Each and every organism can live only for a certain period of time. The period from birth to the natural death of an organism represents its life span.
Several other organisms are drawn for which you should find out their life spans and write in the spaces provided. Isn’t it both interesting and intriguing to note that it may be as short as a few days or as long as a few thousand years? Between these two extremes are the life spans of most other living organisms.
You may note that life spans of organisms are not necessarily correlated with their sizes; the sizes of crows and parrots are not very different yet their life spans show a wide difference. Similarly, a mango tree has a much shorter life span as compared to a peepal tree.
Whatever be the life span, death of every individual organism is a certainty, i.e., no individual is immortal, except single-celled organisms. Why do we say there is no natural death in single-celled organisms? Given this reality, have you ever wondered how vast number of plant and animal species have existed on earth for several thousands of years?
There must be some processes in living organisms that ensure this continuity. Yes, we are talking about reproduction, something that we take for granted.
Reproduction is defined as a biological process in which an organism gives rise to young ones (offspring) similar to itself. The offspring grow, mature and in turn produce new offspring. Thus, there is a cycle of birth, growth and death. Reproduction enables the continuity of the species, generation after generation.
You will study later in Chapter 5 (Principles of Inheritance and Variation) how genetic variation is created and inherited during reproduction. There is a large diversity in the biological world and each organism has evolved its own mechanism to multiply and produce offspring.
The organism’s habitat, its internal physiology and several other factors are collectively responsible for how it reproduces. Based on whether there is participation of one organism or two in the process of reproduction, it is of two types.
When offspring is produced by a single parent with or without the involvement of gamete formation, the reproduction is asexual. When two parents (opposite sex) participate in the reproductive process and also involve fusion of male and female gametes, it is called sexual reproduction.