CHAPTER 16
THE EVOLUTION OF
POPULATIONS
&
SPECIATION
• Is the study of evolution from
a genetic point of view.
• Populations have genetic
differences
• those differences show up in
their phenotypes.
• This bell curve can be used to
show the range of phenotypes
and the relative number of
individuals in the population
w/ the phenotype.
• Ex. Height, fur color, leg
length, neck length
• Certain phenotypes may
increase fitness while some
decrease fitness
Population Genetics
BELL CURVE
Variations in genotype
arise in 3 main ways
1. Mutations of DNA
(s phase)
2. Recombination
(meiosis)
3. Random fusion of
gametes during sexual
reproduction
the gene pool
the total genetic information available in a
population.
This is not a gene pool. It is a pretty pool in Bali.
Better example of gene pool…
• The Darwin Awards.
• Awards to stupid people who have
accidentally killed themselves thus
removing their genes from the gene
pool.
Evolution is the change in allele
frequencies from one generation to the
next.
• ie. In the case of rabbits, the white rabbits are easy to see
because they don’t camouflage well.
• When the recessive phenotype is selected against…
• Fewer homozygous recessive bunnies survive to
reproduce and therefore there is more food to support
heterozygous and homozygous dominant bunnies who
reproduce more.
• So the frequency of recessive alleles decreases and
frequency of dominant alleles increases from one
GENETIC DRIFT is the:
• Random increase or decrease in allele
frequencies. (By chance)
• The effect is more pronounced when
populations are SMALL.
Ex. Flipping a coin. Should be 50% heads &
50% tails. If you flip 1000 or 100 times it
will be. If you flip 10 times it won’t.
Two special kinds of genetic drift
are commonly observed as:
1) The Founder Effect
2) Population Bottleneck
Both reduce the population size significantly, but for
different reasons.
1) The founder effect occurs when the allele
frequencies in a group of migrating individuals are,
by chance, not the same as that of their population
of origin.
Ex. Amish (German immigrants to Pennsylvania). One
founder had an allele for polydactylism.
After 200 years of reproductive isolation
%Amish with polydactylism > % humans with
polydactylism.
FOUNDER EFFECT
• If a population began with a few
individuals — one or more of
whom carried a particular allele
• — that allele may come to be
represented in many of the
descendants.
• Ex. Ellis-Van-Creveld syndrome
results in shortened limbs and six
fingers on each hand.
• Ex. Sickle cell anemia more
prevalent among African
Americans than the general
public-- malaria as NS.
2) A population bottleneck occurs when the
population undergoes a dramatic decrease
in size due to natural catastrophe, disease,
predation, or human influences.
Ex. Ice ages, volcanic eruption, hunting.
Ex. Tay-Sachs more prevalent among
Ashkanazi Jews than the general public.
What’s going to happen to the
diversity of the new Population?
Allele frequencies of new population will be different from the
original population. Some genetic diversity is lost.
POPULATION BOTTLENECKS
put species at risk of extinction.
CHEETAH
Elephant seal
• Cheetahs, the fastest of the land animals, seem
to have passed through a similar period of small
population size with its accompanying genetic
drift.
• Examination of 52 different loci has failed to
reveal any polymorphisms; that is, these
animals are homozygous at all 52 loci.
• The lack of genetic variability is so profound
that cheetahs will accept skin grafts from
each other just as identical twins & inbred
mouse strains do.
• Whether a population with such little genetic
diversity can continue to adapt to a changing
environment remains to be seen.
• By 1900 hunting of the northern elephant
seal off the Pacific coast had reduced its
population to only 20 survivors.
• Since hunting ended, the population has
rebounded from this population
bottleneck to some 100,000 animals today.
• However, these animals are homozygous at
every one of the gene loci that have been
examined.
Disruptive Selection could lead
to SPECIATION
• SPECIATION is the formation new species.
