Download Population Genetics

Population Genetics
Chapter 23
Population
• Group of
organisms that
can interbreed to
produce fertile
offspring
• Gene pool – total
alleles in a
population
Hardy-Weinberg Equilibrium
• Evolution does NOT
occur if the gene
pool remains
constant from one
generation to the
next.
• Outside forces must
act on a population
for there to be
change
Conditions
NO Evolution = genetic
equilibrium
• No Mutations
• Extremely Large population
size – no genetic drift
• No gene flow – no migration
• Random Mating
• No Natural selection
•
•
•
•
•
Evolution = no genetic
equilibrium
Mutations
Small populations – genetic
drift
Gene flow – migration
Non Random mating
Natural Selection
Equation - Hardy-Weinberg
Equilibrium
•
•
•
•
•
•
•
p2 + 2pq + q2 = 1
p+q=1
p = dominant allele
q = recessive allele
p2 = homozygous dominant
2pq = heterozygous
q2 = homozygous recessive
Example Problem #1
• In a population the frequency of the recessive
allele is 0.4.
– What is the frequency of the dominant allele?
– What frequency of the population will be
homozygous dominant, heterozygous, and
homozygous recessive?
– What frequency of the population will
demonstrate the dominant phenotype?
Example problem #2
• In a population 25% of the individuals
demonstrate the recessive phenotype.
– What is the frequency of the recessive allele?
– What is the frequency of the dominant allele?
– What frequency will be homozygous dominant in
the population?
– What frequency will demonstrate the dominant
phenotype?
Violations to H-W Equilibrium –
cause evolution
•
•
•
•
•
Mutations
Small populations – genetic drift
Non random mating
Natural Selection
Migration – gene flow
1. Mutations
• Change in DNA’s nucleotide sequence.
• Raw source for new genes and alleles
• Most mutations are somatic cell mutations and do
not affect offspring
• Only gametic mutations affect a gene pool.
• Mutation rates
– Lower in organisms with a longer generation span
• Plants and animals – 1/100000 genes
– Higher in organisms with a shorter generation span
• Bacteria and viruses
1. Mutations
• Point Mutations – alter
one nucleotide base only
– Usually neutral
– Sickle cell anemia
• Chromosomal Mutations –
alter many regions or loci
of the entire chromosome
– Gene duplication
• Usually harmful, but when
beneficial act as an important
source of variation in a
population
2. Nonrandom Mating – sexual
selection
• Creates sexual recombination – joining of
different alleles in a gene pool
– Huge source of variation in a population
• Crossing over
• Independent assortment
• Random fusion
• Gametes from different organisms contribute
different alleles to the next generation.
3. Natural Selection
• Differential success in reproduction based on
variation in a population.
• Better suited organisms in an environment tend
to produce more offspring than less suited
organisms.
– Fitness
• Types
– Directional
– Disruptive
– Directional
– Sexual
– Artificial
4. Genetic Drift
• Random fluctuation of allele frequencies from
one generation to the next.
• Greater occurrence in smaller populations
CWCW
CRCR
CRCR
Only 5 of
10 plants
leave
offspring
CRCW
CWCW
CRCR
CRCR
CRCW
CWCW
CRCR
CRCW
CRCW
CRCR
CWCW
CRCW
CRCR
CRCR
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
Only 2 of
10 plants
leave
offspring
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
CRCW
Generation 2
p = 0.5
q = 0.5
Figure 23.7
CRCR
CRCR
Generation 3
p = 1.0
q = 0.0
4. Genetic Drift
• Bottleneck effect
– Sudden environmental change can drastically
reduce the size of a population – only some
survive
• Fire, flood etc.
– Reduces the genetic variation in a population
(a) Shaking just a few marbles through the
narrow neck of a bottle is analogous to a
drastic reduction in the size of a population
after some environmental disaster. By chance,
blue marbles are over-represented in the new
population and gold marbles are absent.
Figure 23.8 A
4. Genetic Drift
• Founder effect
– Occurs when a few individuals become isolated
from the original population
– The smaller population may not have the same
gene pool as the original
5. Migration – Gene Flow
• Addition or loss of alleles to or from a gene
pool.
• Caused by the movement of fertile individuals
to or from a population - migration
Genetic Variation
• Differences in phenotypes between members of a
population
• Inherited in genotype
• The raw source for natural selection within a
population
Figure 23.1
Sources of Genetic Variation
• Mutations
• Sexual Reproduction
– Crossing over
– Independent
assortment
– Random fusion of
gametes
• Diploidy – recessive
allele does not show
Sources of Genetic Variation
• Outbreeding – mating with
unrelated partners
• Balanced polymorphism
– Heterozygote advantage –
sickle cell carriers
– Hybrid vigor – plant species