Transcript Population Genetics

Population Genetics
 Populations:
Group of individuals of
the same species living in the same
area.
 Species: Group of populations the
interbreed and provide offspring.
 Gene Pool : Aggregate of genes in a
population.
– In diploid species each individual
represents a double dose of genes.
 Allele
Frequency: How often an allele
appears in a population.
– Lets examine a population of 500 wild
flowers.
– RR, Rr – red
rr - white
– 480 flowers are red
 320
RR
 160 Rr

320 x 2 = 640
160 x 1 = 160
800 R
– 20 flowers are white
 20
rr
 160 Rr
–
20 x 2 =
40
160 x 1 = 160
200 r
800/1000 = 0.8
R occurs 80%
r occurs 20%
200/1000 = 0.2
Hardy Weinberg Theorem
Creates a theoretical non-evolving model
population to compare other evolving
populations to.
 A population in Hardy Weinberg
equilibrium is a stable population in which
the allele frequencies do not change.
 Shuffling of alleles by Meiosis and random
fertilization have no effect on the overall
gene pool.

 So
the chance thath the offspring will
receive an R is 0.8 or 80% and the
chance the offspring will receive an r
is 0.2 or 20%.
 So what are the chances that an R
will end up with another R?
– Rule of multiplication:
– 0.8 x 0.8 = .64
64% should be RR
– 0.2 x 0.2 = 0.04
4% should be rr
– That means 32% should be Rr
Rule of Addition Applies to
Heterozygotes
 There
are two ways in which a
heterozygote can occur.
 Maternal
R
r
Paternal
r
R
– Rr + Rr = 2Rr
– 2(0.8 x 0.2) = 0.32 or 32 %
 The
dominant allele can be
represented by P and the recessive
allele by p.
P + q = 1
 P2
 P2
=RR
and q2 = rr
so 2pq = Rr
+ 2pq + q2 = 1
 These equations can be used to
predict allele frequencies in a given
population.
To Maintain Hardy Weinberg
Equilibrium we must Assume:
–
–
–
–
–

The population is very large
There is no migration
There are no net mutations
Random mating occurs
There is no natural selection occurring.
Microevolution – small changes in
populations from generation to
generation.
– Two primary causes of microevolution are
genetic drift and natural selection.
Two Main causes for
Microevolution
Genetic Drift: Change in allele frequencies
due to chance.
 Natural Selection: Differential reproductive
success.
 Two main mechanisms that cause genetic
drift.

– Bottleneck effect: Most individuals of the
population die off leaving behind an
overrepresentation of some alleles.
– Founder Effect: a few individuals leave the
population to create a new population.
– Both reduce the population number so
that genetic drift is significant.
 Other
Mechanisms that change allele
frequencies:
– Natural Selection
 Works
on genetic genetic variation within
the population.
Gene Flow - migration
– Mutation – change the DNA itself.
 Only
germ line mutations are significant.
Polymorphisms
 Not
all phenotypic variations in a
population are heritable.
 Environmental factors can affect
phenotype.
 Polymorphisms are two or more
distinct phenotypes within a species
often called morphs.
Morphs Are Created by Several
Factors
 Nucleotide
sequence variation.
– RFLP’s
 Environmental
– Cline – gradual phenotypic change due
to climate change.
 Variations
are created by:
– Mutation
– Sexual recombination
Balanced Polymorphism
 Natural
Selection maintains several
morphs within a population.
– Heterozygote advantage
– Sickle Cell Anemia
 Frequency
Dependent Selection
– One morph becomes too common and it is
selected against.
 Neutral
Variation
No effect on selection.
Nucleotide sequence different(fingerprinting)
“Dawinian Fitness”
Contribution of genes relative to next
generation.
 Relative Fitness - contribution of alleles
to the next generation compared to the
other individuals in the population.

– A relative fitness of 1 is assigned to the
individual that contributes the most alleles.
– Red flowers produce the most offspring that survive.
– White flowers produce 80% of the offspring that the
red flowers do.
– White flower allele has a fitness of 0.8
 Relative
fitness of a sterile organism is 0
Factors That Affect Allele
Frequencies
 Directional
Selection
– One extreme morph is favored.
 Diversifying
Selection
– Both extreme phenotypes are
favored.
 Stabilizing
Selection
– The most predominant phenotype is
favored.
 Reduces
variation
 Sexual
Dimorphism
– Sexes look different
 Assorative
mating
– Animals actively select phenotypes
 Intrasexual
Selection
– Same sex competes for the opposite
sex.
 Intersexual
Selection
– Choosey female chooses best male.
Four Main Reasons Natural
Selection Does not Create Perfect
Individuals
Ancestors not sraped, nature works on
preeisting species.
 Adaptations are a compromise

– Sea flippers land and sea.

Not all evolution is adaptive.
– Best insects may not get blown by the storm.

Selection can only edit what already
exists.
– Humans have back problems due to
bipedalism.