AP Biology
Microevolution
Part 1
Important concepts from previous
units:
• Alleles are differing versions
of a gene.
– bb-blue
– BB/Bb brown
• Most organisms are diploid in terms of genetic
content within the genome.
• Fertilization is the combining of parental genes
in the hopes of reproducing the next generation
of the species.
You are what you inherit in terms of DNA.
The egg surrounded by sperm.
Population
Natural Selection & Evolution
•
POPULATIONS evolve not individuals.
–
–
This is because we “are” what we “are” because of the
genetics we inherited.
You can’t change your somatic cells’ DNA by choice,
only by random mutation.
•
•
If a mutation occurs in the DNA that is located in
the gametes (sperm and eggs), then those
changes may affect the next generation of
offspring and therefore a change in traits has
occurred.
In other words, the “population” is evolving from
generation to generation. “Evolve” just means
“change over time” and that is what has
occurred.
Evolution-change (modifications) over time (descent)
•
Individuals “suffer” or “benefit” as a result
of the traits they inherited or mutations
they acquired during their life.
–
“Weak” vs. “strong” genes is the way it is
usually communicated. “Weak” are considered
detrimental traits and “strong” are considered
favorable traits in terms of survival and
reproduction.
•
Population Genetics
–
–
This is the field of science that studies the trait
variation rates over time within a population.
It basically is following allele frequency rates in
a gene pool. (A.K.A. a population.)
•
Species
–
Mostly defined as organisms that are so
genetically similar in genome that there exists
the potential to breed and produce viable
(living) fertile (able to reproduce eventually
themselves) offspring.
Species
Male and Female Blue
Footed Boobies
• Genetics are very similar is important to defining a
species since it is the “blueprint” for “constructing”
an organism. The “plans” must be very similar or
there will be confusion in “construction” and
problems will arise during development. Problems
are a terrible thing to encounter since we are
discussing the making of a living organism.
Horse (f)+ donkey (m)=mule
Geographic range vs. population
• A population is in one specific given area; but in the case
of organisms that are quite common (For example, grey
squirrels or humans.), we may have several populations
that cover a wider expanse of territory.
• In the case of humans, as a species we are global in our
range; but we have millions of different populations,
such as the population of Montgomery or the
population of Birmingham. “Range” refers to
everywhere where that species may be found.
Geographic Range of west coast salamanders
Geographic Range of Humans
• Gene pools may or may not interact; it depends on the
species and if any geographic barriers (such as large
mountains or large bodies of water) interfere with the ability
to interact.
One species, two populations
• Allele frequency (Remember, an allele is a version
of a gene.)
– “Frequency” refers to “how many” are present at that
time within the population (gene pool).
– It is considered fixed, if there is no change in frequency—
no evolution is evident. (Basically, a state of equilibrium
is occurring.)
– It is considered evolving, if frequency is changing—
evolution is occurring. (Basically, a state of change is
occurring over time from generation to generation.)
•
Hardy-Weinberg Theorem
–
This set of equations is used to follow allele
frequency within a population (also considered a
gene pool)
•
•
If the numbers (rates) change from generation to
generation, the population is evolving over time.
If the numbers (rates) do not change from generation to
generation, the population is not evolving over time and
is then said to be in a state of equilibrium.
– Equation #1: p + q = 1 This equation is for alleles. “p”
refers to the “dominant” allele percentage and “q” refers
to the recessive allele percentage. Together p + q
percentages must equal 100% of the gene pool or 1.
Equation 1 (p + q= 1)
– Equation #2: p2 + 2pq + q2 = 1 This equation refers to the
percent composition/number of organisms within the
population (gene pool) at that time. It is essentially a
Punnett square, but in math format.
• p2 = the homozygous dominant percentage of
organisms within the population at that time.
• 2pq = the heterozygous percentage of organisms within
the population at that time.
• q2 = the homozygous recessive percentage of
organisms within the population at that time.
Hardy – Weinberg Theorem
Equation 2 (p² + 2pq + q² = 1)
– These equations are mainly used in health
sciences to explain the frequency of genetic
conditions.
– These equations can be used to show how or if
variation is preserved over time.
– Five conditions must be met for a population to
be in Equilibrium (Allele Frequency is not
changing):
– Five conditions:
• Large population must exist. (This dilutes any nonrandom processes that are occurring.)
• No migration in or out of the population is occurring at
that time. (The population is not being influenced by
outside environmental factors.)
