The number of individuals in the population is the population's size. If a population is small enough you will be able to determine the size by counting the individuals.
Emigration.
The group of individuals used to represent a population is called the sample. It should have the same statistics as the population, though be of a smaller size.
Over population.
Moving out of a population is called emigration. (Moving into a population is called immigration.)
dispersion
genetic variation
emigration
The individuals of a species that live in a particular area are called a population.
Individuals can leave a population through either death or emigration. Death results in a permanent loss of individuals from the population, while emigration involves individuals moving out of the population to join another population or habitat.
population
sample.