Gene Pool and Evolution Explained

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MADE BY MOHD.TATHEER RAZA
GUIDED BY PANKAJ SIR

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The Gene Pool
•Members of a
species can
interbreed & produce
fertile offspring
•Species have a
shared gene pool
•Gene pool – all of
the alleles of all
individuals in a
population

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The Gene Pool
•Different species
do NOT exchange
genes by
interbreeding
•Different species
that interbreed
often produce
sterile or less viable
offspring e.g. Mule

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Populations
•A group of the
same species living
in an area
•No two individuals
are exactly alike
(variations)
•More Fit
individuals survive &
pass on their traits

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Speciation
•Formation of new
species
•One species may
split into 2 or more
species
•A species may
evolve into a new
species
•Requires very long
periods of time

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Modern Synthesis Theory
•Combines Darwinian
selection and
Mendelian inheritance
•Population genetics -
study of genetic
variation within a
population
•Emphasis on
quantitative
characters

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•1940s –
comprehensive theory
of evolution (Modern
Synthesis Theory)
•Introduced by Fisher
& Wright
•Until then, many did
not accept that
Darwin’s theory of
natural selection could
drive evolution
S. Wright
A. Fisher

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•Today’s theory on evolution
•Recognizes that GENES are responsible
for the inheritance of characteristics
•Recognizes that POPULATIONS, not
individuals, evolve due to natural
selection & genetic drift
•Recognizes that SPECIATION usually is
due to the gradual accumulation of small
genetic changes

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Microevolution
•Changes occur in gene pools due to
mutation, natural selection, genetic
drift, etc.
•Gene pool changes cause more
VARIATION in individuals in the
population
•This process is called
MICROEVOLUTION
•Example: Bacteria becoming unaffected
by antibiotics (resistant)

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Allele Frequencies Define Gene Pools
As there are 1000 copies of the genes for color,
the allele frequencies are (in both males and females):
320 x 2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0.8
(80%) R
160 x 1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0.2
(20%) r
500 flowering plants
480 red flowers 20 white flowers
320 RR 160 Rr 20 rr

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Species & Populations
•Population - a localized group of
individuals of the same species.
•Species - a group of populations whose
individuals have the ability to breed and
produce fertile offspring.
•Individuals near a population center are,
on average, more closely related to one
another than to members of other
populations.

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Gene Pools
•A population’s gene pool is the total
of all genes in the population at any
one time.
•If all members of a population are
homozygous for a particular allele,
then the allele is fixed in the gene
pool.

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The Hardy-Weinberg Theorem
•Used to describe a non-evolving
population.
•Shuffling of alleles by meiosis and
random fertilization have no
effect on the overall gene pool.
• Natural populations are NOT
expected to actually be in Hardy-
Weinberg equilibrium.

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The Hardy-Weinberg Theorem
•Deviation from Hardy-Weinberg
equilibrium usually results in
evolution
•Understanding a non-evolving
population, helps us to understand
how evolution occurs

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Assumptions of the H-W Theorem
1.Large population size
- small populations can have chance
fluctuations in allele frequencies (e.g., fire,
storm).
2.No migration
- immigrants can change the frequency of an
allele by bringing in new alleles to a
population.
3.No net mutations
- if alleles change from one to another, this
will change the frequency of those alleles

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Assumptions of the H-W Theorem
3.Random mating
- if certain traits are more desirable,
then individuals with those traits will be
selected and this will not allow for random
mixing of alleles.
4.No natural selection
- if some individuals survive and
reproduce at a higher rate than others,
then their offspring will carry those
genes and the frequency will change for
the next generation.

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Hardy-Weinberg Equilibrium
The gene pool of a non-evolving population remains
constant over multiple generations; i.e., the allele
frequency does not change over generations of time.
The Hardy-Weinberg Equation:
1.0 = p2 + 2pq + q2
where p2 = frequency of AA genotype; 2pq = frequency of
Aa plus aA genotype; q2 = frequency of aa genotype

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1) Genetic drift
Genetic drift = the alteration of the gene pool of a small
population due to chance.
Two factors may cause genetic drift:
a) Bottleneck effect may lead to reduced genetic variability
following some large disturbance that removes a large
portion of the population. The surviving population often
does not represent the allele frequency in the original
population.
b) Founder effect may lead to reduced variability when a few
individuals from a large population colonize an isolated
habitat.

