Announcements

1
Announcements

• Lab 2 handout ?print from course web page
• Questions to do before lab.
• Readings for Lab. 2
• HWE (pp. 142-
  154)
• Selection (pp. 154
  177)
• Migration (pp. 197
  204)
• Genetic Drift (pp. 204
  217)
• http//www.mun.ca/biology/dinnes/B2900/B2900.html
• Text Readings on webpage

2
Evolution in the News

• Worlds smallest fish?
• http//www.nhm.ac.uk/about-us/news/2006/jan/news_7
  501.html
• Proc. Roy. Soc. B.
• http//www.journals.royalsoc.ac.uk/openurl.asp?gen
  rearticleiddoi10.1098/rspb.2005.3419

3
Biology 2900Principles of Evolution and
Systematics

• Topics
• - the fact of evolution
• - natural selection
• - population genetics
• - natural selection and adaptation
• - speciation, systematics and
• phylogeny
• - the history of life

4
Topic

• Population genetics (Ch. 5, 6)
• - gene frequencies within and between
  populations
• - changes in gene frequencies over time

5
Hardy-Weinberg
p2 2pq q2 AA Aa aa

• Relax Assumptions
• ? - Mutation
• ? - Migration
• ? - Non-random mating
• ? - Finite population size
• - Selection - differential survival,
• fecundity etc. among genotypes

6
Selection

• Selection occurs when
• some phenotypes have higher survival and/or
  reproduction than other phenotypes
• Selection -----gt Evolution
• when phenotypes heritable
• (change in allele frequencies)

7
Selection

• - Random drift-------gt stochastic
• - Selection------------gt deterministic
• Fitness differences
• differences in the potential to donate genes to
  future generations among phenotypes
• 
  (genotypes)
• Fitness values relative

8
Selection

• Differential fitness ? change in allele freq.
• q gt 0 q ? 1 fixation q 1.0
• q lt 0 q ? 0 loss q 0.0
• q 0 q equilibrium 1
  gt q gt 0

Outcomes
v
9
Selection

• Differential fitness
• differences among phenotypes (genotypes) in
  survival, fertility, fecundity, mating success,
  etc.
• 
  
• Example differential survival
• survival rate ( U )
• relative fitness (w)

10
Selection

• Differential survival
• 1. average survival rate (U) for each
  genotype
• 2. relative fitness w wmax
  1.0

U
Umax
11
Selection

• Genotype A1A1 A1A2
  A2A2
• Survival (U) 0.8 0.6
  0.2
• Fitness(w) w11 w12
  w22
• 1.00 gt 0.75
  gt 0.25

12
Selection

• Lab. 2 Population genetics simulation
• http//darwin.eeb.uconn.edu/simulations/simulation
  s.html
• Selection (fitness of
  phenotype)
• Favoured allele
• 1) Dominant w11 w12
  gt w22
• 2) Recessive w11
  w12 lt w22

13
Selection

• Lab. 2 Simulation
• Selection
• 3) overdominance w11 lt w12 gt
  w22
• 4) underdominance w11 gt w12 lt
  w22

14
SimulationExample of Selection

• Genotype A1A1 A1A2
  A2A2 Fitness(w) w11
  w12 w22
• 1.00 gt 0.75
  gt 0.25
• Box 5.3 Population Mean fitness
• w p2 w11 2pq w12 q2 w22

15
w111.0 w12 .75 w22 .25
Freq(A1) allele
Directional Selection
16
Initial p 0.40
A1A1 A1A2
A2A2
17
? p rate of change of allele freq.
Box 5.8
Maximum rate
18
w p2 w11 2pq w12 q2 w22
19
Strength of Selection
Fig. 5-12
20
Examples of Selection

• Single gene polymorphisms
• Colour Polymorphisms
• British School of Ecological Genetics
• (Snails, Butterflies)

21
Cepaea nemoralis
Snail
Butterflies
Peppered moth Biston betularia
22
Peppered Moth
Cryptic coloration
23
Mytilus edulis
Cepaea nemoralis
24
Examples of Selection

• Single gene polymorphisms
• 1966 Lewontin and Hubby
• Protein electrophoresis
• Many polymorphic enzyme loci
• Variation neutral or maintained by selection ?

