Evolution of Populations
Chapter 16
Genetic Variation
• Heterozygotes make up between 4-8% in
mammals and 15% in insects.
• The gene pool is total of all the alleles in a
population. All the genes.
• Relative Frequency is the number of times
a particular allele appears in a gene pool.
• In genetic terms evolution is any change in
relative frequency in a gene pool.
Relative Frequencies of Alleles
Sample Population
allele for
brown fur
48%
heterozygous
black
36%
homozygous
brown
Frequency of Alleles
allele for
black fur
16%
homozygous
black
Brown is the recessive trait but
more common in frequency.
Sources of Variation
• The two main sources for genetic variation
are Mutation and Gene Shuffling.
• Mutations occur as DNA is changed by
mistakes in replication, radiation or
chemicals in the environment.
• Gene Shuffling occurs as meiosis creates
gametes. Crossing Over and Independent
Assortment creates new combinations.
Single Gene & Polygenic Traits
• Single-gene traits are controlled by one
gene with two phenotypes.
• The variation leads to only two
phenotypes.
• Polygenic traits are controlled by two or
more genes and have variable
phenotypes.
• The phenotypes usually fall in a bell curve.
Frequency of Phenotype
Generic Bell Curve for
Polygenic Trait
Phenotype (height)
Natural Selection on Single-gene
Traits
• Evolutionary fitness can be measured by
the ability to reproduce and pass genes.
• Natural selection works only on the
organism and its phenotype to survive.
• Natural selection on a single-gene trait can
change gene frequencies and cause
evolution. Fig 16-5
• Red, Brown, & Black Lizards
Natural Selection on Polygenic
Traits
• Natural Selection on Polygenic traits is more
complex.
• Directional Selection: occurs when members on
one end of the curve have greater fitness and
create a shift in that direction
• Stabilizing Selection: the middle of the curve is
most fit and the ends shrink.
• Disruptive Selection: the lower and upper ends
of the curve have greater fitness and the middle
declines.
Graph of Directional Selection
Key
Directional Selection
Food becomes scarce.
Low mortality,
high fitness
High mortality,
low fitness
Graph of Stabilizing Selection
Stabilizing Selection
Key
Low mortality,
high fitness
High mortality,
low fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
Graph of Disruptive Selection
Disruptive Selection
Low mortality,
high fitness
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Key
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
Genetic Drift
• In small populations random mating leads
to a change in gene frequency called
Genetic Drift.
• By chance in a small population some
organisms might leave more offspring thus
increasing their gene frequency.
• The founder effect occurs when a new
population is created from an existing one
and the founding gene frequency is
different than the original population due to
chance.
Genetic Drift
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Evolution vs. Genetic Equilibrium
• Hardy-Weinberg Principle states that allele
frequencies will remain in genetic equilibrium
and unchanged unless acted upon by
evolutionary forces.
• There are five conditions for Hardy-Weinberg to
work:
–
–
–
–
–
Random mating
Very large population
No movement in or out of a population
No mutations
No natural selection
Speciation
• Speciation is the process of a species
evolving into a new species.
• Separation of the gene pool is necessary
for speciation to occur.
– Behavioral Isolation: differences in mating
rituals lead to isolation
– Geographic Isolation: a population is divided
by a geographic boundary. River, mountains
– Temporal Isolation: different breeding times
separate a population.
Concept Map
Reproductive Isolation
results from
Isolating mechanisms
which include
Behavioral isolation
Geographic isolation
Temporal isolation
produced by
produced by
produced by
Behavioral differences
Physical separation
Different mating times
which result in
Independently
evolving populations
which result in
Formation of
new species
Testing Natural Selection
• Peter & Rosemary Grant tested Natural
Selection on the Galapagos Islands.
• Variation in the Finches followed the bell
curve and was genetic.
• During drought bigger beaked birds were
naturally selected, also a behavior for big
beaked birds mating with like birds led to
directional selection.
• The finches’ beaks became bigger in only
a few decades.
Speciation in Darwin’s
• The species of Galapagos finches evolved
by:
– Founding population from South America
– Separation by geographic boundaries
– Changes in gene pool by directional selection
– Reproductive Isolation keeps birds separate
– Ecological Competition may change the
original population
– Evolution continues on the islands
– Discuss Limits and Questions to finch
evolution