Age Structure and Sex Ratio . 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.
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. Female number plays a dominant role in a population since they are the ones to produce the offspring. Males may mate with several females, diminishing them to a subordinate reproductive role.