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
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
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
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
Modern Evolutionary Thought
Modern Synthesis Theory
Combines Darwinian selection and Mendelian inheritance
Population genetics - study of genetic variation within a
population
Emphasis on quantitative characters
Modern Synthesis Theory
1940s comprehensive theory of evolution (Modern Synthesis
Theory)
Introduced by Fisher & Wright
Until then , many did not accept that Darwins theory of natural
selection could drive evolution
S. Wright A. Fisher
Modern Synthesis Theory
Todays 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
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)
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
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.
Gene Pools
A populations 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.
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 .
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
.
Assumptions of the H-W Theorem
Large population size - small populations can have chance
fluctuations in allele frequencies ( e.g. , fire, storm).
No migration - immigrants can change the frequency of an allele
by bringing in new alleles to a population.
No net mutations - if alleles change from one to another, this
will change the frequency of those alleles
Assumptions of the H-W Theorem
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.
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.
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 = p 2 + 2 pq + q 2 where p 2 =
frequency of AA genotype; 2 pq = frequency of Aa plus aA genotype;
q 2 = frequency of aa genotype
But we know that evolution does occur within populations .
Evolution within a species/population = microevolution.
Microevolution refers to changes in allele frequencies in a
gene pool from generation to generation. Represents a gradual
change in a population.
Causes of microevolution :
1) Genetic drift
Natural selection (1 & 2 are most important)
Gene flow
Mutation
1) Genetic drift
Genetic drift = the alteration of the gene pool of a small
population due to chance.
Two factors may cause genetic drift:
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.
Founder effect may lead to reduced variability when a few
individuals from a large population colonize an isolated
habitat.
*Yes, I realize that this is not really a cheetah.
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.
4) Mutation Mutation is a change in an organisms 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.
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 individuals phenotype [ e.g. , the hydrangea we
saw before, and Map butterflies (color changes are due to seasonal
difference in hormones)].
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.
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.
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.
Cline , a type of geographic variation, is a graded variation
in individuals that correspond to gradual changes in the
environment. Example: Body size of North American birds tends to
increase with increasing latitude. Can you think of a reason for
the birds to evolve differently? Example: Height variation in
yarrow along an altitudinal gradient. Can you think of a reason for
the plants to evolve differently?
Mutation and sexual recombination generate genetic variation a.
New alleles originate only by mutations (heritable only in gametes;
many kinds of mutations; mutations in functional gene products most
important). - In stable environments, mutations often result in
little or no benefit to an organism, or are often harmful. -
Mutations are more beneficial (rare) in changing environments.
(Example: HIV resistance to antiviral drugs.) b. Sexual
recombination is the source of most genetic differences between
individuals in a population. - Vast numbers of recombination
possibilities result in varying genetic make-up.
Diploidy and balanced polymorphism preserve variation a.
Diploidy often hides genetic variation from selection in the form
of recessive alleles. Dominant alleles hide recessive alleles in
heterozygotes. b. Balanced polymorphism is the ability of natural
selection to maintain stable frequencies of at least two
phenotypes. Heterozygote advantage is one example of a balanced
polymorphism, where the heterozygote has greater survival and
reproductive success than either homozygote (Example: Sickle cell
anemia where heterozygotes are resistant to malaria).
Frequency-dependent selection = survival of one phenotype
declines if that form becomes too common. (Example: Parasite-Host
relationship. Co-evolution occurs, so that if the host becomes
resistant, the parasite changes to infect the new host. Over the
time, the resistant phenotype declines and a new resistant
phenotype emerges.)
Neutral variation is genetic variation that results in no
competitive advantage to any individual. - Example: human
fingerprints.
A Closer Look: Natural Selection as the Mechanism of Adaptive
Evolution Evolutionary fitness - Not direct competition, but
instead the difference in reproductive success that is due to many
variables. Natural Selection can be defined in two ways: a.
Darwinian fitness - Contribution of an individual to the gene pool,
relative to the contributions of other individuals. And,
b. Relative fitness
- Contribution of a genotype to the next generation, compared
to the contributions of alternative genotypes for the same
locus.
Survival doesnt 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.
