Effective Population Size

Effective population size concept was introduced in population genetics by Sewall Wright, an American geneticist. Sewall Wright defined the concept as a number of the breeding individuals that would show similar amount of allele frequencies dispersion under a random genetic drift or similar number of the inbreeding as population being considered in idealized population set up.

Generally, effective population size can be defined as the total number of the individuals in the idealized population being considered that have value of a given genetic quantity that equals the value of the quantity on the population. Both genetic quantities in the population according to Wright were a single generation increase in the variance across the replicate populations and one generation change in inbreeding coefficient.

Variance increase across the replica populations refers to variance that is effective to the population size whole inbreeding coefficient refers to inbreeding that is effective to population size. The two are linked closely and they are derived from the F-statistics although not identical.

Today, effective population size concept is used more often and it is estimated empirically in respect to coalescence time or sojourn which is estimated as genetic diversity within the species divided by the rate of mutation. Selection effective population size is another essential effective population size indicated as 1/S _critical, where S _critical refers to crucial value of selection coefficient where selection becomes important that the genetic drift. Regardless of how it is defined, effective population size is always lees than census population size.

Due to the fact that most species have large populations, it deems fair to question the importance of genetic drift. Although it is true that most species have large population, they do not necessarily act large. This implies that genetic drift rate is not actually proportional to the census population size N _c. Instead, it is proportional to a more abstract something, specifically effective population size. Ideal population with sexually reproducing organisms or individuals, N_e equals N _c. In addition, ideal population has other characteristics and deviations will mostly decrease effective population size.

Among the characteristics of an ideal population include the equality in the number of females and males that are able reproductive. Individuals that are equally capable of producing offspring and offspring number produced by each are not more than the expected as per chance. In addition, mating is random in an ideal population and breeding individuals have a constant number across the generations.

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Sources

http://en.wikipedia.org/wiki/Effective_population_size

http://www.nature.com/scitable/topicpage/genetic-drift-and-effective-population-size-772523

http://darwin.eeb.uconn.edu/eeb348/lecturenotes/drift/node7.html