Selection

Selection is differential fitness (survival and reproduction) over generations. It can take two forms: natural selection (differential survival) and sexual selection (differential reproduction). It is the process by which certain traits enable an organism to survive and reproduce in a given environment, thus increasing the frequency of those traits (and genes that code for them) in future generations. In order for selection to operate, it needs three things: 1) variation in the trait, 2) differential fitness, and 3) heredity (traits passed from parent to offspring). 

While selection operates on the individual’s phenotype, evolutionary change influences the population over time. Selection can act in multiple ways - the most common is ‘directional selection,’ whereby the occurrence of a trait increases or decreases in a population over time.