Transcript Population Genetics

Population Genetics
 Population-all the members of a
single species that occupy a particular
region
 Population genetics-studies the
genetic diversity of a population
 Single nucleotide polymorphisms
(SNPs)-variation in DNA sequence at a
single nucleotide, important in
human diversity
 Haplotypes: haplotype is a set of
single-nucleotide polymorphisms (SNPs)
on a single chromatid that are
statistically associated. It is thought that
these associations, and the
identification of a few alleles of a
haplotype block, can unambiguously
identify all other polymorphic sites in its
region. Such information is very
valuable for investigating the genetics
behind common diseases, and has been
investigated in the human species by
the International HapMap Project
Microevolution and Population Genetics
• Evolutionary changes within a population
• Gene pool- all the various alleles at all the gene loci in a
population
• Can study the allelic frequencies of particular loci look
at the % who are heterozygous, homozygous
• Peppered Moths
Microevolution and Population
Genetics
• After 1 generation, the
allelic frequencies are still
the same in equilibrium
• Sexual reproduction alone
cannot bring about a
change in genotype and
allele frequencies
• What other factors must
influence change in
genotype?
Hardy Weinberg Equations
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P+Q=1
P2 + 2PQ + Q2 = 1 (100%)
P2=frequency of homozygous dominant
P=frequency of dominant allele
2PQ= frequency of heterozygous dominant
Q2=frequency of homozygous recessive
Q=frequency of recessive allele
Frequency is a
number between
0 and 1
 16% of a population has a recessive disease. Calculate the
allelic frequencies
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check your work
 Q2=.16, Q=0.4 (take square root)
Q2 = 16%
 P + (0.4)=1, P = 1- 0.4 = 0.6
+ P2 = 36%
 P2=(0.6)2=0.36 or 36%
+2PQ= 48%
 2PQ= 2(0.4)(0.6)=0.48 or 48%
1 = 100%
Hardy-Weinberg Equations
 Equilibrium of gene pool frequencies will remain in effect if
there are no pressures on the population
 Determines allelic frequencies of genes
 If frequencies don’t change over time, evolution is not
occurring population in equilibrium
 Conditions for HW equations to work
 Large gene pool (no genetic drift)
 Isolation of population (no gene flow)
 No mutations can occur
 Random mating
 No selective pressure for or against traits (no natural selection)
Processes That Lead To Microevolution
 Mutations-change in the
DNA, low rate, not
“directed”
 Non-random matingorganisms pick their mate,
sexual selection, assortative
mating
 Gene flow-genes move with
individuals when they move
out or into a population;
reduces genetic differences
between populations
 Genetic Drift-natural
disaster causes a crash in
population size, allele
frequency changes due to
chance events
Calculate the changes in allele frequencies
Processes That Lead To Microevolution
• Gene flow-genes move with
individuals when they move
out or into a population
• Mutations-change in the DNA
• Non-random matingorganisms pick their mate
Processes That Lead To
Microevolution
 Genetic drift-random fluctuations
in allelic frequencies due to chance
occurrences, natural disasters
 2 types
 Bottleneck effect-stressful
situation greatly reduces size of
population
 Founder effect-a few individuals
leave original colony to establish a
new one
 Both can result in inbreeding,
homozygosity, loss of variability
3 Types of Natural Selective
Types of Natural Selection
• Stabilizing
Selection-favors
most common
(intermediate)
phenotype
Human birth
weight average of
7 lbs
Seven-foot-six Yao Ming and his
wife, six-foot-two Ye Li, had a
baby girl …She weighed seven
pounds, six ounces.
Types of Natural Selection
• Directional Selection-shift in allelic frequency in a consistent
direction in response to environmental pressures: peppered
moths, pesticide/antibiotic resistance, guppy color
Types of Natural Selection
Forest
• Disruptive Selectionfavors the extreme
phenotypes; eliminates
the intermediate. Finch
beak size large and
small beaks because
only have large, small
seeds, predation favors
2 types of snail shells
Open
Sexual Selection
• Adaptive changes in males
and females that lead to an
increased ability to secure a
mate
• Female choice
– Good gene hypothesis
– Runaway (sexy son)
hypothesis, the term runaway
refers to an exaggeration of
the trait until checked by
survival cost
• Sexual dimorphism
– Males larger, more colorful
than females
Raggiana Bird-of-Paradise
Sexual Selection:
Male Competition
 Cost-benefit analysis
benefit of mating worth the
cost of competition among
males
 Dominance hierarchies 
higher ranking individuals
have greater access to
resources vs lower ranking
individuals, cost/benefit of
dominance
 Territoriality  types of
defense behaviors needed
to defend a territory
Natural Selection Favors Diversity
 Environments change, it
would not be beneficial
to contain all the alleles
that allow an organism to
fully adapt to 1 particular
environment
 Maintenance of variation
among a population has
survival and
consequently
reproductive advantages
Subspecies of rat snakes represent separate populations
Heterozygous Advantage
 Heterozygote is favored over the 2
homozygotes
 Sickle Cell Anemia, Cystic Fibrosis
 Sickle cell mutation in hemoglobin
protein is maintained at a high
frequency in populations where
malaria is prevalent
 Recall: 1 copy offers resistance to
malaria, but 2 copies results in
sickle cell anemia
 SS-normal, not resistant
 Ss-normal, resistant
 ss-sickle cell, resistant
 What happens in the US where
malaria is not prevalent?