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2) Natural selection
As previously stated, differential success in reproduction
based on heritable traits results in selected alleles being
passed to relatively more offspring (Darwinian
inheritance).
The only agent that results in adaptation to environment.
3) Gene flow
-is genetic exchange due to the migration of fertile
individuals or gametes between populations.

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4) Mutation
Mutation is a change in an organism’s DNA and is
represented by changing alleles.
Mutations can be transmitted in gametes to offspring,
and immediately affect the composition of the gene pool.
The original source of variation.

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Genetic Variation, the Substrate for Natural Selection
Genetic (heritable) variation within and between
populations: exists both as what we can see (e.g., eye
color) and what we cannot see (e.g., blood type).
Not all variation is heritable.
Environment also can alter an individual’s phenotype [e.g.,
the hydrangea we saw before, and…
…Map butterflies (color changes are due to seasonal
difference in hormones)].

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Variation within populations
Most variations occur as quantitative characters (e.g.,
height); i.e., variation along a continuum, usually
indicating polygenic inheritance.
Few variations are discrete (e.g., red vs. white flower
color).
Polymorphism is the existence of two or more forms of
a character, in high frequencies, within a
population. Applies only to discrete characters.

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Variation between populations
Geographic variations are differences between gene pools
due to differences in environmental factors.
Natural selection may contribute to geographic variation.
It often occurs when populations are located in different
areas, but may also occur in populations with isolated
individuals.

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Geographic variation
between isolated
populations of house
mice.
Normally house mice are
2n = 40. However,
chromosomes fused in
the mice in the example,
so that the diploid
number has gone down.

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b. Relative fitness
- Contribution of a genotype to the next generation,
compared to the contributions of alternative genotypes
for the same locus.
- Survival doesn’t necessarily increase relative fitness;
relative fitness is zero (0) for a sterile plant or animal.
Three ways (modes of selection) in which natural selection
can affect the contribution that a genotype makes to the
next generation.
a. Directional selection favors individuals at one end of
the phenotypic range. Most common during times of
environmental change or when moving to new habitats.

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Diversifying selection favors extreme over intermediate
phenotypes.
- Occurs when environmental change favors an extreme
phenotype.
Stabilizing selection favors intermediate over extreme
phenotypes.
- Reduces variation and maintains the current average.
- Example = human birth weights.

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Natural selection maintains sexual reproduction
-Sex generates genetic variation during meiosis and
fertilization.
-Generation-to-generation variation may be of greatest
importance to the continuation of sexual reproduction.
-Disadvantages to using sexual reproduction: Asexual
reproduction produces many more offspring.
-The variation produced during meiosis greatly outweighs
this disadvantage, so sexual reproduction is here to
stay.

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All asexual individuals are female (blue). With sex,
offspring = half female/half male. Because males
don’t reproduce, the overall output is lower for sexual
reproduction.

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Sexual selection leads to differences between sexes
a. Sexual dimorphism is the difference in appearance
between males and females of a species.
-Intrasexual selection is the direct competition between
members of the same sex for mates of the opposite sex.
-This gives rise to males most often having secondary
sexual equipment such as antlers that are used in
competing for females.
-In intersexual selection (mate choice), one sex is choosy
when selecting a mate of the opposite sex.
-This gives rise to often amazingly sophisticated
secondary sexual characteristics; e.g., peacock feathers.

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Natural selection does not produce perfect organisms
a. Evolution is limited by historical constraints (e.g., humans
have back problems because our ancestors were 4-legged).
b. Adaptations are compromises. (Humans are athletic due
to flexible limbs, which often dislocate or suffer torn
ligaments.)
c. Not all evolution is adaptive. Chance probably plays a huge
role in evolution and not all changes are for the best.
d. Selection edits existing variations. New alleles cannot
arise as needed, but most develop from what already is
present.