• Happens if populations become ISOLATED- no
gene flow
1) geographic isolation… ie. islands, separate
continents
2) reproductive isolation
a) prezygotic… ie. mechanical incompatibility
b) postzygotic… ie. miscarriage or sterile
offspring
What is the
morphological/biological
SPECIES concept?
Same species IF individuals are
capable of reproducing to
produce fertile offspring.
HARDY-WEINBERG genetic
equilibrium
is a “negative proof” for evolution.
When allele frequencies change- evolution is
occurring… HW conditions prevent changes in
ALLELE frequencies.
Allele frequencies remain the same if:
1. There are no mutations- a dominant allele doesn’t become
recessvie
2. There is no gene flow- immigration or emigration to upset the
balance of alleles in the population
3. The population is large- to avoid genetic drift, change due to
random events or chance
4. Mating is random- no preference for mates
5. Natural Selection does not occur- all phenotypes have the
same chance for survival.
p2 + 2pq + q2 = 1
p+q=1
•
•
•
•
•
P = frequency of dominant alleles
q = frequency of recessive alleles
P2 = frequency of homo dominant genotypes
2pq = frequency of heterozygous genotypes
q2 = frequency of homo recessive genotypes
What is the relationship between
phenotype and genotype?
• Organisms with the recessive phenotype definitely have
the homozygous recessive genotype.
• Organisms with the DOMINANT phenotype may be
heterozygous or homozygous dominant.
• You can’t be sure of their genotype w/out doing a test
cross or looking at a family pedigree.
• We can use this fact to solve Hardy-Weinberg math
problems.
• % recessive phenotype= % recessive genotype = q2
PRACTICE PROBLEM:
In a population of 100 individuals 16 display
the recessive phenotype while the other 84
display the dominant phenotype.
Use the HW equations to calculate the allele
frequencies for the dominant and recessive
alleles.
What do you know? Phenotypes!!!
• q2= recessive phenotype= 16/100
• p2 + 2pq = dominant phenotype = 84/100
• LET’S SOLVE FOR q
• If q2 = 0.16 then q= the square root of .16 is 0.4
• q = 0.4
• LET’S SOLVE FOR p
• p = 1 - 0.4
• p = 0.6
Can you calculate the # of
homozygous dominants and the #
of heterozygous individuals?
• Heterozygotes are represented by…
• 2pq
• 2 x 0.6 x 0.4= .48
• Homozygous dominants are represented by…
• p2
• (0.6)2 = 0.36
• Double check… do those numbers add up?
• Homo rec + hetero + Homo dom = 100%
In the population of 400, 100
exhibit the recessive phenotype.
• Calculate the allele frequencies. (p and q)
• Calculate the genotype frequencies.
• 100/400= recessive phenotype frequency
• Recessive phenotype frequency=recessive genotype
frequency
• Homozygous dominant genotype frequency = q2 = .25
• q2 = .25
• What is the square root of .25? = .5
• Recessive allele frequency = q =. 5
• P+q = 1 so P = 1-.5
• Dominant allele frequency = P = .5
• Homozygous dominant genotype frequency = P2 = .52 =
.25
• Heterozygous genotype frequency = 2pq = 2x.5x.5= .50
TYPES OF NATURAL
SELECTION
TYPES OF NATURAL
SELECTION
• Stabilizing selection
• Directional selection
• Disruptive selection
• Sexual selection
• Artificial selection
STABILIZING
SELECTION
Selects against the extreme phenotypes
Heterozygous individuals are favored
DIRECTIONAL
SELECTION
Selects against an extreme phenotype
Favors the other extreme
DISRUPTIVE
SELECTION
Selects against the median phenotype
Favors phenotypes at both extremes
The evolutionary process is sped up
hrough ARTIFICIAL SELECTION- humans are the
agent of selection… choosing which individuals mate
Sexual Selection
traits that decrease survival but increase
chances for reproduction increase.
• Intersexual
selection =
females
choosing males
based on
appearance or
behavior.
Male vs. Male competition =
intrasexual selection
Intrasexual selection =
males competing with one
another for access to mates.
VIDEO: WHY SEX?