What are the different ways in which individuals with a particular trait may increase in a population?
Individuals in a population may have different traits than others. These traits develop over a long time and are present evenly. If some environmental factors come into play and favour the particular trait; it will result in an increase of that particular trait. If the environment selectively eliminates other contrasting traits, it will also result in the same. For example, There are red and green bottles. Both live in a green plant. Birds can easily distinguish the red beetle from the green background. As a result, the population of red beetles will decrease with time while that of the green beetles will increase. Here we say a natural selection is going on which favours the green beetle.
In a small population, some accidental mishap may result in the survival of the particular trait. This particular trait later passes their trait into their progeny and increase their amount in population. We call this change in gene frequency as genetic drifting. For example, there is a tree full of red and blue beetles; an elephant comes and uproots the plant. Uprooting result in the death of most of the red beetles. Now there are more blue beetles in the population. These beetle will now reproduce and increase the amount of the particular trait.