Directional selection
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.
Diversifying selection
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.
All asexual individuals are female (blue). With sex, offspring
= half female/half male. Because males dont reproduce, the overall
output is lower for sexual reproduction.
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.
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.
Genes Within Populations Chapter 21
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.
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
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
Hardy-Weinberg Principle
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.
Hardy-Weinberg Equilibrium 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?
Hardy-Weinberg Principle
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
Hardy-Weinberg Principle
Calculate genotype frequencies with a binomial expansion
(p+q) 2 = p 2 + 2pq + q 2
p2 = individuals homozygous for first allele
2pq = individuals heterozygous for alleles
q2 = individuals homozygous for second allele
p 2 + 2pq + q 2
and
p+q = 1 (always two alleles)
16 cats white = 16bb then ( q 2 = 0.16 )
This we know we can see and count!!!!!
If p + q = 1 then we can calculate p from q 2
Q = square root of q 2 = q .16 q=0.4
p + q = 1 then p = .6 (.6 +.4 = 1)
P 2 = .36
All we need now are those that are heterozygous (2pq) (2 x .6 x
.4)= 0.48
.36 + .48 + .16
Hardy-Weinberg Principle
Hardy-Weinberg Equilibrium
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
Five Agents of Evolutionary Change
Nonrandom mating
assortative mating - phenotypically similar individuals
mate
Causes frequencies of particular genotypes to differ from those
predicted by Hardy-Weinberg.
Five Agents of Evolutionary Change
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
Genetic Drift - Bottleneck Effect
Five Agents of Evolutionary Change
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
Five Agents of Evolutionary Change
Selection pressures:
avoiding predators
matching climatic condition
pesticide resistance
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.
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.
Natural Selection Can Maintain Variation
Frequency-dependent selection
Phenotype fitness depends on its frequency within the
population.
900/2000 = 0.45 r 1100/2000 = 0.55 R total = 2000 alleles allele
frequencies: = 400 r = 500 r = 500 R = 600 R Describing genetic
structure
for a population with genotypes: 100 GG 160 Gg 140 gg Genotype
frequencies Phenotype frequencies Allele frequencies
calculate:
for a population with genotypes: 100 GG 160 Gg 140 gg Genotype
frequencies Phenotype frequencies Allele frequencies 100/400 = 0.25
GG 160/400 = 0.40 Gg 140/400 = 0.35 gg 260/400 = 0.65 green 140/400
= 0.35 brown 360/800 = 0.45 G 440/800 = 0.55 g calculate: 0.65
260
another way to calculate allele frequencies: 100 GG 160 Gg 140
gg Genotype frequencies Allele frequencies 0.25 GG 0.40 Gg 0.35 gg
360/800 = 0.45 G 440/800 = 0.55 g OR [0.25 + (0.40)/2] = 0.45 [0.35
+ (0.40)/2] = 0.65 0.25 0.40/2 = 0.20 0.40/2 = 0.20 0.35 G g G
g
Population genetics Outline What is population genetics?
Calculate Why is genetic variation important?
genotype frequencies
allele frequencies
How does genetic structure change?
Genetic variation in space and time Frequency of Mdh-1 alleles
in snail colonies in two city blocks
Changes in frequency of allele F at the Lap locus in prairie
vole populations over 20 generations Genetic variation in space and
time
Why is genetic variation important? potential for change in
genetic structure
adaptation to environmental change
- conservation
Genetic variation in space and time
divergence of populations
- biodiversity
Why is genetic variation important? variation no variation
EXTINCTION!! global warming survival
Why is genetic variation important? variation no variation
north south north south
Why is genetic variation important? variation no variation
divergence NO DIVERGENCE!! north south north south
Natural selection Resistance to antibacterial soap Generation
1: 1.00 not resistant 0.00 resistant
Natural selection Generation 1: 1.00 not resistant 0.00
resistant Resistance to antibacterial soap
Natural selection Resistance to antibacterial soap mutation!