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GENES WITH IN
POPULATION
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Gene Variation is Raw Material
Natural selection and evolutionary change
Some individuals in a population possess certain inherited
characteristics that play a role in producing more surviving
offspring than individuals without those characteristics.
The population gradually includes more individuals with
advantageous characteristics.

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Gene Variation In Nature
Measuring levels of genetic variation
blood groups – 30 blood grp genes
Enzymes – 5% heterozygous
Enzyme polymorphism
A locus with more variation than can be explained by mutation is
termed polymorphic.
Natural populations tend to have more polymorphic loci than can be
accounted for by mutation.
15% Drosophila
5-8% in vertebrates

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Hardy-Weinberg Principle
Population genetics - study of properties of genes in
populations
blending inheritance phenotypically intermediate (phenotypic
inheritance) was widely accepted
new genetic variants would quickly be diluted

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Hardy-Weinberg - original proportions of genotypes in
a population will remain constant from generation to
generation
Sexual reproduction (meiosis and fertilization) alone will not
change allelic (genotypic) proportions.

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Population of cats
n=100
16 white and 84 black
bb = white
B_ = black
Can we figure out the allelic frequencies of individuals BB and Bb?

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Necessary assumptions
Allelic frequencies would remain constant if…
population size is very large
random mating
no mutation
no gene input from external sources
no selection occurring

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Calculate genotype frequencies with a binomial expansion
(p+q)2 = p2 + 2pq + q2
p2 = individuals homozygous for first allele
2pq = individuals heterozygous for alleles
q2 = individuals homozygous for second allele

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p2 + 2pq + q2
and
p+q = 1 (always two alleles)
16 cats white = 16bb then (q2 = 0.16)
This we know we can see and count!!!!!
If p + q = 1 then we can calculate p from q2
Q = square root of q2 = q √.16 q=0.4
p + q = 1 then p = .6 (.6 +.4 = 1)
P2 = .36
All we need now are those that are heterozygous (2pq)
(2 x .6 x .4)=0.48
.36 + .48 + .16

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Five Agents of Evolutionary Change
Mutation
Mutation rates are generally so low they have little effect on
Hardy-Weinberg proportions of common alleles.
ultimate source of genetic variation
Gene flow
movement of alleles from one population to another
tend to homogenize allele frequencies

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Nonrandom mating
assortative mating - phenotypically similar individuals mate
Causes frequencies of particular genotypes to differ from those
predicted by Hardy-Weinberg.

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Genetic drift – statistical accidents.
Frequencies of particular alleles may change by chance alone.
important in small populations
founder effect - few individuals found new population (small allelic pool)
bottleneck effect - drastic reduction in population, and gene pool size

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Genetic Drift - Bottleneck Effect

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Selection – Only agent that produces adaptive
evolutionary change
artificial - breeders exert selection
natural - nature exerts selection
variation must exist among individuals
variation must result in differences in numbers of viable offspring
produced
variation must be genetically inherited
natural selection is a process, and evolution is an outcome

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Selection pressures:
avoiding predators
matching climatic condition
pesticide resistance

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Measuring Fitness
Fitness is defined by evolutionary biologists as the
number of surviving offspring left in the next
generation.
relative measure
Selection favors phenotypes with the greatest fitness.

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Interactions Among Evolutionary Forces
Levels of variation retained in a population may be
determined by the relative strength of different
evolutionary processes.
Gene flow versus natural selection
Gene flow can be either a constructive or a constraining force.
Allelic frequencies reflect a balance between gene flow and natural
selection.

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Natural Selection Can Maintain
Variation
Frequency-dependent selection
Phenotype fitness depends on its frequency within the population.
Negative frequency-dependent selection favors rare phenotypes.
Positive frequency-dependent selection eliminates variation.
Oscillating selection
Selection favors different phenotypes at different times.

