Effective Population Size ($N_e$) vs Total Population Size ($N$)

The total population size, represented by $N$, is the count of all individuals within a population. The effective population size ($N_e$) refers to the size of an idealized population that would experience the same rate of genetic drift or increase in inbreeding as the real population. It essentially reflects the number of individuals that successfully contribute genes to the next generation.

Understanding the $N_e$ and $N$ Relationship

In virtually all naturally occurring eukaryotic populations, the effective population size ($N_e$) is smaller than the total population size ($N$). This discrepancy arises because real populations deviate from the idealized model in several ways that reduce genetic contribution:

• Unequal Sex Ratio: If the number of breeding males and females is unequal, the rarer sex limits the effective size.
• Variation in Offspring Number: Differences in reproductive success among individuals (some have many offspring, others few or none) reduce $N_e$.
• Population Size Fluctuations: If the population size varies over generations, $N_e$ is more strongly influenced by the smallest population sizes experienced (bottlenecks).
• Overlapping Generations: When individuals from multiple generations breed simultaneously, it affects the genetic contribution dynamics.

Consequently, the relationship is consistently:

$ N_e < N $

This inequality highlights that only a fraction of the total population typically contributes effectively to the gene pool of the next generation, influencing evolutionary processes like genetic drift and the maintenance of genetic diversity.