A population is a summation of all the organisms of the same group or species, which live in a particular geographical area, and have the capability of interbreeding.

In ecology, the population of a certain species in a certain area is estimated using the Lincoln Index. The area that is used to define a sexual population is defined as the area where inter-breeding is potentially possible between any pair within the area. The probability of interbreeding is greater than the probability of cross-breeding with individuals from other areas. Under normal conditions, breeding is substantially more common within the area than across the border.