To be genetically viable, having a realistic chance of avoiding the problems of inbreeding

Inbreeding

Inbreeding is the reproduction from the mating of two genetically related parents. Inbreeding results in increased homozygosity, which can increase the chances of offspring being affected by recessive or deleterious traits. This generally leads to a decreased fitness of a population, which is...

, a population of plants or animal

Animal

Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

s requires a certain amount of genetic diversity

Genetic diversity

Genetic diversity, the level of biodiversity, refers to the total number of genetic characteristics in the genetic makeup of a species. It is distinguished from genetic variability, which describes the tendency of genetic characteristics to vary....

, and consequently a certain minimum number of members. See effective population size

Effective population size

In population genetics, the concept of effective population size Ne was introduced by the American geneticist Sewall Wright, who wrote two landmark papers on it...

. The minimum is normally somewhere in the region of a hundred unrelated individuals. Where a population has become extremely small in a population bottleneck

Population bottleneck

A population bottleneck is an evolutionary event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing....

, due for example to near-extinction

Extinction

In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...

 of the species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

, it may have lost its genetic viability, and if numbers recover it will be through inbreeding, possibly leaving an unhealthy population. But in fact it provided a more diverse population thus limiting the amount of genetic disorder or repetition.