The individuals with a particular trait will increase in number if the trait provides:

More Food: The trait helps in obtaining more food that leads to increased growth and reproduction.

Useful Variations: The trait helps the individuals to adapt to environment and achieve greater success in struggle for existence.

Genetic Drift: It causes genetic fixation of a trait which, therefore, occurs, in whole of the progeny.

Differential Reproduction: The trait gives extra benefit to the individuals in survival and reproduction.