Selection Pressures Acting on 2 Alleles
• HBSHBS
• Selection against these genotypes b/c they become seriously anemic
• HBAHBA
• Selection against these genotypes b/c they are more likely to die from malaria
HBAHBs
• Strong selection advantage
• Do not suffer from sickle cell anemia
• Less likely to suffer from malaria
• Both alleles remain in population where MALARIA is an
important environmental factor
Genetic Drift
More noticeable when a small # of
individuals are separated from the rest of
the population
• Form small sample of original population
• Not likely to have same allele frequencies
as large population
• Further genetic drift in small population 
alters allele frequencies even more
• Evolution of small population may be very
different from evolution of larger population
•
Process called FOUDNERS EFFECT (occurs
in small, isolated populations)
Genetic Drift
• Founder Effect
• When allele frequency of group
of migrating individuals, by
CHANCE, are different from
their original population
• Bottleneck Effect
• Occurs when a population
undergoes a dramatic decease
in size
• Regardless of cause of
bottleneck (natural disaster,
predation, disease), small
population is now very
vulnerable to genetic drift
• Certain alleles have greater effect
on population than other alleles
Genetic Drift
• changes in the allele frequency within a population that
occur by chance
Before
After
• genes of the next generation will be the genes of the
“lucky” individuals, not necessarily the healthier or
“better adapted” individuals
• No guarantee that the new population will be better
suited to its environment than the original population
• Effect of Genetic drift is very strong and dramatically
influences evolution of small populations (fewer than
100)
Allele frequencies measure genetic variation.
– how common allele is in population
– can be calculated for each allele in gene pool
1.
Calculate the allele frequency for G(Green frogs) in the population
2.
Calculate the allele frequency for g (brown frogs) in the population
How to calculate the ALLELE Frequency in a
Gene Pool
Allele X or Allele x for certain TRAIT
Allele frequency X =
# of allele X in population (gene pool)
total # of alleles (X + x) in population (gene pool)
Hardy Weinberg Principle (HWP)
• Phenotypes controlled by 2 alleles only (A and a)
• Three possible genotypes:
• Homozygous DOMINANT (AA)
• Heterozygous DOMINANT (Aa)
• Homozygous RECESSIVE (aa)
• Two phenotypes:
• Dominant (can be AA or Aa)
• Recessive (HAS to be aa)
• HWP allows the proportions of each genotype in
a large, randomly mating population to be
calculated
• Total population = 100% or 1
• Frequencies within population are rations or
percentages of whole population (decimals or
percentages)
P = frequency of dominant allele in population
Q = frequency of recessive allele in population
P + Q = 1 (all the alleles in the population)
P2 = frequency of homozygous dominant allele in
population
Q2 = frequency of homozygous recessive allele in
frequency in population (easy to recognize)
2pq = frequency of heterozygous genotypes in
population
Condition when the HWP
does NOT apply to a
population
• Small populations
• Significant selective
pressures against one
particular genotype
• Migration of
Individuals carrying
one of the two alleles
into OR out of
population
• Non-random mating
Purpose of HWP
• When to use HWP:
• Determining ratios of different
genotypes in population allows
predication of their ratios in the next
generation to be compared with
observed results
• Use Chi-Squared test to analyze
significance
• Significant differences mean:
• Evidence of directional selection
occurring in population (only if
migration and non-random
mating can be discounted)
Practice Problem
Practice Problem