A single characteristic may increase a population in any of the following two ways:
(a) Natural selection: If a population considers an attribute helpful, it will grow naturally.
For example, mosquitoes that are resistant to a certain pesticide will pass on their genes, making their progeny resistant as well. The chemical kills the mosquitoes that it affects.
(b) Genetic drift: If a species experiences a catastrophic catastrophe that kills off the majority of the population, the remaining group may pass on its characteristics to succeeding generations. This can cause the population to have more people with the trait.