In biology, a population is defined as the number of organisms of the same species that live in the same geographic area at the same time, being also able to interbreed.

For this interbreeding to be possible, it is necessary that the individuals have the ability to mate with other members of the population and produce fertile offspring. However, sometimes within a population, there are certain genetic variations that will determine the chances of survival and reproduction.

There different scales at which populations can be measured. For instance, a local population can be restricted to a small area in terms of space (the fish in a pond), however, this locality can operate on a larger scale; such as regional, countrywide, island or continental.

It may, in some cases, even count the entire species. When individuals of local populations have the ability to disperse to other populations, it is known as a metapopulation.

The branch of biology that studies the characteristics and factors that play a role in their dynamics, such as size and distribution is called population biology. Some of the areas studied in this field include migratory patterns, demographics (birth and death rates, sex ratio and age distribution), population density, life-history traits, population genetics and the group dynamics in terms of how they interact within and between populations. The term population ecology refers to the study of how populations interact with the environment.

In reality, populations are not usually stable, since they are fluctuating in size as time goes by. These changes come by thanks to variations in biotic and abiotic factors that also act as limiting factors against indefinite exponential growth of populations.

For example, in environmental conditions and food resources are plentiful, the size of the population may grow. On the other hand, when predation is high, a population may become depleted.

Some characteristics of a population can have determined effects depending on certain factors. Density-dependent factors and its effects depend on the size of the population, so for instance, a denser population will be decimated faster by disease than a population whose individuals are more widely dispersed.

Life history traits also play a role in the growth rate and life span of a population, if for example, the maturation rate of individuals happens later in life, this population will grow more slowly than a population that matures at a younger age.

Another important aspect about population is that the genetic variations within them, the larger ones are normally more resilient to stochastic changes in their environment. This happens because the chance of a genetic mutation in the gene pool that will lead to a better adaptation is higher.

Sometimes when an extreme event takes place, where a large number of the population cannot survive, the ones that are left are considered to have a genetic advantage, so they will reproduce and therefore increase the frequency of this genetic advantage in their offspring. This is known as a population bottleneck.

While the survivors of this bottleneck are better suited to thrive in these new conditions, the long term success for the population isn’t always guaranteed. Since the genetic diversity for the group is low, this increases the vulnerability to further changes to the environment, as well as the consequences of inbreeding. If a bottleneck results in too few individuals, then the chances of extinction are high after a few generations.