• No mutations are occurring within the genome. (No
random, unforeseen change due to an environmental
stress.)
• Random mating is occurring (No preferences are being
displayed for one trait over another trait…everyone is
equal in fitness.)
• No natural selection is occurring on the population at
this time. (Nature favors all equally in terms of fitness.)
• All three must add up to 100% (1) of the population.
• The five conditions for non-evolving populations
are rarely met in nature:
– Extremely large population size
– No gene flow
– No mutations
– Random mating
– No natural selection
Flowers
Red (RR) White (WW)
Pink (RW)
• Total population size: 500 flowers (1000 alleles)
– 320 red
– 160 pink
– 20 white
320*2=640 alleles for red
160 white alleles; 160 red alleles
20*2=40 alleles for white
• Calculate the allele frequency (p+q=1)
– Red (640+160)÷1000 = 800÷1000=0.8 or 80%
– White (40+160) / 1000 = 200/1000=0.2 or 20%
p2+2pq+q2 = 1
(0.8)^2 + 2(0.8)(.02) + (0.2)^2
0.64 + 0.32 + 0.04 = 1
64% Red (RR)
32% Pink (RW)
4% White
100%
• Albinism is a rare genetically inherited trait that is only expressed in the
phenotype of homozygous recessive individuals (aa). The most characteristic
symptom is a marked deficiency in the skin and hair pigment melanin. This
condition can occur among any human group as well as among other animal
species. The average human frequency of albinism in North America is only
about 1 in 20,000.
– What is the frequency of homozygous recessive individuals (aa) in a
population is q²?
q² = 1/20,000 = .00005
q = .007
• p+q=1
• p=1-q
• p = 1 - .007
• p = .993
p² + 2pq + q² = 1
(.993)² + 2 (.993)(.007) + (.007)² = 1
.986 + .014 + .00005 = 1
p² = predicted frequency
of homozygous
dominant individuals
2pq = predicted frequency
of heterozygous
individuals
q² = predicted frequency
of homozygous
recessive individuals
(the albinos)
= .986 = 98.6%
= .014 = 1.4%
= .00005
• http://www.kstate.edu/parasitology/biology198/hardwein.
html
• PROBLEM #1: 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."
• PROBLEM #1: 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: Answer: 36%, as given in the
problem itself.
– The frequency of the "a" allele: q2 = 0.36; If q2 = 0.36, then q = 0.6
=60%
– The frequency of the "A" allele: 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.“: 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."
• 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%.
• The frequency of the "aa" genotype. Answer: 36%, as given in the problem
itself.
• The frequency of the "a" allele. Answer: 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%.
Problem 2: Sickle-cell anemia is an interesting genetic disease.
Normal homozygous individials (SS) have normal blood cells that are
easily infected with the malarial parasite. Thus, many of these
individuals become very ill from the parasite and many die.
Individuals homozygous for the sickle-cell trait (ss) have red blood
cells that readily collapse when deoxygenated. Although malaria
cannot grow in these red blood cells, individuals often die because of
the genetic defect. However, individuals with the heterozygous
condition (Ss) have some sickling of red blood cells, but generally not
enough to cause mortality. In addition, malaria cannot survive well
within these "partially defective" red blood cells. Thus, heterozygotes
tend to survive better than either of the homozygous conditions. If
9% of an African population is born with a severe form of sickle-cell
anemia (ss), what percentage of the population will be more
resistant to malaria because they are heterozygous (Ss) for the sicklecell gene?
• If 9% of an African population is born with a
severe form of sickle-cell anemia (ss), what
percentage of the population will be more
resistant to malaria because they are
heterozygous (Ss) for the sickle-cell gene?
Answer:
• 9% =.09 = ss = q2.
• To find q, simply take the square root of 0.09 to get 0.3.
• p+q=1
– Since p = 1 - 0.3, then p must equal 0.7.
• 2pq = 2 (0.7 x 0.3) = 0.42 = 42% of the population are
heterozygotes (carriers).
There are 100 students in a class. Ninety-six did well in the
course whereas four blew it totally and received a grade of
F. Sorry. In the highly unlikely event that these traits are
genetic rather than environmental, if these traits involve
dominant and recessive alleles, and if the four (4%)
represent the frequency of the homozygous recessive
condition, please calculate the following:
– The frequency of the recessive allele.
– The frequency of the dominant allele.
