Directional selection is a type of natural selection that favors individuals with an extreme phenotype, leading to a shift in the average phenotype of a population over time. It is also known as progressive selection. In this process, the frequency of individuals with a particular trait increases or decreases in a population due to the selective advantage or disadvantage associated with that trait. Let's delve into the details of why directional selection is referred to as progressive selection.

Directional Selection and Progressive Selection
Directional selection occurs when individuals with traits that deviate from the average phenotype have a higher fitness and survival advantage in a changing environment. This results in a shift in the distribution of phenotypes towards one extreme over successive generations. The key point to note here is that the average phenotype of the population changes over time in response to the selective pressure.

Progressive Selection
The term "progressive selection" is used to describe this phenomenon because the population is progressing or moving towards one extreme of the trait spectrum. For example, if a population of birds with varying beak sizes experiences a decrease in the availability of large seeds, individuals with smaller beaks may have a higher chance of survival and reproduction. Over time, the average beak size in the population will decrease, illustrating a progressive shift towards the smaller beak phenotype.

Other Types of Selection
It is important to note that there are other types of selection as well, such as stabilizing selection and disruptive selection. Stabilizing selection favors individuals with average or intermediate phenotypes, maintaining the status quo of a population. Disruptive selection, on the other hand, favors individuals with extreme phenotypes at both ends of the spectrum, leading to the diversification of a population into two or more distinct phenotypic groups.

Conclusion
In summary, directional selection is referred to as progressive selection because it leads to a progressive shift in the average phenotype of a population towards one extreme. This shift occurs due to the differential survival and reproductive success of individuals with extreme phenotypes, which are favored in a changing environment. Understanding these concepts is crucial in comprehending the mechanisms of evolution and how populations adapt to their surroundings.