Population Genetics. Hardy-Weinberg Equilibrium Determination a)A b)B c)both A and B d)neither A nor...
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Transcript of Population Genetics. Hardy-Weinberg Equilibrium Determination a)A b)B c)both A and B d)neither A nor...
Population Genetics
Hardy-Weinberg Equilibrium Determination
a) A
b) B
c) both A and B
d) neither A nor B
Which of these populations are in Hardy-Weinberg equilibrium?
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
Generation 1p (frequency of CR) = 0.7 q (frequency of CW) = 0.3
CRCR CRCR
CRCW
CWCW CRCR
CRCW
CRCR CRCW
CRCR CRCW
Genetic Drift
5plantsleaveoff-
spring
Generation 1p (frequency of CR) = 0.7 q (frequency of CW) = 0.3
CRCR CRCR
CRCW
CWCW CRCR
CRCW
CRCR CRCW
CRCR CRCW
CRCRCWCW
CRCW
CRCR CWCW
CRCW
CWCW CRCR
CRCW CRCW
Generation 2p = 0.5 q = 0.5
Genetic Drift
5plantsleaveoff-
spring
Generation 1p (frequency of CR) = 0.7 q (frequency of CW) = 0.3
CRCR CRCR
CRCW
CWCW CRCR
CRCW
CRCR CRCW
CRCR CRCW
CRCRCWCW
CRCW
CRCR CWCW
CRCW
CWCW CRCR
CRCW CRCW
Generation 2p = 0.5 q = 0.5
2plantsleaveoff-
spring
CRCR
CRCR CRCR
CRCRCRCR
CRCR CRCR
CRCR
CRCR CRCR
Generation 3p = 1.0 q = 0.0
Population Bottleneck
Originalpopulation
Originalpopulation
Bottleneckingevent
Originalpopulation
Bottleneckingevent
Survivingpopulation
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 Population size
Number of alleles per locus
Percentage of eggs hatched
93<50
5.23.7
5.8
5.8
99
96
1,000–25,000 <50
750,000
75,000–200,000
Nebraska, 1998 (no bottleneck)
(b)
Kansas, 1998 (no bottleneck)
Illinois 1930–1960s 1993
Pre-bottleneck(Illinois, 1820)
Post-bottleneck(Illinois, 1993)
Greater prairie chicken
Range of greater prairie chicken
(a)
Location Population size
Number of alleles per locus
Percentage of eggs hatched
93<50
5.23.7
5.8
5.8
99
96
1,000–25,000 <50
750,000
75,000–200,000
Nebraska, 1998 (no bottleneck)
(b)
Kansas, 1998 (no bottleneck)
Illinois 1930–1960s 1993
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
Population in which the surviving females eventually bred
Central
Eastern
Su
rviv
al r
ate
(%)
Females born in central population
Females born in eastern population
Parus major
60
50
40
30
20
10
0
Central population
NORTH SEA Eastern population
Vlieland, the Netherlands
2 km
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