A group of organisms that changes over time is said to be evolving. These gradual evolutionary changes occur at the genetic level and are passed on as inheritable traits to succeeding generations.
Evolution can occur within a group due to a number of reasons, including migration of genes from another group, genetic drift within the group, natural selection of favorable traits and random mutation. The term "evolution" is applicable only to groups. When an individual organism changes over the course of its lifetime, it is said to be developing. Unlike in evolution, developmental changes do not occur at the genetic level.