Many populations have overlapping generations where individuals of more than one generation coexist. This situation produces an age structure in most populations. Every age group has a characteristic birth and death rate. A standard rule of thumb sets a high mortality for the lower and upper age groups and a low mortality rate for the intermediate age group. Generation time refers to the average time span between the birth of an individual and the birth of their offspring.
Body size has a major effect on generation time.
Mice,which are small,can produce a litter of individuals ever 21 days. Their offspring usually are ready to reproduce in 3 to 4 weeks after birth. On the other hand, the Elephant takes 2 years to produce an offspring and several more years before the offspring can successfully reproduce. The sex ratio plays an important role in the development of the population. Female number plays a dominant role in a population since they are the ones to produce the offspring.