Creatures that have sexual reproduction are often only females. In addition to birth and death, species that reproduce less frequently from generation to generation also include emigration and emigration. Sometimes it refers to the ratio of the number of individuals in the same development stage between one generation and the next generation, which is called population trend index. The net reproductive rate (net reproductive rate and net reproductive rate) commonly used in demographic statistics should be defined as the average number of females produced by each female in a certain generation, or the number of females in the next generation produced by each female of childbearing age. The expression is as follows:

Where lx is the female survival rate at t age (at birth, assuming that lx of x=0 is 1.0). Mx is the average litter size of X-year-old females (only females are counted). If R0= 1, the number of individuals between generations will not increase or decrease, and R0 >；; 1 Increase the number of individuals. When R0< is 1, the number of individuals decreases. On the contrary, the sum of mx is the average female post-algebra of a woman without considering death, which is called the crude reproduction rate. Inherent natural reproduction rate refers to the highest instantaneous reproduction rate per unit time when the population is not limited by space or food and the age composition of the population is relatively stable. R can be obtained by the following formula:

R represents the inherent proliferation ability of a species under given environmental conditions, so it is appropriate to compare the species from the average value measured in unit time. In population genetics, it can be used to measure the fitness of a population (Darwinian fitness). λ=ex, called the finite natural growth rate, represents the multiple of the increase in the number of individuals per unit time. In addition, when λt=2, the time required to double the number of individuals is called doubling time (time doubling).