Generation time

In population biology and demography, the generation time is the average time between two consecutive generations in the lineages of a population. In human populations, the generation time typically ranges from 22 to 32 years.Historians sometimes use this to date events, by converting generations into years to obtain rough estimates of time.

The existing definitions of the generation time fall into two categories: those that treat the generation time as a renewal time of the population, and those that focus on the distance between individuals of one generation and the next. Below are the three most commonly used definitions:

The net reproductive rate R₀ is the number of offspring or molecules of life an individual is expected to produce during its lifetime (a net reproductive rate of 1 means that the population is at its demographic equilibrium). This definition envisions the generation time as a renewal time of the population. It justifies the very simple definition used in microbiology ("the time it takes for the population to double", or doubling time) since one can consider that during the exponential phase of bacterial growth mortality is very low and as a result a bacterium is expected to be replaced by two bacteria in the next generation (the mother cell and the daughter cell). If the population dynamic is exponential with a growth rate r (i.e. n(t) ~ α.e^rt, where n(t) is the size of the population at time t), then this measure of the generation time is given by:

Indeed, ${\displaystyle \textstyle T}$ is such that n(t + T) = R₀n(t), i.e. e^rT = R₀.

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