– The frequency of heterozygous individuals.
– The frequency of the recessive allele.
• Answer: Since we believe that the homozygous
recessive for this gene (q2) represents 4% (i.e. =
0.04), the square root (q) is 0.2 (20%).
– The frequency of the dominant allele.
• Answer: Since q = 0.2, and p + q = 1, then p = 0.8
(80%).
– The frequency of heterozygous individuals.
• Answer: The frequency of heterozygous individuals
is equal to 2pq. In this case, 2pq equals 0.32, which
means that the frequency of individuals
heterozygous for this gene is equal to 32% (i.e. 2
(0.8)(0.2) = 0.32).
• Within a population of butterflies, the color brown (B)
is dominant over the color white (b). And, 40% of all
butterflies are white. Given this simple information,
which is something that is very likely to be on an exam,
calculate the following:
– The percentage of butterflies in the population that
are heterozygous.
– The frequency of homozygous dominant individuals.
Answers:
• The first thing you'll need to do is obtain p and q.
• So, since white is recessive (i.e. bb), and 40% of the
butterflies are white, then bb = q2 = 0.4.
• q = 0.63.
• Since p + q = 1, then p must be 1 - 0.63 = 0.37.
• what is the percentage of butterflies in the population that
are heterozygous?
– 2pq so the answer is 2 (0.37) (0.63) = 0.47.
• what is the frequency of homozygous dominant individuals?
– That would be p2 or (0.37)2 = 0.14.
• PROBLEM #6.A very large population of
randomly-mating laboratory mice contains
35% white mice. White coloring is caused by
the double recessive genotype, "aa".
Calculate allelic and genotypic frequencies
for this population.
Answer:
• 35% are white mice, which = 0.35 and represents the
frequency of the aa genotype (or q2).
• The square root of 0.35 is 0.59, which equals q.
• p = 1 - q then 1 - 0.59 = 0.41.
• AA and Aa individuals
– AA = p2 = 0.41 x 0.41 = 0.17;
– Aa = 2pq = 2 (0.59) (0.41) = 0.48;
– aa = q2 = 0.59 x 0.59 = 0.35.
– If you add up all these genotype frequencies, they
should equal 1.
• PROBLEM #9.Cystic fibrosis is a recessive
condition that affects about 1 in 2,500 babies
in the Caucasian population of the United
States. Please calculate the following.
– The frequency of the recessive allele in the
population.
– The frequency of the dominant allele in the
population.
– The percentage of heterozygous individuals
(carriers) in the population.
Answer:
– q2 is 1/2,500 or 0.0004.
– The frequency of the recessive allele in the population
• q is the square root, or 0.02.
• the frequency of the cystic fibrosis (recessive) allele in the
population is 0.02 (or 2%).
– The frequency of the dominant (normal) allele in the
population
• p is simply 1 - 0.02 = 0.98 (or 98%).
– The percentage of heterozygous individuals (carriers) in the
population.
• since 2pq equals the frequency of heterozygotes or
carriers, then the equation will be as follows: 2pq =
(2)(.98)(.02) = 0.04 or 1 in 25 are carriers.
AP Biology
Microevolution– Part 2
Evolution of Populations
Important concepts from previous
units:
• Alleles are differing versions of a gene.
• Most organisms are diploid in terms of genetic
content within the genome.
• Fertilization is the combining of parental
genes in the hopes of reproducing the next
generation of the species.
• Variation (Different traits exist within a given species or
population.)
• Variation is key to surviving in a changing environment. (This
is because you have “options”.)
• Perhaps some of the members of that species or population
will survive and reproduce.
• These “options” are the raw building materials of evolution to
utilize. If there is no variation or “option” from which to
utilize, a species is confined to what is available; even if it is
weak or unfavorable. Variation, on the most basic level, will
only come into existence with a change in the DNA nucleotide
sequence, what we refer to as a mutation. Some mutations
are favorable, but most are harmful.
• Variation exists between individuals and populations unless
the population is composed of clones.
Variation Exists
Variation Exists
“Creating” Variation for evolution
to build upon:
• Through mutations
These changes are rare and random in gametes.
(Because gamete cells are normally not exposed to
the environmental stresses an organism may
encounter in their existence.)
Mutations mostly occur in somatic cells because these
cells are exposed to the environmental stresses.
Most mutations, unfortunately, are harmful to the cell or
organism, so it usually dies.
