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Chapter 19: Population genetics
Fig. 19-1
Population genetics
Population: interbreeding members of a species
Three major principles of Darwinian evolutionary theory:
• variation for traits exists within populations
• selection applies to a subset of those traits
(selection can act only upon variations)
• traits are genetically transmitted
Polymorphism: multiple forms of a gene are
commonly found in a population
(all studied populations are “wildly” polymorphic)
• chromosomal polymorphisms
• immunological polymorphisms
• protein polymorphisms
• nucleic acid sequence/structure polymorphisms
p = (2 x MM) + (1 x MN) = frequency of M allele
q = (2 x NN) + (1 x MN) = frequency of N allele
Therefore, frequency of MN reflects the genetic variation in a population
Fig. 19-2
“Factoids” regarding protein polymorphisms
in most species:
• structural polymorphisms are displayed by
about one-third of all proteins
• typically, about 10% of the individuals in
a large population are heterozygous for
polymorphisms of an average gene
Therefore, enormous protein-level variation
exists in most populations
Electrophoretic allelic variants of esterase-5 in Drosophila
Fig. 19-2
Electrophoretic variants of hemoglobin A in humans
Fig. 19-3
Enormous naturally-occuring variation (polymorphism)
in protein sequence
Enormous naturally-occuring variation (polymorphism)
in chromosomal rearrangements
Restriction sites within Drosophila xdh gene
(58 wild chromosomes sampled)
4-base sites in 4.5 kb DNA
* Site present in minority
of chromosomes
0/1 Site in ½ of chromosomes
Fig. 19-5
Enormous naturally-occuring variation (polymorphism)
in nucleotide sequence
Enormous naturally-occuring variation (polymorphism)
in tandem repeat arrays (VNTRs)
Hardy-Weinberg Equilibrium
Random mating within a large population assures a stable
equilibrium of genetic diversity in subsequent generations
provided that certain assumptions apply:
Mating is random
(no biased mating, infinite population size)
Allele frequencies do not change
(no selection, no migration, etc.)
Hardy-Weinberg Equilibrium
For a two-allele system, all genotypes exist as a simple
product of the frequency of each allele:
homozygotes = p2 or q2
heterozygotes = 2pq
p2 + 2pq + q2 = 1
Box 19-2
Box 19-2
Allele frequencies determine frequencies
of homozygotes and heterozygotes
Fig. 19-6
Rare alleles are almost always found in heterozygotes,
almost never homozygous
Another measure of heterozygosity is
haplotype diversity
Haplotype: combination of non-allelic alleles on a single chromosome
MN allele and genotype frequencies
reflect Hardy-Weinberg assumptions
Allele and genotype frequencies can vary between populations,
while exhibiting H-W equilibria within each population
Non-random mating: inbreeding (mating among relatives)
Positive inbreeding:
Mating among relatives
is more common than
random
Increases frequencies
of homozygotes in a
population
Fig. 19-7
Extreme inbreeding: self-fertilization results in loss
of heterozygosity
No change in p or q; change only in heterozygosity and diversity
Fig. 19-8
Negative inbreeding (enforced outbreeding)
-barriers to inbreeding are common attributes
of successful populations
Positive assortative mating
- individuals chose “like” mates
(not necessarily relatives)
Negative assortative mating
- individuals choose dissimilar mates
Sources of variation
• Mutation – very slow
•
Mutation is the ultimate source of variation
But spontaneous mutations occur at extremely low frequencies
Mutation frequency is influenced by allele frequency
Mutation alone is a very slow evolutionary force and
cannot directly account for diversity observed in populations.
Box 19-3
Sources of variation
• Mutation – very slow
• Recombination – rapidly mixes genes to
provide new genetic combinations in
a population
• Migration – gene flow among different
populations changes gene frequencies
Selection: directed change in genotypes
in a population
Fitness: survival and reproduction success;
function of genotype and environment
Fitness can be obvious (mortality, sterililty)
• HbS/HbS: severe anemia, low survival
• HbS/HbA: apparent resistance to malaria
or more subtle/partial/conditional
Fitness (viability) of
various homozygotes
as a function
of temperature
Drosophila pseudoobscura
Fig. 19-9
Enhanced fitness of a genotype will enrich those genes
in subsequent generations of that population
Frequencies of positively selected genes increase over time
Frequencies of negatively selected genes decrease over time
Change in A frequency (p) is greatest where p = q
Fig. 19-11
For a two-allele system, mean fitness (W)
in a population is the proportional contribution
of fitness by each genotype (A/A, A/a, a/a)
W = p2WA/A + 2pqWA/a + q2Wa/a
WA/A and WA/a > Wa/a
p should increase
q should decrease
Wa/a > WA/A and WA/a
q should increase
p should decrease
Fitness can account for allele frequency changes over time
Fig. 19-12
p for malic dehydrogenase electrophoretic mobility
variant MDHF where WS/S=1, WS/F=0.75, WF/F=0.4
Selection: directed change in genotypes
in a population
Fitness: survival and reproduction success;
function of genotype and environment
Frequency independent selection: fitness is
independent of genotype frequency
Frequency dependent selection: fitness
changes as genotype frequency changes
Random genetic drift: random changes in gene
frequency that can lead to extinction/fixation
of genes
•
Requires no selection
•
Essentially “sampling error” inherent in
each generation in achieving HardyWeinberg equilibrium
•
Most exaggerated in small populations
(especially “founder effects”)
•
Allows isolated populations to diverge
without differential selection (each
experiences its own drift history)
Model: history of emergence of ten mutations
and their drift in a population over time
Fig. 19-13
Drift to extinction for nine; drift to p=1 in one
Drift explains differences in unselected allele
frequencies in isolated populations
Genetic change is directed by diverse evolutionary forces
which tend to increase (blue) or decrease (red) variation
Recommended problems in Chapter 19: 3, 5, 12, 17
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