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Evolution: it’s all
about mutations
that aid survival
and reproductive
selection
Natural selection…
is the process by which those
______________that make it more
likely for an ______________ to survive
and successfully ______________
become more common in a
______________ over successive
generations. It is a key mechanism of
______________.
Natural selection…
is the process by which those heritable
traits that make it more likely for an
organism to survive and successfully
reproduce become more common in a
population over successive generations.
It is a key mechanism of evolution.
Let’s start out with the gene pool
• The gene pool is the combined genetic info of
all the members of a population
• The gene pool can consider all of the traits of
the population, or just one of the traits.
Let’s try looking at our gene pool…
Some traits in the
gene pool are easier
to study than others:
• The # of phenotypes of
a given trait indicate
the # of genes
controlling that trait
– How many genes
control this trait?
Some traits in the
gene pool are easier
to study than others:
• The # of phenotypes of
a given trait indicate
the # of genes
controlling that trait
– How many genes
control this trait?
1, it is a single gene trait
Polygenic traits are represented by a
bell-shaped curve
How and why
does the gene
pool change?
The two main sources of genetic variation:
1. Mutations
2. Gene shuffling, that results from sexual
reproduction during meiosis
Natural Selection on polygenic traits can happen in
3 basic patterns.
Which areas on the map have the highest fitness?
The highest mortality/selection pressure?
• Which graphs of natural
selection best relate to
Darwin’s tortoises?
Finches?
Genetic drift:
Random change/event
affects allele frequency
in a population
Founder effect…
can result in genetic drift, when a sub-population
becomes isolated. What is an example?
More genetic drift
Evolution vs. genetic equilibrium
5 factors must exist to maintain genetic
equilibrium from generation to generation
1.
2.
3.
4.
5.
Random mating
The population must be very large
No movement into or out of the population
No mutations
No natural selection
The Hardy-Weinberg principle states that allele frequency in a
population will remain constant unless one or more of these
factors cause change
Hardy-Weinberg and genetic equilibrium
explained mathematically
Early in the twentieth century mathematician Godfrey Hardy and
physician Wilhelm Weinberg independently developed a model
describing the relationship between the frequency of the dominant and
recessive alleles (hereafter, p and q ) in a population.
p 2 + 2 pq + q 2
They reasoned that the combined frequencies of p and q must equal 1,
since together they represent all the alleles for that trait in the
population:
Hardy and Weinberg represented random
mating in the population as the product (p +
q)(p + q), which can be expanded to
p 2 + 2 pq + q 2 .
This corresponds to the biological fact that,
as a result of mating, some new individuals
have two p alleles, some one p and one q,
and some two q alleles.
P 2 then represents the fraction of the
population that is homozygous dominant
while 2 pq and q 2 represent the
heterozygous and homozygous recessive
fractions, respectively.
How do new species come to be?
Speciation
1. Founders arrive
Speciation
1. Founders arrive
2. Isolation of populations
a. Geographic
b. Behavioral
c. Temporal
Speciation
1. Founders arrive
2. Isolation of populations
a. Geographic
b. Behavioral
c. Temporal
3. Changes in gene pool
a. Shuffling
b. Mutations
Speciation
1. Founders arrive
2. Isolation of populations
a. Geographic
b. Behavioral
c. Temporal
3. Changes in gene pool
a. Shuffling
b. Mutations
4. Become reproductively
different
Speciation
1. Founders arrive
2. Isolation of populations
a. Geographic
b. Behavioral
c. Temporal
3. Changes in gene pool
a. Shuffling
b. Mutations
4. Become reproductively different
5. Continued evolution
• Speciation video (Crash Course in Biology)
• Natural selection in action (U. of Colorado)