Download Natural Selection Depends on Genetic Variation

Natural Selection
EU 1.A: Change in the genetic makeup of a population
over time is evolution
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Darwin’s Theory of Natural :
◦ There is variation within a population.
◦ Competition for limited resources results in
differential survival.
◦ Individuals with more favorable phenotypes
are more likely to survive and produce more
offspring, thus passing traits to subsequent
generations
Natural Selection is a Major
Mechanisms of Evolution
Evolutionary Fitness
Fitness = Reproductive Success
Genetic Variation is essential for the
survival of a species in a changing
environment
 Sources of Variation
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◦ Mutation
◦ Sexual Reproduction
 Segregation – Anaphase 1
 Independent Assortment – Metaphase 1
 Crossing Over – Prophase 1
 Random Fertilization
Natural Selection Depends on
Genetic Variation

Some phenotypic variations significantly
increase or decrease fitness of the
organism and the population
◦ Sickle Cell anemia
◦ DDT resistance in insects
◦ Antibiotic resistance in bacteria
Favored Phenotypic Variation

Environments can be more or less stable or
fluctuating, and this affects evolutionary rate and
direction; different genetic variations can be selected
each generation.
◦ Peppered Moth
 Bernard Kettlewell vindicated by Michael Majerus
 8 February 2012 as "Selective bird predation on the peppered
moth: the last experiment of Michael Majerus.“
◦ Global warming and flowering times in Thoreau's Concord: a
community perspective.
 “Using a subset of 43 common species, we determined that
plants are now flowering seven days earlier on average than
they did in Thoreau's times.” Miller-Rushing & Primack,
Ecology. 2008 Feb;89(2):332-41
http://www.ncbi.nlm.nih.gov/pubmed/18409423
Environmental Impact

Genetic variation that is favored by
selection & is manifested as a trait that
provides an advantage to an organism in
a particular environment
Adaptation

Evolution (especially in small populations)
can be influenced by:
◦ Genetic drift – chance events causing changes
in allele frequency
 Founder effect
 Bottleneck effect
◦ Mutations
◦ Human Impact
 Artificial selection
 Loss of genetic diversity
 Overuse of antibiotics
Other Influences

Conditions:
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Large population size
Absence of migration
No net mutations
Random mating
Absence of selection
Rarely are these conditions met…
Hardy-Weinberg Equilibrium

Mathematical approach to calculate allele
frequency, providing evidence for the
occurrence of evolution in a population
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p+q=1
p2 + 2pq + q2 = 1
p = frequency of dominant allele
q = frequency of recessive allele
p2 = frequency of homozygous dominant
2pq = frequency of heterozygous
q2 = frequency of homozygous recessive
Hardy-Weinberg Equilibrium

Example: A fruit fly population has a gene
with two alleles A1 and A2. Tests show that
70% of the gametes produced in the
population contain the A1 allele. If the
population is in Hardy-Weinberg Equilibrium,
what proportion of the flies carry both the A1
and the A1 alleles?
◦ What is given?
 p or q (this problem doesn’t tell you enough to know
which for sure)
◦ What are you finding?
 2pq (so it doesn’t matter if you are given p or q – the
answer is still the same – 0.42)
Hardy-Weinberg Equilibrium

A second mathematical approach to
calculate changes in allele frequency
provides evidence for evolution.
Graphical Analysis