Generation 1: 1.00 not resistant 0.00 resistant Generation 2: 0.96
not resistant 0.04 resistant
Natural selection Resistance to antibacterial soap Generation
1: 1.00 not resistant 0.00 resistant Generation 2: 0.96 not
resistant 0.04 resistant Generation 3: 0.76 not resistant 0.24
resistant
Natural selection Resistance to antibacterial soap Generation
1: 1.00 not resistant 0.00 resistant Generation 2: 0.96 not
resistant 0.04 resistant Generation 3: 0.76 not resistant 0.24
resistant Generation 4: 0.12 not resistant 0.88 resistant
Natural selection can cause populations to diverge divergence
north south
Selection on sickle-cell allele aa abnormal hemoglobin
sickle-cell anemia very low fitness intermed. fitness high fitness
Selection favors heterozygotes ( Aa ). Both alleles maintained in
population ( a at low level). Aa both hemoglobins resistant to
malaria AA normal hemoglobin vulnerable to malaria
sampling error
genetic change by chance alone
misrepresentation
small populations
How does genetic structure change?
mutation
migration
natural selection
genetic drift
non-random mating
Genetic drift 8 RR 8 rr Before: After: 2 RR 6 rr 0.50 R 0.50 r
0.25 R 0.75 r
mutation
migration
natural selection
genetic drift
non-random mating
cause changes in allele frequencies How does genetic structure
change?
mutation
migration
natural selection
genetic drift
non-random mating
non-random mating
non-random
allele combinations
mating combines alleles into genotypes How does genetic structure
change?
AA x AA AA aa x aa aa genotype frequencies: AA = 0.8 x 0.8 =
0.64 Aa = 2(0.8 x0.2) = 0.32 aa = 0.2 x 0.2 = 0.04 allele
frequencies: A = 0.8 A = 0.2 A A A A A A A A a a AA 0.8 x 0.8 Aa
0.8 x 0.2 aA 0.2 x 0.8 A 0.8 A 0.8 a 0.2 a 0.2 aa 0.2 x 0.2
Example: Coat color
B__ = black
bb = red
Herd of 200 cows:
100 BB, 50 Bb and 50 bb
Allele frequency
No. B alleles = 2(100) + 1(50) = 250
No. b alleles = 2(50) + 1(50) = 150
Total No. = 400
Allele frequencies:
f(B) = 250/400 = .625
f(b) = 150/400 = .375
genotypic frequencies
f(BB) = 100/200 = .5
f(Bb) = 50/200 = .25
f(bb) = 50/200 = .25
phenotypic frequencies
f(black) = 150/200 = .75
f(red) = 50/200 = .25
Previous example: counted alleles to compute
frequencies. Can also compute allele frequency
from genotypic frequency.
f(A) = f(AA) + 1/2 f(Aa)
f(a) = f(aa) + 1/2 f(Aa)
Previous example, we had
f(BB) = .50, f(Bb) = .25, f(bb) = .25.
allele frequencies can be computed as:
f(B) = f(BB) + 1/2 f(Bb)
= .50 + 1/2 (.25) = .625
f(b) = f(bb) + 1/2 f(Bb)
= .25 + 1/2 (.25) = .375
Mink color example:
B_ = brown
bb = platinum (blue-gray)
Group of females (.5 BB, .4 Bb, .1 bb) bred to
heterozygous males (0 BB, 1.0 Bb, 0 bb).
Allele frequencies among the females?
f(B) = .5 + 1/2(.4) = .7
f(b) = .1 + 1/2(.4) = .3
Allele frequencies among the males?
f(B) = 0 + 1/2(1) = .5
f(b) = 0 + 1/2(1) = .5
Expected genotypic, phenotypic and allele frequencies in the
offspring?
Expected frequencies in offspring
Genotypic
.35 BB
.50 Bb
.15 bb
Phenotypic
.85 brown
.15 platinum
Allele frequencies in offspring
f(B) = f(BB) + .5 f(Bb)
= .35 + .5(.50) = .6
f(b) = f(bb) + .5 f(Bb)
= .15 + .5(.50) = .4
Also note: allele freq. of offspring = average of sire and dam
f(B) = 1/2 (.5 + .7) = .6
f(b) = 1/2 (.5 + .3) = .4
Hardy-Weinberg Theorem
Population gene and genotypic frequencies dont change over
generations if is at or near equilibrium.
