This is one of the fundamental definitions for comprehending evolution.
Fig. A group of squirrelfish (Holocentrus ascensionis
) that are shoaling, not schooling: though swimming as a group, their speed and direction are not synchronised. Source: 
A population is a group of individuals of one species that live in the same geographical area at the same time.
- ↑ http://en.wikipedia.org/wiki/Fish