Transcript Mendelian Genetics in Populations – 1

Selection against heterozygotes =
underdominance (Fig. 5.19a-c)
1
Selection against heterozygotes =
underdominance (Fig. 5.19d-e)
2
Change in chromosome
frequency in
populations that are a
mixture of normal 2nd
chromosomes - N(2) and compound 2nd
chromosomes - C(2)
Heterozygotes die.
Graph legend gives
proportion of viable
zygotes in matings
between C(2)
homozygotes, between
C(2) and N(2)
homozygotes, and
between N(2)
homozygotes.
Frequency of C(2)
Selection against heterozygotes =
underdominance (Fig. 5.19f)
3
Selection against heterozygotes =
underdominance – 2
• When C(2) is common (> 0.90), most matings are between
C(2) homozygotes, which produce 25% viable offspring,
and C(2) increases toward fixation
• When C(2) is less common (< 0.80), enough matings occur
between N(2) homozygotes, which produce 100% viable
offspring, that N(2) chromosomes can increase in
frequency and C(2) is driven out of the population
• Main point: the outcome of underdominant selection
depends upon the initial allele frequencies
• Other forms of chromosome rearrangements
(translocations, inversions) can also result in
underdominant selection
4
Frequency dependent selection: flower color
in Elderflower orchids
• Orchids have yellow or purple flowers
• Bumblebees visit flowers and act as pollinators, but receive
no nectar
• Less common color morph will receive more bee visits per
flower because bees tend to alternate between the two
color morphs
• Flowers that are visited more frequently donate more
pollen (= greater male reproductive success) and receive
more pollen (= greater female reproductive success)
• Therefore natural selection favors the rare color morph
5
Frequencydependent
selection:
Elderflower
orchids (Fig.
5-21)
Natural
selection (via
pollinators)
favors yellowflowered plants
when they are
rare but not
when they are
common
6
Adding mutation to the Hardy-Weinberg
analysis
• Mutation is the ultimate source of new
genetic variation
• However, by itself, mutation is a weak force
for changing allele frequencies
7
Mutation is a weak force of evolution (Fig. 5.23)
8
Mutation-selection balance – 1
• Mutation constantly introduces new (mostly less
fit) alleles into populations
• Selection tends to remove less fit alleles
• Could the observed frequency of low fitness
alleles in populations be the result of a balance
between mutation and selection?
• For a recessive allele with a selection coefficient
against homozygotes of s, the equilibrium
frequency, qeq = √(µ/s), where µ is the mutation
rate to the recessive allele
• Rearranging this equation: µ = s (qeq)2
9
Mutation-selection balance and spinal
muscular atrophy
• Autosomal recessive lethal
• Affects about 1 in 10,000 Caucasians (q2 = 0.0001,
therefore q = 0.01, which is the collective frequency of
disease causing alleles)
• If q = 0.01 and s = 0.9 (i.e., 90% mortality for
homozygotes), then µ = 0.9 x (0.01)2 = 0.9 x 10-4
• This estimate of the mutation rate to disease causing alleles
of the telomeric survival motor neuron gene (telSMN)
agrees well with direct estimates from sequencing alleles
in affected individuals and their parents (1.1 x 10-4)
• It appears that mutation-selection balance can explain the
observed frequency of spinal muscular atrophy
10