A population has certain attributes that an individual organism does not. An individual may have births and deaths, but a population has birth rates and death rates.

• In a population these rates refer to per capita births and deaths, respectively. The rates, hence, expressed as change in numbers (increase or decrease) with respect to members of the population.
• Another attribute characteristic of a population is sex ratio. An individual is either a male or a female but a population has a sex ratio (e.g., 60 per cent of the population are females and 40 per cent males).
• A population at any given time is composed of individuals of different ages. If the age distribution (per cent individuals of a given age or age group) is plotted for the population, the resulting structure is called an age pyramid. For human population, the age pyramids generally show age distribution of males and females in a combined diagram.

The shape of the pyramids reflects the growth status of the population

(a) whether it is growing, (b) stable or (c) declining.