Download Changes in Gene Frequencies

Hardy-Weinberg
• The Hardy-Weinberg theorem (p2+2pq+q2 = 1) describes gene frequencies
in a stable population that are well adapted to the environment. It
assumes the following:
–
–
–
–
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Extremely large populations
No gene flow between populations
Random mating
No natural selection
No mutation
• As you can see the chance for a these conditions leading to a stable
population is entirely improbable. Any departure in these conditions leads
to evolution.
Why it
doesn’t
work
Application of Hardy Weinberg
• An investigator has determined by inspection that
16% of a human population has a recessive trait
(tt). Complete all the genotype and allele
frequencies for this population, assuming that it
is in Hardy-Weinberg equilibrium.
• p2+2pq+q2 = 1
•
•
•
•
&
p+q=1
Since the trait is expressed in the homozygous form (tt) .16 = q2
q: t2 = 0.16, √t2 = √16 = 0.4 so p + q = 1, p + 0.4 = 1, p = 0.6
p2(TT)= .62 = 0.36
2pq (Tt) = 2(0.6)(0.4) = 0.48
• Genetic Drift - deviations from expected
(Hardy-Weinberg) frequencies
– bottle-neck effect
• caused by a disaster suddenly altering the
environmental pressures causing the genes of a few
survivors to dominate
– founder effect
• when a few individuals move and start a new
population allowing their genes to dominate
– gene flow
• changes to a gene pool as individuals move out and
into a breeding population
• tends to reduce differences between populations
Agents of
Evolution
Stabilization of a gene pool
• Directional selection
– selection toward a phenotype that is best adapted to the environment
– most common type of selection
• Disruptive selection
– occurs when environmental factors favor individuals on both extremes
of the phenotypic range
– often to facilitate different food sources
Stabilization of a gene pool
• Stabilizing selection
– selects against the extremes of
phenotypes
– humans
• Balancing Selection
– occurs when nature allows 2 distinctly
different polymorphisms to exist in
relatively equal frequencies (balanced
polymorphism)
– leads to heterozygous advantage and
frequency dependent selection
• heterozygous advantage - heterozygous
individuals exhibit less genetic disorders
• frequency dependent selection frequency of the dominant phenotype
becomes less fit over time (prey species)