Microevolution
Chapter 16
Under Stalin the teaching of genetics
was forbidden. One scientist was
imprisoned and died for teaching it.
After Stalin’s death the Premier,
Khrushev had to buy wheat from the
U.S. He even came to Morris, IL to
see how closely corn rows could be
planted.
Darwin did not know where
variation came from. He and most
other scientists believed
inheritance was blended. Ex. Zebra
and horse produce a zebroid but it
is sterile.
Blending inheritance and natural
selection are incompatible.
Embryologists noted that tissue
that forms ovaries or testes
separates from the human body
early in development –
differentiation takes place in the 4th
week.
Weisman concluded that nothing in
development or the environment of
the body influences the germ cells
or (DNA). Genetic material, he
thought was passed from generation
to generation.
In 1883 biologists observed that
sexual reproduction provides new
combinations – but it wasn’t known
where new traits came from.
In 1900 3 biologists in different
countries rediscovered Mendel’s
work, de Vries, Correns, and
Tschermak rediscovered 3:1 ratios in
monohybrid crosses.
In the 1930’s a population genetics
arose. It described in mathematical
terms processes by which organisms
generate new traits and pass them
on – called microevolution - changes
in the frequencies of genes in a
population.
Ex. In flowers if the number of red
flowers exceeds the number of
white eventually the population will
contain all red. Genetic variation
begins with mutation that changes
1 or a few nucleotides and alters a
single protein.
Chromosome mutations change the
number of chromosomes or
arrangement of genes. If harmless
the mutation is passed on. But
inheritance in a diverse population
can have many alleles. If these are
2 or more alleles the gene is
polymorphic such as eye color.
Most traits are also polygenic (eye
color) 2 genes with 2 alleles can
result in at least 5 phenotypes. AABB
= dark brown. Grey, green hazel,
and blue are caused by different
combinations. Environmental
influences (internal or external) can
also change phenotype slightly.
Other traits influenced by polygenes
are height, weight, bone structure,
hormone levels, personality,
intelligence, and skin color. Genetic
variation is different from population
to population.
What determines genetic variability in
a Population?
1) How fast the mutations accumulate
in DNA.
2) How fast mutations spread through
a population.
3) How fast selection eliminates
mutations from a population.
Each new baby carries one or two
new mutations.
Mutations arise constantly. The rate
at which a mutation spreads through
a population determines the
evolutionary rate of a population. In
humans about 7% of an individuals
genes are heterozygous. In most
natural populations heterozygosity is
between 4-15%.
Natural selection acts on phenotype.
Selection eliminates organisms less
able to survive but environments can
also quickly change.
Genetic Equilibrium-
In 1908 Hardy and Weinberg
considered the idea that sexual
reproduction eliminates variation.
Sexual reproduction does not
eliminate the frequency of alleles in a
population. This idea have become
known as the “Hardy-Weinberg
principle”.
All the genes in a population are the
gene pool. The frequency of a
given allele within the gene pool is
the number of times the allele is
present divided by the number of
times it could be present if every
person had the allele in both
chromosomes.
Consider a gene with 2 alleles; A and
a. AA and Aa are blue, aa is brown.
The fraction of the population
containing gene A is p.
The fraction of the population
containing gene a = q.
If p = .6, q=.4 There are only 2
alleles for the gene so p + q = 1.0. p
and q are allele frequencies. If
mussels release sperm into sea water
60% carry an A allele. 40% carry an
a allele.
The frequency that a AA homozygote
= .6 x .6 = .36.
The frequency that an aa
homozygote = .4x .4 = .16.
The frequency of an Aa = .6 x.4 =
.24 and the frequency of aA = .4 x .6
= .24.
The probability of p + 2 pq + q = 1.0
(.36 + .24 + .24 + .16 = 1.0.
Despite sexual reproduction allele
frequencies do not change. If there
are 3 alleles A, a, and a’, p + q + r
=1. A = p, a = q, and a” = r.
A= .5, a =.3, a’ =.2
A = .5 x .5 = .25
A = .3 x.3 = .09
A’ = .2 x .2 = .04
Aa = ,5 x .3 = .15 (x2) = .30
Aa’ = .5 x .2 = .10 (x2) = .20
aa’ = .3 x .2 = .06 (x2) = .12
.25 + .09 + .04 + .30 + .20 + .12 = 1.0
The stability of genotype frequencies is called
Hardy-Weinberg Equilibrium. It exists when
populations meet all of the following 5
requirements:
1. Random mating
2. Large population size.
3. No mutations
4. No gene flow (interbreeding with another
population)
5. No selection
In reality this does not exist in
nature. Gene frequencies change so
evolution does occur. The HardyWeinberg Principle showed sexual
reproduction does not dilute
variation – supporting Darwinism.
Factors that change allele
frequencies are nonrandom mating,
small population size, mutation,
interbreeding between populations
(gene flow), and natural selection.
Random mutations can generate
every possible change in an amino
acid sequence.
Mutations have little immediate
effect on gene frequencies. One or
2 mutations out of 3 billion
nucleotides isn’t going to change
overall allele frequencies in an entire
population of thousands of people.
Genetic drift can be demonstrated by
flipping a coin. If you flip it 100 x
you may get close to 50 H : 50 T.