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Heterozygote Advantage
Heterozygote advantage will favor heterozygotes, and
maintain both alleles instead of removing less successful
alleles from a population.
Sickle cell anemia
Homozygotes exhibit severe anemia, have abnormal blood cells, and
usually die before reproductive age.
Heterozygotes are less susceptible to malaria.

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Forms of Selection
Disruptive selection
Selection eliminates intermediate types.
Directional selection
Selection eliminates one extreme from a phenotypic array.
Stabilizing selection
Selection acts to eliminate both extremes from an array of
phenotypes.

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Selection on Color in Guppies
Guppies are found in small northeastern streams in
South America and in nearby mountainous streams in
Trinidad.
Due to dispersal barriers, guppies can be found in pools below
waterfalls with high predation risk, or pools above waterfalls
with low predation risk.

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Evolution of Coloration in Guppies

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Selection on Color in Guppies
High predation environment - Males exhibit drab
coloration and tend to be relatively small and
reproduce at a younger age.
Low predation environment - Males display bright
coloration, a larger number of spots, and tend to be
more successful at defending territories.
In the absence of predators, larger, more colorful fish may
produce more offspring.

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Evolutionary Change in Spot Number

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Limits to Selection
Genes have multiple effects
pleiotropy
Evolution requires genetic variation
Intense selection may remove variation from a population at a
rate greater than mutation can replenish.
thoroughbred horses
Gene interactions affect allelic fitness
epistatic interactions

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Variations in genotype arise by random
fusion of gametes, mutation, and
______.
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Recom
bination
Translation
Transcription
Sorting by phenotype
25% 25%25%25%
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1.Recombination
2.Translation
3.Transcription
4.Sorting by phenotype

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The total genetic information in a
population is called the
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Allele
frequency
Phenotypefrequency
Gene
pool
Distribution
oftraits
25% 25%25%25%
20
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
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1.Allele frequency
2.Phenotype frequency
3.Gene pool
4.Distribution of traits

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Saint Bernards and Chihuahuas can’t mate normally
owing to great differences in size. What type of
reproductive isolationg mechanism is operating
here?
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Developm
ental
Prezygotic
Postzygotic
Geographic
25% 25%25%25%
20
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
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1.Developmental
2.Prezygotic
3.Postzygotic
4.Geographic

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If a population is in genetic equilibrium,
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Evolution
isoccuring
Speciation
isoccuring
Allelicfrequenciesare
c...
Allelicfrequenciesare
r...
25% 25%25%25%
20
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30
1.Evolution is occuring
2.Speciation is occuring
3.Allelic frequencies are
changing
4.Allelic frequencies are
remaining stable

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Mutations affect genetic
equilibrium by
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M
aintaining
it
Introducingne...
Causing
im
m
igr...
Causing
em
igra...
25% 25%25%25%
20
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30
1.Maintaining it
2.Introducing new
alleles
3.Causing immigration
4.Causing emigration

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Directional selection, disruptive
selection, and stabilizing selection
are all examples of
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Geneticequilibrium
Naturalselection
M
utation
Speciation
25% 25%25%25%
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30
1.Genetic equilibrium
2.Natural selection
3.Mutation
4.Speciation

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The most common way for new species
to form is through
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M
utation
Stabilizingselection
Geographicand
reprod...Geneticequilibrium
25% 25%25%25%
1.Mutation
2.Stabilizing selection
3.Geographic and
reproductive isolation
4.Genetic equilibrium
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30

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Population genetics
• genetic structure of a population
group of individuals
of the same species
that can interbreed
• alleles
• genotypes
Patterns of genetic variation in populations
Changes in genetic structure through time

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• mutation
• migration
• natural selection
• genetic drift
• non-random mating
cause changes in
allele frequencies
How does genetic structure
change?

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Forces that affect allele freq.
1. Mutation
2. Migration
3. Selection
4. Random (genetic) drift
Selection and migration most important for livestock
breeders.

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Selection
Some individuals leave more offspring than others.
Primary tool to improve genetics of livestock.
Does not create new alleles. Does alter freq.
Primary effect  change allele frequency of desirable
alleles.

Gene Pool and Evolution Explained

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