Q.List any three important characteristics of a population and explain.
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Start your 14-day free trial to unlock the full solution →A population is not just a collection of individuals — it has specific measurable properties that define its structure, growth, and potential for change over time.
When we study a population in biology or demography, we are not simply counting heads. A population is a group of individuals of the same species living in a particular area at a given time. But what makes it a meaningful unit for study is that it possesses population-level properties — characteristics that emerge only when you look at the group as a whole, not at any single individual. These properties help us understand how populations grow, shrink, interact, and respond to their environment.
The NCERT textbook for Class 12 Biology (Chapter 11: Organisms and Populations) highlights several such properties. Among them, three stand out as fundamental: population size (or density), birth rate (natality), and death rate (mortality). Let us examine each in turn.
1. Population Size or Density
This is the most basic property of any population. Population size simply means the total number of individuals in a given area at a given time. However, because counting every individual is often impossible (think of fish in a lake or bacteria in a petri dish), we usually measure population density — the number of individuals per unit area or volume.
Why is this important? Population size determines the amount of resources a population needs, its vulnerability to extinction, and its potential to influence other species. A small population, for example, is more prone to genetic drift and inbreeding depression. A very large population may exhaust its food supply or spread disease more rapidly. The NCERT emphasises that population density is a dynamic property — it changes over time due to births, deaths, immigration, and emigration.
Population density is not always measured by counting individuals. For some organisms, we use indirect measures — like the number of nests, the area covered by a plant species, or even the number of faecal pellets per unit area. These are called population indices.
2. Birth Rate (Natality)
Birth rate refers to the number of live births per unit time per individual (or per unit population). In ecology, it is often expressed as the number of births per 1,000 individuals per year, but for simpler calculations, it can be given as births per individual per unit time.
Birth rate is a measure of a population's reproductive output. It tells us how quickly a population can add new individuals. A high birth rate, combined with favourable conditions, can lead to rapid population growth — as seen in bacteria, insects, or even human populations in certain historical periods. But birth rate alone does not determine growth; it must be balanced against death rate.
The NCERT points out that natality is not constant — it depends on factors like age structure, availability of mates, food supply, and environmental conditions. For instance, a population with many young, reproductive-age individuals will have a higher birth rate than one dominated by old or very young individuals.
Birth rate is always expressed per unit time — without a time dimension, it is meaningless. A population that produces 100 offspring in a day is very different from one that produces 100 offspring in a year.
3. Death Rate (Mortality)
Death rate is the number of deaths per unit time per individual (or per unit population). Like birth rate, it is usually expressed per 1,000 individuals per year, or as deaths per individual per unit time. …
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