generationstid

Generationstid, often translated as generation time, is a measure used in biology to describe the interval between the birth of parents and the birth of their offspring in a population. In demography and life-history research, it commonly refers to the average age of mothers at the birth of their offspring, weighted by fertility at each age; in stable populations this approximates the time between generations. In microbiology and ecology, generation time can also denote the doubling time of a population under exponential growth—the time required for the population to double.

Generation time varies widely across species and conditions. It is influenced by genetics, age at first reproduction, fecundity, survivorship, and environmental factors such as food availability and temperature. Short generation times enable faster evolutionary responses but can trade off with longevity or juvenile survival. Long generation times are typical of many large vertebrates, while many microbes and some insects reproduce rapidly.

Calculations: For age-structured populations, generation time G can be estimated as G = (sum over ages of x × l_x × m_x) / (sum over ages of l_x × m_x), where l_x is survivorship to age x and m_x is maternity at age x, and R0 = sum l_x × m_x. In microbes, generation time Td is the doubling time: N = N0 × 2^(t/Td). It can also be stated as Td = ln 2 / r, where r is the intrinsic rate of increase.

Applications and limitations: Generation time is central to demographic modeling, evolutionary biology, and population ecology. It is not fixed; it varies with environment, health, and life-history strategy, so estimates are context-dependent.