Natural Selection Acts On Phenotype

Natural selection acts on phenotypicvariations 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.

Changes in genetic information may be silent (with no observable phenotypic 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.

Wood FrogBreeds in early April

Leopard FrogBreeds in mid-April

Example: Wood Frog and Leopard Frog

geographic variation – difference in variation between population subgroups in different

areas

A cline is a graded change in a trait along a geographic axis.