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
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
 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
 Isolation of population
 No mutations can occur
 Random mating
 No selective pressure for or against traits
Processes That Lead To Microevolution
 Mutations-change in the
DNA
 Non-random matingorganisms pick their
mate, sexual selection
 Gene flow-genes move
with individuals when
they move out or into a
population
 Genetic Drift-natural
disaster causes a crash in
population size
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,
homozygousity, 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
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
• 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
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
• Sexual dimorphism
– Males larger, more colorful
than females
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
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?