Life span of organisms
After birth, each organism lives upto a certain period of time. This span of time from birth till natural death of an organism is called as its life span. Often, people are filled with misconceptions about life span. They often correlate life span with size of the organisms, which is not true. Life spans can be as short as few days or it can be as long as few hundred years and has got nothing to do with sizes of the organisms. For example, parrot and crow are almost similar in size. However, their life spans differ alot. Crow has an average life span of about 15 years, while parrot has an average life span of about 140 years. Another example is that of crocodile and tortoise, where crocodile lives only for an average of 60 years, while tortoise lives for an average of 100 to 150 years, which is astonishing. Thus, it is evident that life span has got nothing to do with size of the organism.
Now, we all know that death is certain for all organisms. Then, there must be a process that enables the species to exist continuously, even after the death of an organism belonging to that species. This process, which enables continuity of species generation after generation is called reproduction.
Hence, reproduction is defined as a biological process which enables the species to continue their existance on this earth. In other words, it is the process in which an organism gives rise to young ones or offsprings. Anybody can observe that there is a cycle of birth, growth and death in reproduction. That is, an organism gives rise to an offspring, which in turn grows, matures and produces another offspring. This cycle in reproduction ensures the continuity of species on the earth.
Just like the diversity in the organisms existing in this world, the methods of reproduction also shows large diversity. However, there is a simple classification for reproduction too. This classification is based on whether a single organism is involved in sexual reproduction or two organisms are involved. Based on this criteria, we can classify reproduction into two types, asexual and sexual. However, in this article we are discussing only about asexual reproduction.
Asexual reproduction in lower organisms
When a single organism is involved in the reproductive process, it is called asexual reproduction. That is, a single organism of a species is capable of giving rise to offsprings. Thus, in asexual reproduction, the offsprings produced will be identical to each other and also exactly identical to their parents. That is, they are clones, morphologically and genetically similar individuals.
Talking about single-celled organisms, asexual reproduction is a common phenomenon among them. We can also find asexual reproduction in plants and animals with relatively simple organisations.
For single-celled organisms, that is, Monerans and Protists, the parent cell divides into two giving rise to new individuals. Thus, cell division itself is a mode of asexual reproduction for there organisms. This is why many scientists believe that single-celled organisms are immortals. Thus, single-celled organisms can be taken as an example for organisms who are excepted from death. Many single-celled organisms also reproduce by binary fission. Binary fission is a method of asexual reproduction in which the parent cell at first divides into two halves, and then, each half rapidly grows into a new cell. A common example for binary fission is Amoeba and Paramecium. However, yeast reproduce asexually by budding. In this method, the parent cell divides unequally producing small buds. These small buds initially remain attached to the parent and eventually gets seperated, after which they mature into adults.
Organisms belonging to Kingdom Fungi and also plants with simple organisations such as algae reproduce asexually. However, their asexual reproduction is with the help of special asexual reproductive structures. Of these structures, zoospores are the most common one. They are nothing but microscopic motile structures. An example for organism reproducing with the help of zoospore is Chlamydomonas. Another one of these structures is gemmules, which are endogenous buds. They are commonly seen in the case of sponges. Conidia and buds are also examples for asexual reproductive structures and are common method of asexual reproduction in Penicillium and Hydra respectively. Plants also have the ability to reproduce asexually while, most of the animals exhibit sexual reproduction.