Answer: D. A gradual change of the traits of an organism over time.
Step-by-step explanation:
A natural selection is a phenomena that was proposed by Charles Darwin. In this he proposed the fact that of differential survival and reproduction of living beings who have better phenotypic traits over the others. These traits are passed from the parents to their offsprings. These traits are beneficial for the survival in adverse environments. The development of new traits with respect to the environment occurs gradually with time.