Download Conclude population genetics - April 13

Population Genetics
Hardy-Weinberg Equilibrium Determination
Which of these populations are in HardyWeinberg equilibrium?
a)
b)
c)
d)
A
B
both A and B
neither A nor B
Question 6 – Chap. 23
• Researchers examining a particular gene in a fruit fly
population discovered that the gene can have either of
two slightly different sequences, designated A1 and
A2. Further tests showed that 70% of the gametes
produced in the population contained the A1
sequence. If the population is at Hardy-Weinberg
equilibrium, what proportion of flies carries both A1
and A2?
• A 0.7
B 0.49 C 0.21 D 0.42
E 0.09
Question from an earlier edition of Campbell
• At a locus with a dominant and recessive allele
in Hardy-Weinberg equilibrium, 16% of the
individuals are homozygous for the recessive
allele. What is the frequency of the dominant
allele in the population?
• A 0.84 B 0.36 C 0.6 D 0.4
E 0.48
Hardy-Weinberg Equilibrium
Hardy-Weinberg Equilibrium is based on:
1. A very large population where all genotypes
are equally viable
2. Random mating (panmixia)
3. No mutations
4. No gene flow (dispersal of individuals and
their genes)
5. No natural selection
Evolutionary Change
• Evolution is a generation to generation change
in a population’s frequencies of alleles –
change in proportions of alleles in the gene
pool is evolution at its smallest scale and is
often referred to as microevolution
• The two main causes of microevolution are
genetic drift and natural selection
Natural Selection
Genetic Drift
• Random changes in gene frequency in a
population – this can lead to losses in genetic
diversity – the population becomes more
homozygous
• this is most important in small populations
Genetic Drift
CRCR
CRCR
CRCW
CWCW
CRCR
CRCW
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
Genetic Drift
CRCR
CRCR
CRCW
CWCW
5
plants
leave
offspring
CRCR
CWCW
CRCW
CRCR
CWCW
CRCR
CRCW
CRCW
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CWCW
CRCW
CRCR
CRCW
Generation 2
p = 0.5
q = 0.5
Genetic Drift
CRCR
CRCR
CRCW
CWCW
5
plants
leave
offspring
CRCR
CWCW
CRCW
CRCR
CWCW
CRCR
CRCW
CRCW
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CWCW
CRCW
2
plants
leave
offspring
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
Generation 2
p = 0.5
q = 0.5
CRCR
CRCR
CRCR
CRCR
Generation 3
p = 1.0
q = 0.0
Population Bottleneck
Original
population
Original
population
Bottlenecking
event
Original
population
Bottlenecking
event
Surviving
population
Northern Elephant Seal
Northern Elephant Seal Population
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
Greater prairie chicken
Range
of greater
prairie
chicken
(a)
Location
Illinois
1930–1960s
1993
Population
size
Percentage
Number
of alleles of eggs
per locus hatched
1,000–25,000
<50
5.2
3.7
93
<50
Kansas, 1998
(no bottleneck)
750,000
5.8
99
Nebraska, 1998
(no bottleneck)
75,000–
200,000
5.8
96
(b)
Pre-bottleneck
(Illinois, 1820)
Greater prairie chicken
(a)
Range
of greater
prairie
chicken
Post-bottleneck
(Illinois, 1993)
Location
Illinois
1930–1960s
1993
Population
size
Number Percentage
of alleles of eggs
per locus hatched
1,000–25,000
<50
5.2
3.7
93
<50
Kansas, 1998
(no bottleneck)
750,000
5.8
99
Nebraska, 1998
(no bottleneck)
75,000–
200,000
5.8
96
(b)
Founder effect – founder population and
three possible new populations
Mal de Meleda – founder effect
Serial Founder Effect
• Serial founder effects have occurred when
populations migrate over long distances. Such long
distance migrations typically involve relatively rapid
movements followed by periods of settlement. The
populations in each migration carry only a subset of
the genetic diversity carried from previous
migrations. As a result, genetic differentiation tends
to increase with geographic distance.
Movement of mitochondrial genes
out of Africa
‘Wisteria vine’ model of human
genetic diversity
Gene Flow
• Gene flow is the movement of alleles in and
out of a population
• Gene flow occurs because gametes or fertile
individuals move from one population to
another and take their genes with them
Gene flow
Gene Flow in Conifers
60
Survival rate (%)
50
Population in which the
surviving females
eventually bred
Central
Eastern
Central
population
NORTH SEA
Eastern
population
Vlieland,
the Netherlands
40
2 km
30
20
10
0
Females born
in central
population
Females born
in eastern
population
Parus major
Non-Random Mating
• Hardy-Weinberg assumes random mating – if mating
is not random then the population may change in the
short term – the most common form of non-random
mating is in-breeding – the mating of closely related
individuals
• In fact inbreeding is very common – many mammals
probably mate with first or second cousins in the
wild; many plants self-pollinate – the ultimate form
of inbreeding
• Inbreeding tends to produce homozygous populations
Inbreeding and White Squirrels
Mutations
• Mutations are the ultimate source of new
genetic variations – a new mutation that is
transmitted in gametes immediately changes
the gene pool of a population by inserting a
new allele into the gene pool