Natural Selection Acts On Phenotype
Natural selection acts on phenotypic
variations in populations.
Environments change and act as
selective mechanism on populations.
The environment does not directly cause
changes in DNA, but acts upon phenotypes
that occur through random changes in DNA
Example: Flowering time in relation to
global climate change
Crop production is sensitive to climate
change; temperature has a large impact on
the rate of plant development.
Warmer temperatures will mean reduced
crop yields.
“Earlier crop flowering
and maturity have been
observed and documented
in recent decades, and
these are often associated
with warmer (spring)
temperatures.”
―Climate change and the flowering time of annual crops by P.Q. Craufurd and T.R. Wheeler, Journal of Experimental Botany, 2009.
Example: Peppered moth
The light phenotype was favored before
the Industrial Revolution. The color
blended with the tree bark.
After the Industrial Revolution, mostly dark
colored moths were seen. They had an
advantage on dark tree bark.
Phenotypic variations are not directed
by the environment but occur through
random changes in the DNA and
through new gene combinations.
effects) or result in a new phenotype.
Some phenotypic variations significantly
increase or decrease fitness of the
organism and the population.
Example: Sickle cell anemia and
Heterozygote Advantage
Sickle cell gene allele frequency
Sickle cell disease is caused by a single
base substitution mutation. It is
maintained in the population in a state
of balanced polymorphism because of
the protective effect against severe
forms of malaria conferred by the
heterozygous states.
Example: The Peppered moth
Example: DDT resistance in insects
Humans impact variation in other species.
Example: Artificial selection
Example: Loss of genetic diversity
within a crop species
Example: Overuse of antibiotics leading to
increase in antibiotic resistant bacteria.
Biggest problem: overuse and improper use of
antibiotics, especially in livestock.
Directional Selection is most common
when an environment changes. One
phenotype favored over another.
Example: the peppered moth
Stabilizing selection maintains the status
quo by favoring the mean phenotype.
Example: human birth weight
Distruptive selection occurs when the
extreme phenotypes are favored. May
lead to speciation.
Example: Wood Frog and Leopard Frog
Wood Frog
Breeds in early April
Leopard Frog
Breeds in mid-April
geographic variation – difference in
variation between population subgroups in
different areas
A cline is a graded change in a trait along a
geographic axis.