Topic 10.3 – Gene Pools and Speciation
Essential Idea: Gene Pools change over time.
A Gene Pool is the collection of
genes (and the alleles for them)
present in an interbreeding
population.
Speciation is the process by
which small differences between
organisms in a single species lead to
changes within a gene pool over time.
A species is a set of interbreeding
organisms that produce fertile
offspring.
The changes within
a population are representative
of allele frequency shifts, and
these occur over time.
Evolution requires that allele frequencies change with time in populations.
This concept is known as
Genetic Drift.
Allele Frequencies are
represented by a number:
0.0 – 1.0
The higher the number, the
greater the frequency within
a population.
In the same way that the alleles shift
in this population of rabbits, so did
the alleles shift in our population of
hand grabby aliens.
Ex. .3 = 30% of the
population carry the allele.
The Hardy – Weinberg Equation is a way to represent the change in allele frequency over time.
In general, we represent the dominant allele as p, and the recessive allele as q.
Total Frequency of all alleles must equal 1.0
Frequency of dominant allele(p) + frequency of recessive allele(q) = 1.0
p +
q = 1.0
However, we have a combination of two alleles for each trait, so the frequency with respect to a
trait is
Example:
Allele #1
(p+q)
Allele #2
X
(p+q) = 1
Dominant - R (p)
Recessive – r (q)
Hardy – Weinberg Equation
p2 + 2pq + q2=1
Number of
Homozygous
Dominant
Individuals
Number of
Heterozygous
Individuals
Number of
Homozygous
Recessive
Individuals
Remember: Individuals who are RR
(q2) and Rr (2pq) will have the same
phenotype, so it is important to start
with the recessive phenotype.
The Hardy – Weinberg Equation is a way to represent the change in allele frequency over time.
Sample Problem: There are a population of 1000 penguins, in which 12 penguins have blue
feet (recessive trait). Calculate the allele frequency of the foot color alleles, and the number
of individuals with each genotype in the population.
Four Steps to solving any Hardy-Weinberg Problem
1. Assign alleles
1. Frequency of dominant is ‘p’
2. Frequency of recessive is ‘q’
2. Calculate q by taking the square root of the
number of homozygous recessive individuals.
3. Calculate p (the allele frequencies must equal 1, so
p = 1-q)
4. Use p and q to calculate the other genotype
frequencies:
1. Freq. of homo. Dominant = p2
2. Freq. of heterozygous = 2pq
3. Freq. of homo. Recessive = q2
1. Yellow Feet = p
Blue Feet = q
2. q2 =
3. p = 1-q
4. p2 =
2pq =
q2 =
Sample Problem: There are a population of 2,732 ogres, in which 248 ogres have layers
(recessive trait). Calculate the allele frequency of the ‘having layers’ alleles, and the number of
individuals with each genotype in the population.
Work Space:
1.
2.
3.
4.
Four Steps to solving any Hardy-Weinberg Problem
1. Assign alleles
1. Frequency of dominant is ‘p’
2. Frequency of recessive is ‘q’
2. Calculate q by taking the square root of the
number of homozygous recessive individuals.
3. Calculate p (the allele frequencies must equal 1, so
p = 1-q)
4. Use p and q to calculate the other genotype
frequencies:
1. Freq. of homo. Dominant = p2
2. Freq. of heterozygous = 2pq
3. Freq. of homo. Recessive = q2
The Hardy – Weinberg Equation is a way to represent the change in allele frequency over time.
Often times, the
representation of
allele frequency is
done in a pie chart,
since it is being
discussed out of the
total population.
Reproductive isolation of populations can be temporal, behavioral or geographic.
Any of these mechanisms can lead to
reproductive isolation, which in turn
leads to speciation.
This process of speciation can either
occur abruptly or gradually
SPECIATION
Isolated species reproduce
independently, leading to either:
Behavioral Isolation
Changes of
courtship rituals,
mating calls
Sympatric Speciation:
Still live in same area
Allopatric Speciation:
Live in different areas
Temporal Isolation
Difference of
mating seasons
Geographic Isolation
Barriers between
portions of
population
Speciation can either be gradual or abrupt.
Punctuated Change
Gradual Change
-Due to extreme environmental
changes
-Long periods where fossils are unable
to be collected
-Long periods of no change
-Small changes over long periods of
time
-Each phenotype is represented as a
part of the whole
Devonian
Extinction
(75% Life)
End
Permian
Extinction
(97% Life)
Triassic
Extinction
(50% Life)
Selection as a result of isolation can be either directional, stabilizing or
disruptive.
Directional Selection:
Pushes the phenotypes
towards one end of a
continuum
Stabilizing Selection:
Removes individuals at
the extremes of the
phenotypes
Disruptive Selection:
Removes individuals
from the middle of the
phenotype.
This pushes the species
often into two new
species.
Identify the type of selection present in the following scenarios.
Giraffes used to have shorter necks. As time went on, extremely long
necks were selected for and short or medium necks were selected against.
A species of tropical land snail
shows a polymorphism of
opposite-coiling shells. Some
snails in the species have
shells that coil clockwise, and
others are counter-clockwise.
A species of birds is used to laying lightly colored eggs on the
white pebbles of a beach, and has done so for many generations.
The light color helps to hide the eggs from predators and it is
extremely rare to see any dark eggs produced. Because of a
volcanic eruption that covered the beach in black particles, this
species of bird is now laying more and more dark colored eggs.
Human babies demonstrate different body masses at birth, but a body
mass that is too small would be disadvantageous to the survival of the
baby. Too great of a body mass makes childbirth too difficult and can
also reduce survival chances.
Polyploidy is a condition in which organisms contain more than two homologous sets of
chromosomes.
Organisms that exhibit
polyploidy tend to be larger.
This also holds true for
plants.
The genus Allium is
comprised of many
flowering plants, all
stemming from the true
organism, garlic.
This polyploidy in the genus Allium leads to all of the different variety of plants we see
currently.
Garlic
Chives
Onion
Shallots
Leeks
Diploid
2n = 16
Pentaploid
5n = 40
Diploid
2n = 16
Triploid
3n = 24
Quadraploid
4n = 32
Polyploidy can be used by cultivators to generate larger and larger offspring from their plants.
Farmers use diploid cells from parent
plants and combine them to add
genetic material to the progeny.
More DNA = more chance for
mutations.
It’s only a matter of time until the plants start
to cultivate us!
Practice Problem – Data Analysis
When a person’s blood type is O+, the positive sign refers to the
presence of a characteristic known as the Rhesus factor, Rh. Having
Rh factor (Rh+) is dominant to not having Rh factor (Rh-). This trait
follows normal Mendelian Inheritance, in that persons with
genotypes BB and Bb will be Rh+, and bb persons will be Rh-. There
was a study conducted recently on the frequency of Rh factor in two
geographically separated populations. Data shown below.
In Lagos, Nigeria, a study with 23,382 people revealed 3% of the population was Rh-.
In Abha, Saudi Arabia, a study with 944 males revealed 7.2% of the population was Rh-.
1. Use the Hardy –Weinberg equation to calculate the frequencies of the two alleles, B and
b, in the two countries.
2. In the south-west of France, a study of 127 French Basque people found that the allele
frequency for b to be 0.51. How does this compare with the frequency of b in the other
two populations?