Natural Selection on 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 grater
survival/reproduction, while phenotypes at both extremes lead to decreases 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 middle range
are a disadvantage.
Directions:
Your assignment is to invent realistic examples of how different types of natural selection might
work in the following scenarios. Write all of your examples on a separate paper
Scenarios:
1. Tongue length in anteaters is polygenic and therefore tongues come in a range of
lengths. Give an example of how directional selection might be acting upon this trait.
Give another example of how stabilizing selection might be acting upon this same trait.
2. Wing color in western moths range from white to dark gray. Give and example of how
directional selection might be acting upon this trait. Give another example of how
disruptive selection might be acting upon this same 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. Give an example of how stabilizing selection might
be active upon this trait. Give another example of how disruptive selection might be
acting upon this trait.
4. A species wolf hunts a variety of prey at different times of day. Give and example of how
directional selection might be acting upon this trait. Give another example of how
disruptive selection might be acting upon the same trait.
5. Wing span in great blue herons is a polygenic trait and therefore wing span varies
among these birds. Give an example of how directional selection might be acting upon
this trait. Give another example of how stabilizing selection might be working upon the
same trait.