Directional selection is a type of natural selection in which individuals that display extreme phenotypes are favored over individuals with intermediate or opposite phenotypes. This leads to a shift in the population's average phenotype in one direction. Directional selection is also known as progressive selection.

Explanation:
- Directional selection: Directional selection occurs when individuals with a specific extreme phenotype have a higher fitness and reproductive success compared to individuals with a different phenotype. As a result, the frequency of the extreme phenotype increases in the population over time, leading to a shift in the average phenotype towards that extreme.
- Extreme phenotypes: In directional selection, extreme phenotypes are those that deviate significantly from the average or intermediate phenotype. These extreme phenotypes may possess certain advantageous traits or adaptations that make them better suited to their environment and increase their chances of survival and reproduction.
- Fitness and reproductive success: Fitness refers to an individual's ability to survive and reproduce, passing on their genes to the next generation. In directional selection, individuals with the extreme phenotype have a higher fitness and reproductive success compared to individuals with the average or opposite phenotype. This can be due to various factors such as increased competitiveness, better adaptation to environmental conditions, or improved ability to acquire resources.
- Shift in average phenotype: As individuals with the extreme phenotype have higher fitness, their frequency in the population increases over generations. This leads to a shift in the average phenotype of the population towards the extreme phenotype favored by natural selection. The direction of the shift depends on the specific selective pressure and the nature of the advantageous traits associated with the extreme phenotype.
- Progressive selection: The term "progressive selection" is another name for directional selection. It reflects the fact that directional selection leads to progress or change in the population's average phenotype over time, as it moves towards the extreme phenotype favored by natural selection.

In summary, directional selection, also known as progressive selection, is a type of natural selection where individuals with extreme phenotypes have higher fitness and reproductive success, leading to a shift in the population's average phenotype towards the extreme phenotype favored by selection.