Population in equilibrium means that the populations isnt under
evolutionary forces (Assumptions for Equilibrium*)
Assumptions for equilibrium
large population (no random drift)
Random mating
no selection
no migration (closed population)
no mutation
Hardy-Weinberg Theorem
Under these assumptions populations remains stable over
generations.
It means: If frequency of allele A in a population is =.5, the
sires and cows will generate gametes with frequency =.5 and the
frequency of allele A on next generation will be =.5!!!!!
Hardy-Weinberg Theorem
Therefore:
It can be used to estimate frequencies when the genotypic
frequencies are unknown.
Predict frequencies on the next generation.
Hardy-Weinberg Theorem
If predicted frequencies differ from observed frequencies
Population is not under Hardy-Weinberg Equilibrium.
Therefore the population is under selection, migration,
mutation or genetic drift.
Or a particular locus is been affected by the forces mentioned
above.
Hardy-Weinberg Theorem (2 alleles at 1 locus)
Allele freq.
f(A) = p
f(a) = q
p + q = 1
Sum of all alleles = 100%
Genotypic freq.
f(AA) = p 2 Dominant homozygous
f(Aa) = 2 pq Heterozgous
f(aa) = q 2 Recessive homozygous
p 2 + 2 pq + q 2 = 1
Sum of all genotypes = 100%
Hardy-Weinberg Theorem
Allele freq.
f(A) = p
f(a) = q
p + q = 1
Sum of all alleles = 100%
Genotypic freq.
f(AA) = p 2 Dominant homozygous
f(Aa) = 2 pq Heterozgous
f(aa) = q 2 Recessive homozygous
p 2 + 2 pq + q 2 = 1
Sum of all genotypes = 100%
AA = p*p = p 2 Aa = pq + qp = 2pq Aa = q*q = q 2 Gametes A (p) a(q)
A(p) AA (pp) Aa (pq) a(q) aA (qp) aa (qq)
Example use of H-W theorem
1000-head sheep flock. No selection for color. Closed to
outside breeding.
910 white (B_)
90 black (bb)
Start with known: f(black) = f(bb) = .09 =q 2
Then, p = 1 q = .7 = f(B)
f(BB) = p 2 = .49
f(Bb) = 2pq = .42
f(bb) = q 2 = .09
In summary:
Allele freq.
f(B) = p = .7 (est.)
f(b) = q = .3 (est.)
Phenotypic freq.
f(white) = .91 (actual)
f(black) = .09 (actual)
Genotypic freq.
f(BB) = p 2 = .49 (est.)
f(Bb) = 2pq = .42 (est.)
f(bb) = q 2 = .09 (actual)
Mink example using H-W
Group of 2000 (1920 brown, 80 platinum) in equilibrium. We know
f(bb) = 80/2000 = .04 = q 2
f(b) = (q 2 ) = .04 = .2
f(B) = p = 1- q = .8
f(BB) = p 2 = .64
f(Bb) = 2pq = .32
Forces that affect allele freq.
1. Mutation
2. Migration
3. Selection
4. Random (genetic) drift
Selection and migration most important for livestock
breeders.
Mutation
Change in base DNA sequence.
Source of new alleles.
Important over long time-frame.
Usually undesirable.
Migration
Introduction of allele(s) into a population from an outside
source.
Classic example: introduction of animals into an isolated
population.
others:
new herd sire.
opening herd books.
under-the-counter addition to a breed.
Change in allele freq. due to migration
p mig = m(P m -P o ) where
P m = allele freq. in migrants
P o = allele freq. in original population
m = proportion of migrants in mixed pop.
Migration example
100 Red Angus (all bb, p 0 = 0)
purchase 100 Bb (p m = .5)
p = m(p m - p o ) = .5(.5 - 0) = .25
new p = p 0 + p = 0 + .25 = .25 = f(B)
new q = q 0 + q = 1 - .25 = .75 = f(b)
Random (genetic) drift
Changes in allele frequency due to random segregation.
Aa .5 A, .5 a gametes
Important only in very small pop.
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.