25
Protein Electrophoresis
Pgm
Origin
26
Examples of Selection

• 1. Laboratory natural selection experiments

27
Directional selection
AdhF allele
Fig. 5.13
28
Examples of Selection

• 2. Geographic clines in allele frequency
• - gradient due to migration history
  (neutral) ?
• - selection due to environmental gradient ?

29
Geographic clines

• Migration history
• mixing of alleles
• (neutral)

30
Freq. B Human Blood Group Allele
31
Six enzyme loci
insecticide
none
32
Geographic clines

• Mosquito enzyme genes
• cline for AceR allele correlated with
• pesticide usage
• Selection ?
• Five control genes no cline
• What type of experiment would be useful ?

33
Selection for Pesticide Resistance

• Chemical Year Deployed Resistance observed
• DDT 1939 1948
• 2,4-D 1945 1954
• Dalapon 1953 1962
• Atrazine 1958 1968
• Picloram 1963
  1988
• Trifluralin 1963
  1988
• Triallate 1964
  1987
• Diclofop 1980
  1987

34
Selection for Antibiotic Resistance

• Antibiotic Year Deployed
  Resistance observed
• Penicillin 1943
  1946
• Streptomycin 1943
  1959
• Tetracycline 1948
  1953
• 
  

35
Genetic Variation

• Loss of genetic variation
• - random genetic drift
• - inbreeding
• - migration
• - directional selection
• How can genetic variation be maintained ?

36
Maintenance of Genetic Variation

• Balance of gain and loss of alleles
• - balance of forward and reverse mutation
• - selection - mutation balance
• - selection - migration balance
• - heterozygote advantage
• - frequency-dependent selection

37
Mutation Balance

• two-way (reversible)
• v equilibrium
  q 0
• A a
• u q
• p

u u v
V
v u v
V

38
Mutation Balance
Equil. Freq. (A)
v u v
V
V
(equilibrium) p

0.00001
u v
39
Selection - Mutation Balance

• Most mutations deleterious
• Selection acts to remove deleterious alleles
• New mutations created continuously
• Balance - rate mutations added
• - rate selection removes
• q equilibrium frequency of deleterious
• 
  allele

v
40
Selection - Mutation Balance

• A1 dominant, A2 recessive deleterious mutation
• w11 w12 1 w22 1 - s m
  mutation
• 
  rate
• q Ö

s selection coefficient
m
v
s
41
Selection - Mutation Balance
m

• q Ö
• s low and high then q high
• s high and low then q low
• if s 1 then q Ö m
• (lethal)

v
s
v
m
v
m
42
Selection Mutation Balance
? 1.0 x 10-6
v
m
v
q Ö
s
(selection)
lethal
43
Selection - Mutation Balance

• Human genetic diseases
• Cystic fibrosis (recessive allele c)
• f(cc) 1/2500 0.0004 q2 s
  1
• q
  .02
• q Ö

m
m 0.0004
v
s
44
Selection - Mutation Balance

• Mutation - selection balance ?

m 0.0004 unusually high Assumptions
incorrect ??? - selection scheme (Fitness
of CC lt Cc?) - not in equilibrium ( f(c)
allele decreasing?) - genetic drift
increased f(c) allele?
45
Migration - Selection Balance

• Migration - homogenizes allele freq.
• Example
• Lake Erie Island water snakes (Nerodia sipedon)
• Single locus (banded dominant to unbanded)

46
Lake Erie Islands Mainland (banded)
Island (unbanded)

• Selection by gull predators against banded on
  islands
• Why some banded on the islands ??

47
Migration - Selection Balance
Box 6.2

• Phenotype Island Fitness (w)
• Banded (AA, Aa) 0.84
• Unbanded (aa) 1.0
• 
  migration (m) 0.01
• a unbanded allele
• q 0.94 expected
  (balance)
• q 0.73 observed
• (other
  factors ??)

v
48
Migration - Selection Balance

• King and Lawson (1995)
• Estimates Methods
  
• Gene Flow enzyme genes
• Inheritance controlled
  crosses
• Natural selection mark and
  release
• Temporal changes museum
  collections

49
Maintenance of Genetic Variation

• Balance of gain and loss of alleles
• Ö - balance of forward and reverse mutation
• Ö - selection - mutation balance
• Ö - selection - migration balance
• - heterozygote advantage
• - frequency-dependent selection