The document explains selection pressure, which is an interaction between natural variation in a species and environmental factors that drive evolution. It outlines four types of selection pressure: artificial selection, directional selection, stabilizing selection, and disruptive selection, each affecting allele frequencies and trait distributions in different ways. Additionally, it discusses the concept of selection differential, referring to the difference between population averages for traits and those selected for reproduction.

1. Selection pressure and its
types
Submitted By: Abdul Basit
Submitted To: Miss Sadia

2. Selection Pressure
Selection pressure regarded as a force that causes a particular
organism to evolve in a certain direction. It is not a physical
force but an interaction between natural variation in a specie
and factors in its environment that cause a certain form to have
an advantage over from the others.

3. Types of selection pressure
1. Artificial Selection
2. Directional Selection
3. Stabilizing Selection
4. Disruptive Selection

4. Artificial Selection
This is the method thorough which most of modern domestic breeds of
animals and crop plants were created. In artificial selection human
choose specific traits to either retain or eliminate from the gene pool of
breeding population, because this process is closely monitored,
changes in population can be seen easily.

5. Directional Selection
The directional theory states that an extreme phenotype is favored
over other phenotypes and causes allele frequency to shift over time in
favor of extreme phenotype. In other words, If a particular trait is
favorable, it will be expressed at the most beneficial frequency in the
population

6. Stabilizing Selection
Stabilizing selection can be thought of as “middle-of-the-road”
selection, meaning a non-extreme trait is favored instead of one of the
two extreme traits. An example of this is plant height. In a population of
plants, those that are short may not get enough sunlight, but those that
are tall may be subjected to wind damage. This results in an increase in
the number of medium-height plants and a decrease in very tall and
very short plants. Because most traits do not change drastically over
time, stabilizing selection is considered to be the most common
mechanism for natural selection.

7. Disruptive Selection
This type of natural selection is bimodal and favors both extreme traits
in a population. For example, in a population of plants, there are some
pollinators that visit the tallest plants, a different species of pollinator
visits medium-height plants and a third species of pollinator that prefers
the shortest plants. If the pollinator that prefers medium-height plants is
removed, natural selection would select against medium-height plants
and the overall plant population would move toward having only tall and
short plants, the two extreme phenotypes.

8. Selection differential
The difference between the average value of a quantitative character in
a whole population and the average of those selected to reproduce the
next generation