• Through sexual reproduction
– The process of crossover, during Prophase I of meiosis,
“swaps genes from one chromosome to another, its equal
“mate” usually, during gamete formation. This is so that each
sperm or egg is unique in it’s genetic composition.
Variation in Crossover
– The Segregation (means “separation”) of Chromosomes
during Anaphase I and II and Independent Assortment
(lining up of chromosomes on the mid-plane) during
meiosis so as to reduce the genetic content (number of
chromosomes) found within a sperm or egg to one-half
(referred to as “haploid”) the normal content (referred to
as “diploid”) and have a variety of alleles in each.
Variation in Anaphase I
Variation in Anaphase II
• The random fertilization of a sperm by an egg increases
variety. Each sperm and egg are different remember… so each
coming together between egg and sperm (what is referred to
as fertilization) will be different too.
Variation in Egg and sperm
(Which sperm will it be?)
Microevolution (Evolution/change on a small
scale.)
•
•
•
•
This term usually refers to changes in allele frequency
within a population of a species.
Microevolution can eventually lead to macroevolution.
(The evolution of a new species or higher taxon in the
classification system from a pre-existing species.)
Remember: Change over time is referred to as evolution.
Evolution is a scientific law… the environment changes
from minute to minute, hour to hour, day to day; just as a
genome may.
Please do not confuse this “change over time” with the
belief of creationism. These are two different concepts
that are confused with each other because of
misconceptions of the definition. Darwin’s theory is
Natural Selection.
• Genetic Drift causes random change in allele frequency
of a population. (“drift” indicates “random”)
• A. Random (unpredictable) change in allele frequency
that occurs within a population’s gene pool.
• This process is more severe on small parent populations
rather than large populations usually.
In each generation, some
individuals may, just by chance,
leave behind a few more
descendants (and genes, of
course!) than other individuals. The
genes of the next generation will be
the genes of the “lucky” individuals,
not necessarily the healthier or
“better” individuals.
It happens to ALL populations—
there’s no avoiding the vagaries of
chance.
Genetic Drift
“Random change”
Evolution of Allele
frequency
CWCW
CRCR
CRCR
CRCW
Only 5 of
10 plants
leave
offspring
CRCR
CWCW
CRCW
CWCW
CRCR
CRCW
CRCW
CRCR
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CWCW
CRCR
Only 2 of
10 plants
leave
offspring
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
CRCW
Generation 2
p = 0.5
q = 0.5
CRCR
CRCR
Generation 3
p = 1.0
q = 0.0
Two ways that random drift can occur:
•
Bottleneck effect -This has a negative effect by
reducing variation.
–
–
This is usually caused by a random act of nature, such as
a hurricane.
Imagine an island with a parent population consisting of
variation within the gene pool. A hurricane ravishes the
island and most of the individuals of that species die.
The survivors (the individuals who made it out of the
bottle’s neck) are now the sole survivors. They comprise
the new gene pool for that population and the allele
frequency has dramatically changed because of this
unpredictable event.
Genetic Drift –
Bottleneck effect
Original
population
Bottlenecking
event
Surviving
population
• Founder effect -This also has a negative effect by
reducing variation.
– This is also caused by a random event.
– In this situation, a small part of a parent population leaves
or becomes isolated from the larger parent population.
This leaving or isolation “creates” a new founding gene
pool for the area they will occupy.
• Think of the “founding fathers” leaving Europe or
Africa for America. They will “create” a new gene
pool (with a new allele frequency) for their new
environment. The larger older parent population
may have its allele frequency changed as well.
“Founding fathers”
“Founding fathers”
“Founding fathers”
•
Natural Selection
–
Always has a positive effect on variation because
“nature” favors those traits that make a
population or species more able to survive within
an environment and increases their ability to
reproduce and keep the species viable. The weak
traits perish over time.
Gene Flow (Flow indicates purposeful movement
between populations.)
A. This may have a negative effect, a positive effect, or no
effect on allele frequency within a population.
B. Migration in and out of an area may affect the gene
pool.
C. Reduced variation can occur if the interaction
(reproduction) between populations is permitted to
occur. They may begin to merge into one unified
population instead of two distinct populations.
Variation may also increase if new “traits” are introduced
to a stagnate gene pool. Usually seen in captive
breeding of endangered species.
Gene Flow
(Movement on purpose)
Gene Flow
(Movement on purpose)