natural selection acts on phenotype natural selection acts on phenotypic variations in populations
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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.
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