Transcript Chapter 23 Notes: Population Genetics

DEFINITIONS:
● POPULATION: a localized group of individuals
belonging to the same species
● SPECIES: a group of populations whose
individuals have the potential to
interbreed and produce fertile
offspring
● GENE POOL: all alleles at all
gene loci in all individuals in a
population
We all belong to
the same gene
pool!!!
A population of flamingo’s
6 different species of flamingo
The Hardy-Weinberg Theorem:
● a tool that describes a
gene pool of a nonevolving population
● states that allele
frequencies and
genotypes in a
population’s gene pool
remain constant over
the generations unless
acted upon by agents
other than sexual
recombination
● for Hardy-Weinberg equilibrium to occur, the
following conditions must be met:
1) Large population
2) No mutation
3) No gene flow (no immigration or emigration)
4) Random mating (no mating preference for
particular phenotype)
5) No natural selection (all genotypes have an =
chance of surviving & reproducing)
HOWEVER, in nature:
1) most populations are small & may mate with
one another
2) there are always mutations
(chance with every DNA replication)
3) gene flow often occurs between
populations
4) mating is non-random
5) natural selection is always occurring
**Therefore, in nature there will always be
changes in populations (“microevolution”)
**So why study population genetics?
Why use the H-W Theorem?
1) shows how genetics is related to evolution;
2) provides a benchmark genetic equilibrium against
which change can be noted;
3) permits an estimation of gene frequencies;
especially useful in estimating the number of
carriers of lethal alleles in human populations.
Ex: Brachydactyly - fingers are abnormally short in
heterozygotes; condition is fatal during infancy to
homozygous recessive individuals due to major skeletal
defects
Hardy-Weinberg Equation:
● p = frequency of dominant allele (A)
● q = frequency of recessive allele (a)
●p+q=1
● frequency of possible diploid
combinations (AA, Aa, aa):
p2
(AA)
+
2pq
(Aa)
+
q2
(aa)
=
1
Example Problem:
● If the frequency of a recessive allele is
35% in a population of 1500 people,
how many people would you predict
would be carriers of this allele, but
would not express the recessive
phenotype?
Solution:
q = 35% = 0.35
p = 1 - q = 1 - 0.35 = 0.65
p2
+
2pq
+
freq. of Aa genotype =
=
=
# of carriers =
=
q2
=
1
2pq
2(0.65)(0.35)
0.455 = 45.5%
(0.455)(1500)
683 people
Example Problem:
● In a population with 2 alleles for a particular
locus, B and b, the allele frequency of B is
0.78. If the population consists of 172
individuals, how many individuals are
heterozygous? How many will show the
recessive phenotype?
Solution:
p = 0.78
q = 1 - p = 1 - 0.78 = 0.22
p2
+
2pq
+
q2
=
1
freq. of Bb genotype = 2pq
=
2(0.78)(0.22)
=
0.343 = 34.3%
# of heterozygotes =
=
(0.343)(172)
59 individuals
Solution:
p2
+
2pq
+
q2
=
1
freq. of recessive phenotype =
freq. of bb = q2
=
(0.22)2
=
0.0484 = 4.84%
# of recessive ind. =
=
(0.0484)(172)
8.3 individuals
(8 ind.)
DEFINITIONS
● Microevolution = studies
how pop’s of organisms
change from generation to
generation; changes in
allele frequencies in a
population’s gene pool
● Macroevolution = studies
changes in groups of related
species over long periods of
geologic time; determines
evolutionary relationships
among species
Causes of Microevolution:
1) Natural Selection
2) Genetic Drift (changes in the gene pool
of a small population due to chance)
Examples:
-Bottleneck Effect: results from drastic
decrease in population size
-Founder Effect: few individuals in a
population colonize a new habitat
Bottleneck Effect
3) Gene Flow (migration of fertile
individuals between populations)
4) Mutation (introduces new alleles into a
population)
5) Nonrandom Mating (individuals
choose mates based upon their traits)
Ways Natural Selection
Acts on a Population:
1) Stabilizing Selection: eliminates
individuals with extreme or unusual traits;
existing population frequencies of common
traits are maintained
*Example of Stabilizing
Selection in humans:
*human babies most commonly weigh 3-4
kg; babies much smaller or larger have
higher infant mortality rates.
2) Directional Selection:
favors traits at one
extreme of a range of
traits; common during
periods of environmental
change
Examples:
-insecticide resistance
-peppered moth
Peppered Moth example:
● 100 years after the first dark
moth was discovered in 1848,
90% of moths were dark;
● the light variety continued to
dominate in unpolluted
areas outside of London.
3) Diversifying (a.k.a.
Disruptive) Selection:
occurs when
environment favors
extreme or unusual
traits while selecting
against common traits
4) Sexual Selection: differential mating of
males in a population; leads to sexual
dimorphisms
-females tend to increase their fitness by
increasing the quality of their offspring by
choosing superior male mates (and are therefore
“choosier” or more selective when finding a mate)
Sexual Selection (cont.)
-males increase their fitness by maximizing
the quantity of offspring produced
**as a result, in vertebrate species,
the male is typically the
“showier” sex
-colorful plumage
-lion’s mane
-antlers
Sexual Selection:
● INTRASEXUAL SELECTION = direct
competition among individuals of one
sex (males use antlers, aggressive
behavior, etc.)
● INTERSEXUAL SELECTION =
“mate choice”; individuals of one sex
are choosy (usually the females)