Certain insect species have become more and more resistant to insecticides. What type of natural selection is this, and how does it differ from the other types?

This is just an example of simple positive selection for those insects that have greater endogenous resistance. As time goes on, insects would grow more resistant to the insecticides from the small amounts that survive the insecticide in the first place. These insects spread their genes through breeding which can pass the gene along. Soon, more and more of the insects have the insecticide resistant gene.

The type of natural selection you are referring to is called "directional selection." This is a form of natural selection where individuals of a certain population that possess a specific trait that is advantageous in a changing environment are more likely to survive and reproduce, leading to an increase in the frequency of that trait in subsequent generations.

In the case of insects becoming resistant to insecticides, the process of directional selection occurs as follows:

1. Variation: Within a population of insects, there is naturally occurring genetic variation. Some individuals may possess genes that make them less susceptible to the effects of insecticides.

2. Environmental pressure: The introduction and usage of insecticides create an environmental pressure on the insect population. The insecticides kill or reduce the reproductive success of the majority of the insect population that is susceptible to them.

3. Survival and reproduction: The individuals with genetic variations that confer resistance to the insecticide have a survival advantage. They are more likely to survive the exposure to insecticides, reproduce, and pass on their resistant traits to their offspring.

4. Increasing resistance: Over time, as generations of insects with resistant traits reproduce, the frequency of those resistant genes within the population increases. This population-wide increase in resistance is the result of directional selection.

It is important to note that directional selection differs from other types of natural selection, namely stabilizing selection and disruptive selection.

Stabilizing selection occurs when individuals with intermediate traits have a higher fitness than those with extreme traits. This leads to a reduction in genetic variation over time, as the population becomes more homogeneous.

Disruptive selection occurs when individuals with extreme traits have higher fitness than those with intermediate traits. This can lead to the population becoming divided into distinct groups, each favored by different selective pressures.

In the case of insect resistance to insecticides, directional selection is at play because the insecticide exerts selective pressure in favor of individuals with resistance traits, leading to a directional shift in the population over time.