But if you flip it 4x there is a much
higher chance of not getting close to
50/50. Chance fluctuations that
affect small sample sizes are called
“sampling errors”.
In small populations, sampling
errors will cause allele frequencies
to change randomly from generation
to generation. The effects of
chance in small populations lead to
genetic drift, change in gene
frequency due to random events
(not selection).
Population Bottlenecks occur when a
population is reduced by hunting or
other factors. The few remaining
individuals have fewer alleles to
reproduce so they are inbred and
lack genetic diversity. A single
epidemic could wipe them all out.
African cheetahs have the least
heterozygosity of any animal species
, .07%
In bottleneck populations the gene
pool is less than what the
environment and natural selection
chooses. The smaller the
population the less time it takes for
genetic drift to change allele
frequencies.
If a new mutant appears in a gene
pool it can be eliminated by other
things than natural selection
(volcanos). Even in large
populations genetic drift will cause
alleles to go to 0% or 100% slowly
or quickly but leads to a
homozygous population.
Evolution would cease. New
mutations restock genetic variationthe key to evolution. The HardyWeinberg Principle assumes random
mating. However in nature
genotype is chosen as reflected by
phenotype.
Mate selection occurs as “sexual
selection”. Mates are chosen on
basis of similarities to an individual’s
own genotype. Humans prefer
those of similar skin color. When
individuals choose mates from
among their own relatives children
are more likely to carry 2 identical
alleles.
Ex. Ellis – van Creveld syndrome is
more frequent among Amish
because of years of inbreeding.
Recessive genes have negative
effects that are expressed in children
homozygous of that allele. (inbreds
are less healthy). Breeding between
populations changes allele
frequencies.
Hardy –Weinberg equilibrium
requires that a population not
interbreed with other populations.
Animals immigrate to nearby
populations and take new alleles.
The resulting gene flow causes
changes in allele frequencies and is
nearly universal. 2 effects of gene
flow :
1.From the outside it causes genetic
variation.
2.Makes adjacent populations more
alike.
Natural selection can decrease the
frequency of some genes and
increases the frequency of others.
Ex. Tay-Sachs is fatal before age 4
if it is homozygous therefore it
survives in low numbers in
heterozygotes. Lethal alleles persist
indefinitely in low percentages.
Selection increases the frequency of
traits. Insects have developed
resistance to poisons and
reproduced. Ex. DDT was used in
insecticide to kill flies in Lake Tahoe.
Some flies died but the few resistant
ones reproduced and became as
large a nuisance as the original
strain.
DDT is toxic to other animals and
fish there. Because fish consume
many fly larvae they had large
concentrations in their livers. When
humans consumed the fish they
became affected by the carcinogenic
effects of DDT.
When penecillin was developed it
was called a “magic bullet”. Now
days most bacteria are resistant to
it. Some bacteria are resistant to all
known antibiotics.
The peppered moth in Great Britain
existed in 2 forms, light and dark.
They are preyed on by birds. 100
years ago they were mostly light but
after the industrial revolution soot
turned tree trunks dark. 98% of the
population were dark due to light
forms being more visible to birds.
Kettlewell proved this was true. He
marked moths with paint, recaptured
them. Where air was cleaner the
majority of the population was light.
In sooty areas more dark forms were
recaptured. After laws were passed
reducing soot in the air lighter forms
were common.
Types of Natural Selection
Directional- shifts frequency of traits in
a direction. Ex: Industrial melanism in
moths
Stabilizing selection- acts against
extremes in phenotypes so the
average is selected for. EX:
Horseshoe crabs have survived
hundreds of millions of years
unchanged.
Disruptive selection- encourages
development of new species. Ex.
Antibiotic resistant bacteria.
Coevolution – selection from the
environment causes separate species
to mutually benefit one another. EX:
hummingbirds are the sole pollinators
of certain flowers in St. Lucia. Male
birds defend the flowers. Females
must feed from a different species of
flowers. The male coevolved with one
flower and females with another
flower.
Sexual selection - in birds males are
brightly colored and females drab.
Male plumage attracts the females.
Females are camouflaged to guard
nests.
In mammals males are larger than
females and more aggressive. Males
fight for territories. The difference in
appearance of males and females is
called sexual dimorphism. Females
choose males with characteristics she
chooses. Courtship varies with
species.
Maintaining genetic heterogeneity in
a population is not surprising if all
the genes are useful. Ex: orange
ladybugs are favored in the fall and
red in the spring. This creates a
balance of different alleles in a
population called “balanced
polymorphism”.
Populations may contain high
frequencies of alleles lethal to an
organism. In these cases
polymorphism results from the
selective superiority of the
heterozygote. EX: sickle cell anemia.
In the U.S. sickle cell homozygotes
die young.
In Africa homozygotes die in their
teens and 20’s. The BS allele
persists in populations because it
has strong BABS selective value.
Heterozygotes are not sick and
resistant to the parasite that causes
malaria in BABA people.
In Africa the BS allele is 20 % , BA =
80%. Using the Hardy-Weinberg
Principle the frequency of the
heterozygote is 2(pq), 2 x .2 x .8 =
32%. The frequency of the
homozygote is .2 x .2 = 4%.
Homozygotes die without
reproducing. In the U.S. the BS allele
has no selective value and is lower.
Change in allele frequency is
microevolution.