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Chapter 23
Hardy-Weinberg
Microevolution
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Change in a population's alleles over a
period of time.
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It is very difficult to detect changes that occur
on the microscopic level.
These changes must manifest themselves in
the organisms phenotype.
Since individuals do not evolve, one must
keep a close eye on the individuals population
to detect any change in genotypic
modification.
Biologists have a way to help them detect
such changes; called the Hardy Weinberg theorem.
1000 total alleles
(aa)
(AA)
(Aa)
Can these conditions really exist?
NO!!!
Other method
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Large ear allele (L) has a .8 frequency
Small ear (l) allele has a .2 frequency
The percent of each of the three
phenotypes in a given population.
p2 = .64
2pq = .32
q2 = .04 = 1
64%
32%
4% = 100%
If we know the percent of the homozygous
recessive organisms, we can take the
square root of the decimal value and
determine the frequency.
Determine the frequency of the dominant
allele by subtracting it from 1.
Sample Problem #1
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In Drosophila (fruit fly), the allele for
normal wing length is dominant over the
allele for short wings.
In a population of 1000 individuals, 360
show the recessive phenotype.
How many individuals would you expect
to be homozygous dominant for the trait?
Sample Problem #2
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The allele for a widow's peak (hairline) is
dominant over the allele for a straight
hairline.
In a population of 500 individuals, 25%
show the recessive phenotype.
How many individuals would you expect
to be homozygous dominant and
heterozygous for the trait?
Sample Problem #3
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The allele for a hitchhiker's thumb is
dominant over a straight thumb.
In a population of 1000 individuals, 510
show the dominant phenotype.
How many individuals would you expect
for each of the three possible genotypes
for this trait.
Microevolution
Genetic Variation
The substrate for natural
selection
Variations within populations
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Polymorphism-two or more forms of a
characteristic in a population that are
readily noticeable (red and white flowers)
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Gene diversity-Measuring the # of
heterozygous alleles in the entire genome
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Nucleotide diversity- comparing nucleotide
sequences from DNA samples
Variations between populations
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Geographic variation- results from
differences in genetic structure either
between populations that inhabit
different areas. Variation along a
geographic axis is called a cline.
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Average size of yarrow plants (Anchillea),
gradually decreases with increasing variation.
Fig. 23.8
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In contrast to clines, isolated populations
typically demonstrate discrete differences.
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For example, populations of
house mice were first introduced to the island of
Madiera in the 15th century,
but isolated populations
developed that were
separated by mountains.
Some isolated populations
have evolved differences
in karyotypes probably
through genetic drift.
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Fig. 23.9
What is the source of variation?
Mutation
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Mutation - changes in the nucleotide
sequence of DNA.
Mutations of individual genes are rare
and random.
 Mutations in somatic cells are lost
when the individual dies.
 Mutations in gametes can be passed to
offspring.
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Sexual recombination
Random segregation of homologous
chromosomes and random union of
gametes creates
 Sexual reproduction recombines old
alleles into fresh assortments every
generation.
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How is variation preserved?