Final answer:
Directional selection favors an extreme phenotype leading to a shift in population traits, while disruptive selection favors multiple extreme phenotypes resulting in increased population diversity.
Step-by-step explanation:
When discussing the mechanisms of natural selection and the resulting shifts in a population's genetic makeup, two types of natural selection are often highlighted: directional selection and disruptive selection (also known as diversifying selection). While both of these processes result in the selection for certain phenotypes over others, they differ significantly in their impact on the population.
Directional Selection
Directional selection occurs when the environment changes in such a way that one extreme phenotype becomes advantageous over others. As a result, the phenotypic distribution shifts towards that trait. An illustrative example of this is the Galápagos finches, whose beak sizes have evolved to adapt to the availability of different food sources over time. In this type of selection, the variance towards one end increases, creating a directional shift in the population's traits.
Disruptive Selection
Disruptive selection, on the other hand, selects for two or more extreme phenotypes at the expense of the intermediate ones. This is observable in species where different strategies for survival or reproduction are favored, as seen with some animal populations having multiple forms of males. Disruptive selection results in increased genetic variance as it leads to a more diverse population with multiple adapted phenotypes.
In summary, while directional selection results in a shift towards a specific extreme, disruptive selection leads to an increased variety as multiple extremes are favored. This understanding of natural selection helps to explain how populations adapt to their environments and how different forms of natural selection can lead to varying evolutionary outcomes.