# Adaptive Evolution https://openstax.org/books/biology-2e/pages/19-3-adaptive-evolution --- # Types of Natural Selection ## Polygenic Traits Most traits are **polygenic**, meaning that multiple genes influence the expression of the trait. These polygenic traits often result in a bell curve distribution (variations of the trait as opposed to it being present/not present). - **Ex:** Height is the most common example of a polygenic trait. ![[Pasted image 20220606095311.png]] We can study a change in the **distribution** of phenotypes. > [!tip]+ Remember! > Phenotypes don't always need to be externally visible. > > The inner structure of organisms can have many different variations as well. > > **Ex:** Eyesight, Lactose-intolerance, etc. --- ## Types of Selection/Adaptation ![[Types of Natural Selection.png]] ### Stabilizing Selection - **Narrows** the peak of the bell curve. - Occurs when an **intermediate** phenotype is favored - **Ex:** The amount of eggs a bird lays ### Directional Selection - **Shifting** the peak of the bell curve - Occurs when an extreme phenotype is favored - **Ex:** Darwin's Finches and their beak sizes ### Disruptive Selection > *Sometimes referred to as "unstable forces or diversifying selection".* - **Splits** the distribution, creating two new peaks (*It can sometimes simply lower the height of the peak)*. - **Ex:** The height of giraffe necks. Those with short necks can reach food on the ground, long necks can reach food in tall trees, while those in-between often struggle.