SUMMARY OF LAST TIME

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SUMMARY OF LAST TIME

• MUTATIONS
• Extremely rare per gene per generation
• Vary in scale from 1 base to whole genome
• Vary from lethal to advantageous
• Ultimate source of genetic variation
• GENE FLOW
• Only isolated populations receive none
• Can be a powerful evolutionary force
• Can prevent adaptation

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(7) SELECTION GENETIC DRIFT
Altering allele frequencies
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NATURAL SELECTION - change in allele frequency
because some genotypes average more
descendents than others - causes
adaptation GENETIC DRIFT - change on allele
frequency as a result of chance -
only random process in evolution - only
powerful in tiny populations
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NATURAL SELECTION
BETTER ADAPTED VARIANTS LEAVE MORE
DESCENDENTS BUT WHAT IS THE SCOPE FOR
SELECTION? HOW VARIABLE ISLIFETIME
REPRODUCTIVE SUCCESS?
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SPARROWHAWK
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IAN NEWTON
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20 YEARS IN ESKDALE
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OVER 1,000 SPARROWHAWK CHICKS
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SPARROWHAWKS (Newton) chicks with chicks
of own 23 chicks rearing half of next
generation 5
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SPARROWHAWKS (Newton) chicks with chicks
of own 23 chicks rearing half of next
generation 5
23 bird 9 mammal species
13-34 and 3-9
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HUMANS 1734-1861

babies with babies of own 54
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PHENYLKETONURIA (PKU)
One of over 1200 genetic diseases in humans so
far discovered Mental problems Urine smells
mousey
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PHENYLKETONURIA (PKU)
Gene codes for a liver enzyme breaking down amino
acid Phenylalanine High levels of this amino
acid are toxic to nerve cells causing mental
retardation
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PHENYLKETONURIA (PKU)
Gene has two alleles, thus three
genotypes HH Enzyme works normal Hh Half of
enzyme copies work normal hh Enzyme broken
mental retardation unless do not eat
phenylalanine
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PHENYLKETONURIA IN 1930s
HH Hh hh
Relative
1 1 0.2
Fitness
1 1 1-s
Selection Coefficient
0 0 s 0.8
Allele h strongly selected against
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PHENYLKETONURIA TODAY IN UK
HH Hh hh
Relative
1 1 1
Fitness
1 1 1
Selection Coefficient
0 0 0
No selection allele h is increasing slowly by
mutation
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GENETIC DRIFT
RANDOM CHANGES IN ALLELE FREQUENCY
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GENETIC DRIFT
ASSUME NO SELECTION NO MUTATION
NO GENE FLOW ALLELE FREQUENCIES WOULD ONLYSTAY
ABSOLUTELY CONSTANT IN AN INFINITE POPULATION
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THUS, GENETIC DRIFT

• OCCURS IN ALL REAL POPULATIONS
• IS STRONGEST IN SMALL ONES

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BACK TO LION CUBS
AA AB BB
ASSUME NO SELECTION i.e., all genotypes have
same average fitness
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INFINITE POPULATION, initial p 0.5, s 0
Allele frequency of A
0 GENERATIONS
100
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LARGE POPULATION, initial p 0.5, s 0
Allele frequency of A
Different colours are three repeats of exactly
the same simulation
0 GENERATIONS
100
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Allele frequency of A
0 GENERATIONS
100
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Allele frequency of A
Allele fixed
Allele eliminated
0 GENERATIONS
100
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BOTTLENECK EFFECT
A SPECIAL CASE OF GENETIC DRIFT
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BOTTLENECK EFFECT
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(No Transcript)
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FOUNDER EFFECT
A SPECIAL CASE OF GENETIC DRIFT
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FOUNDER EFFECT
Yellow allele is best adapted
MAINLAND
ISLAND
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FOUNDER EFFECT
Allele frequency of yellow 0.999
MAINLAND
ISLAND
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FOUNDER EFFECT
Allele frequency of yellow 0.50
MAINLAND
ISLAND
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A GENERAL POINT ABOUTALL KINDS OF GENETIC DRIFT
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IMAGINE A NEWADVANTAGEOUSMUTANT
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RARE ALLELES ARE AT RISK
!?!