Natural Selection in Polygenic Traits
Most traits are polygenic, meaning they are coded for by more than one gene.
Because many genes influence these traits, polygenic traits come in a range
of phenotypes. For example, height is a polygenic trait. As a result, people
come in a variety of sizes, rather than simply short and tall.
There are three categories of natural selection that act upon polygenic traits.
Directional selection occurs when phenotypes at one end of the spectrum
lead to greater survival and/or reproduction. Stabilizing selection occurs when
phenotypes in the middle range confer greater survival/reproduction, while phenotypes at both
extremes lead to decreased fitness. Disruptive selection is the least common of the three types
and occurs when phenotypes at both ends of the spectrum lead to greater survival and
reproduction, while phenotypes in the mid range are a disadvantage.
Assignment:
Your assignment is to work with your group to prepare a whiteboard presentation. In your
presentation you will invent realistic examples of how different types of natural selection might
work in the assigned situation. Be sure that each group member participates equally.
Scenarios:
1. Tongue length in anteaters is polygenic and therefore tongues come in a range of
lengths. Give (1) an example of how directional selection might be acting upon this
trait, and (2) an example of how stabilizing selection might be acting upon this trait.
2. Wing color in western moths ranges from white to dark gray. Give (1) an
example of how directional selection might be acting upon this trait,
and (2) an example of how disruptive selection might be acting upon this trait.
3. A species of evening primrose is pollinated by a variety of insects, and its tube-shaped
flowers come in a range of lengths (due to flower length being a polygenic trait). Give
(1) an example of how stabilizing selection might be acting upon the trait, and (2)
an example of how disruptive selection might be acting upon this trait.
4. A species of elephant has a gestational period (time it takes for offspring to develop in
the womb) of 24 months. Give (1) an example of how directional selection might be
acting upon this trait, and (2) an example of how stabilizing selection might be acting
upon this trait.
5. A species of wolf hunts a variety of prey at different times of day. Give (1) an
example of how directional selection might be acting upon this trait, and (2) an
example of how disruptive selection might be acting upon this trait.
6. Wing span in great blue herons in a polygenic trait and therefore wing span varies
among these birds. Give (1) an example of how directional selection might be acting
upon this trait, and (2) an example of how stabilizing selection might be acting upon
this trait.
7. A species of frog has a wide range of time in which it mates and lays its eggs. Give an
example of (1) how stabilizing selection might be acting upon this trait, and (2) how
disruptive selection might be acting upon this trait.
8. A species of Rubus (blackberry) has a wide range of thorn length and density. Give an
example of (1) how directional selection might be acting upon this trait, and (2) an example of
how stabilizing selection might be acting upon this trait.