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Transcript
Population Genetics and Evolution
Forces of Evolution
DETERMINISTIC: direction of change predictable
Mutation
Migration
Natural Selection
STOCHASTIC: direction of change unknowable (none exp.)
Genetic Drift
Forces of Evolution
Mutation
strength related to rate & genomic position of mutation
Forces of Evolution
Mutation
strength related to rate & genomic position of mutation
Forces of Evolution
Migration
strength related to directionality, rate of migration; genetic variability
Forces of Evolution
Migration
strength related to directionality, rate of migration; genetic variability
Forces of Evolution
Natural Selection
strength fitness differences; genetic variability
Forces of Evolution
Natural Selection
strength fitness differences; genetic variability
Forces of Evolution
Genetic Drift
strength related to population size (i.e., ‘sampling error’)
Forces of Evolution
Genetic Drift
strength related to population size (i.e., ‘sampling error’)
Forces of Evolution
Genetic Drift
Sampling error: more trials per sample… closer to expectations
Forces of Evolution
Genetic Drift
&'$
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5062-72)819:).1&'%
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Sampling error: more trials per sample… closer to expectations
!
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#
$
()*+,-./(01,2304
%
Hardy-Weinberg Principle
Idealization of (less-than-ideal) populations…
…allow for comparisons…
Assumptions
Random Mating
No Mutation
No Migration
No Natural Selection
No Genetic Drift (exceptionally large population size)
Hardy-Weinberg Equilibrium
Assuming no evolution of the populations…
Hardy-Weinberg Equilibrium
Assuming no evolution of the populations…
… only need to keep track of allele frequencies
heterozygote
homozygotes
allele
Hardy-Weinberg Equilibrium
Assuming no evolution of the populations…
… only need to keep track of allele frequencies
Alleles: come in singles
Genotypes: come in doublets
Hardy-Weinberg Equilibrium
In meiosis, the alleles (B and b) segregate…
… and randomly unite by fertilization.
Hardy-Weinberg Equilibrium
Hardy-Weinberg Equilibrium
To find the genotypic, allelic, or phenotypic proportions in the next
…use the Hardy-Weinberg formula!
generation…
p2 + 2pq + q2 = 1
p = frequency of B
q = frequency of b
random union of gametes
Violations of Hardy-Weinberg Equilibrium
ANY violation of HWE indicates evolution
Industrial Melanism: natural selection in peppered moths
Camouflaged organisms more apt to survive, reproduce
Genetic variability existed (and exists) in some populations
Habitat in some forests of UK modified by Industrial Revolution
hypotheses
Darker moths ought to be more common in heavily sooted forests
Lighter moths ought to be more common in more pristine forests
Industrial Melanism
Kettlewell’s hypotheses (1950s)
Darker moths more common in heavily sooted forests
Lighter moths more common in more pristine forests
Industrial Melanism
Industrial Melanism
Kettlewell’s experiments: a synopsis
Natural Selection the principal violator of HWE
Other forces of evolution (e.g., Drift, Migration) involved
Industrial Melanism
Kettlewell’s experiments: a synopsis
Natural Selection the principal violator of HWE
Other forces of evolution (e.g., Drift, Migration) involved
Industrial Melanism
No change in allele (or phenotypic) frequencies…
… does NOT imply that evolutionary forces are absent!!!
Chi-square test of HWE
OBSERVE:
EXPECT:
24 BB
0.25*100 =
25 BB
52 Bb
2(0.25)*100 =
50 Bb
24 bb
0.25*100 =
25 bb
Find expected proportions:
p2 + 2pq + q2 = 1
p = 0.5; q = 0.5
Chi-square test of HWE
FIND DIFFERENCES:
24 BB - 25 BB = -1… squared = 1
52 Bb - 50 Bb =
2… squared = 4
24 bb - 25 bb =
1… squared = 1
TALLY DIFFERENCES, scaled by expected values:
(1/25) + (4/50) + (1/25) = 0.6
Is the difference ‘real’?
p = 0.74 (attribute difference to chance)
Chi-square test of HWE
OBSERVE:
EXPECT:
49 BB
0.25*100 =
25 BB
1 Bb
2(0.25)*100 =
50 Bb
49 bb
0.25*100 =
25 bb
Chi-square test of HWE
FIND DIFFERENCES:
49 BB - 25 BB = 24… squared = 576
1 Bb - 50 Bb = 49… squared = 2401
49 bb - 25 bb = -24… squared = 576
TALLY DIFFERENCES, scaled by expected values:
(biggish) + (big) + (biggish) = sizable (35.53)
Is the difference ‘real’?
p ~ 0.00 (Something wonky is going on!)
Game plan…
Two beads (alleles) is a single individual (genotype)!
Sample with replacement! (put back individual)
Do 4 (rather than 10) generations per simulation
Exercises
I.
Do all; Chi-square test (Table 3, pp. 208); ASSUMES HWE (ask for help!)
IIA. Choose either Bottleneck model (pp. 211) OR Founder effect model (pp. 214)
IIB. Use 2 populations of 25 individuals each
IIC. Choose either Industrial Melanism (pp. 218) OR Sickle-cell Anemia (pp. 219)
PREPARE to DISCUSS RESULTS with CLASS