In population genetics and population ecology, population size (usually denoted N) is a countable quantity representing the number of individual organisms in a population. Population size is directly associated with amount of genetic drift, and is the underlying cause of effects like population bottlenecks and the founder effect. Genetic drift is the major source of decrease of genetic diversity within populations which drives fixation and can potentially lead to speciation events. An alternate hypothesis posits that while genetic drift plays a larger role in small populations developing complexity, selection is the mechanism by which large populations develop complexity.
Population bottlenecks and founder effect
Population bottlenecks occur when population size reduces for a short period of time, decreasing the genetic diversity in the population.
The founder effect occurs when few individuals from a larger population establish a new population and also decreases the genetic diversity, and was originally outlined by Ernst Mayr. The founder effect is a unique case of genetic drift, as the smaller founding population has decreased genetic diversity that will move alleles within the population more rapidly towards fixation.
Modeling genetic drift
Genetic drift is typically modeled in lab environments using bacterial populations or digital simulation. In digital organisms, a generated population undergoes evolution based on varying parameters, including differential fitness, variation, and heredity set for individual organisms. Although small populations tend to face more challenges because of limited access to widespread beneficial mutation adaptation within these populations is less predictable and allows populations to be more plastic in their environmental responses.
LaBar and Adami use digital haploid organisms to assess differing strategies for accumulating genomic complexity. This study demonstrated that both drift and selection are effective in small and large populations, respectively, but that this success is dependent on several factors.
Contrary to the findings of previous studies, critical mutation rate has been noted to be dependent on population size in both haploid and diploid populations. When populations have fewer than 100 individuals, critical mutation rate can be exceeded, but will lead to loss of genetic material which results in further population decline and likelihood of extinction.
Effective population size (N<sub>e</sub>)
The effective population size (N<sub>e</sub>) is defined as "the number of breeding individuals in an idealized population that would show the same amount of dispersion of allele frequencies under random genetic drift or the same amount of inbreeding as the population under consideration." N<sub>e</sub> is usually less than N (the absolute population size) and this has important applications in conservation genetics.
Overpopulation may indicate any case in which the population of any species of animal may exceed the carrying capacity of its ecological niche.
See also
- Carrying capacity
- Holocene extinction event
- Lists of organisms by population
- Overpopulation
- Population growth rate