Evolution as Genetic Change in Populations
• Single Gene Trait – A trait controlled by only one
gene.
Ex. Snails may have a shell with dark bands, or a shell
without dark bands.
The gene for
has 2 alleles.
Single gene traits have just 2 or 3
.
• Polygenic Trait – A trait controlled by two or more
genes.
Ex. Height in humans
from very short to very tall
Each
of a polygenic trait has 2 or more alleles.
A polygenic trait has many genotypes, and even more
phenotypes.
Single Gene Trait
Polygenic Trait
Evolution as Genetic Change in Populations
Examples of Genetic Change in Populations
1. Pesticide Resistant Insects
-Farmers use pesticides to kill crop eating insects.
-Some insects possess a trait that makes them resistant to the pesticide, so
they are not killed.
-The resistant insects pass the gene for resistance to offspring.
-After a few generations the population is mostly insects that are resistant to
the pesticide. (insects have evolved)
-Pesticide used is no longer effective in killing these insects.
2. Antibiotic Resistant Bacteria
-Dr.s prescribe antibiotics to kill infections caused by bacteria. Common exs.
include strep throat or sinus infections.
-Some bacteria possess a trait making them resistant to the antibiotic and are
not killed.
-Bacteria that survive pass the gene for resistance to offspring.
-Repeated infections can be harder to treat with antibiotics if the infection is
caused by antibiotic resistant bacteria.
How Natural Selection Works
How does natural selection affect single gene traits?
Can lead to changes in allele and
Phenotype
.
Ex. Lizard Color
Normal lizard
is brown.
Mutations in one gene for body color can cause
lizards.
Red lizards are seen and eaten easily by
.
Black lizards absorb more
, move faster to avoid
predators, eat better, and leave more offspring.
Result: there is a change in allele and phenotype frequencies.
Later populations have increased frequency of alleles for black lizards.
How Natural Selection Works
How does natural selection affect polygenic traits?
1. Directional Selection – Occurs when individuals at one end
of the curve have higher
than individuals in the
middle or other end of the curve. Ex. Beak Size
How Natural Selection Works
How does natural selection affect polygenic traits?
2. Stabilizing Selection – Occurs when individuals near the
of the curve have higher fitness than individuals at
either end of the curve. Ex. Mass of human infants at birth.
How Natural Selection Works
How does natural selection affect polygenic traits?
2. Disruptive Selection – Occurs when individuals at the
outer ends of the curve have higher fitness than individuals
near the
of the curve. Ex. Beak Size
Genetic Drift
What is genetic drift?
•
change in allele frequency that occurs in
small populations.
• Individuals with a
allele may leave more
offspring than other individuals, just by chance. (not
due to natural selection)
• Over time, a series of
occurrences can
cause an allele to become more or less common in a
population.
Two Types of Genetic Drift
1. Genetic Bottlenecks
A change in allele frequency
following a dramatic
in the size of a population.
-Genetic diversity is sharply
.
-Caused by disaster, such as
killing many
individuals.
-The smaller population’s
gene pool may have different
allele frequencies than
population’s.
Two Types of Genetic Drift
2. The Founders Effect
Allele frequencies change as a result of the
of a small
subgroup of a population.
-A few individuals colonize a new habitat.
-Founding individuals may carry alleles that differ in frequency to the
main population, just by
. (not due to natural selection)
-Ex. Migration and evolution of fruit flies on Hawaiian islands.
Evolution Versus Genetic Equibrium
What conditions are required to maintain genetic
equilibrium?
• Genetic equilibrium occurs if a population is not evolving,
meaning the allele frequencies in its gene pool do not change.
• Hardy-Weinberg principle predicts that five conditions can
disturb genetic equilibrium and cause evolution to occur:
1. N
(artificial selection, sexual selection
causes evolution)
2. Small
size (genetic drift causes evolution)
3. Immigration or
(loss or gain of alleles causes
evolution)
4. M
(new alleles causes evolution)
5. Natural
(different fitness levels causes evolution)