What is sexual reproduction?
We all know that reproduction is the biological process of giving rise to offsprings or young ones. Out of the two types of reproduction, asexual and sexual (based on the criterion of number of individuals taking part in the reproductive process), sexual mode of reproduction is quite complex and difficult to understand. This is because the method of sexual reproduction varies greatly from species to species. However, the definition and overall process remains the same in all species.
Sexual reproduction can be defined as a 'method of reproduction', in which two individuals take part in the reproductive process to give rise to new similar individuals of the same species. However, there are exceptions to the definition. For example, donkey and horse mate to produce mules. Here, the parents are of different species and the offspring, a whole different one from the parents. This is an example for interspecific hybridization in animal breeding. Even though there are such exceptions, they are very few in number and rarely occurs naturally. In 99% of the cases, such matings are artificial and made to occur by humans for their advantage.
Overall processes involved in sexual reproduction of all organisms
No matter to which species the organism belongs to, in sexual reproduction, these is always the production of gametes by the two parents involved in the reproductive process. Production of gametes by the male and female parent is followed by the fusion of gametes produced. As a result of the fusion, zygote is formed. The formation of zygote is universal in all sexually reproducing organisms. The zygote further develops into a new individual or adult. Even though this sounds simple, it is a very slow process, which shows great differences as we move from one species to another, making it highly complex.
Biggest advantage of sexual mode of reproduction
Sexual mode of reproduction has a great advantage over the asexual mode. We all know that asexual reproduction results in clones, which possess the same genome as that of their parents. This is a great disadvantage at the time of adverse conditions. It is due to the absence of genetic variants which can be selected by the nature (as per natural selection principle of Charles Darwin). This is why organisms with simpler organisation, like fungi and algae switch to sexual mode of reproduction just before the onset of adverse conditions.
Now, why do they do this? What is the advantage of switching over to sexual reproduction just before the onset of adverse conditions? This is because sexual mode of reproduction always favour variations and promotes generation of new genetic combinations (variants). During adverse conditions, these variants get selected by the nature and hence, these variants get selected by the nature, as a result of which, they are able to tide over adverse conditions.
Different phases in the life span of a sexually reproducing organism
It is not possible for an organism to start reproducing sexually all of a sudden. They have to grow and reach a certain state of maturity and development. This period from birth till attainment of maturity is called juvenile phase or vegetative phase (in the case of plants). Only after completing the juvenile phase, the organisms can begin to reproduce sexually.
Followed by the juvenile or vegetative phase, is the reproductive phase. An organism in this phase is completely capable of reproducing sexually. In the case of annual and biennial plants, it is very easy to demarcate these phases. However, in the case of perennials, it is very difficult to demarcate these phases. In the case of animals, before they pass on to reproductive phase from juvenile phase, many changes occur in them. After the end of reproductive phase, the senescent phase begins, which is the final phase in the life span of an organism reproducing sexually. All the three phases shows different duration from species to species.
Events of sexual reproduction
Even though the method of sexual reproduction varies greatly from organism to organism, the overall process remains the same in all organisms. The major trademark of sexual mode of reproduction is the fusion of male and female gametes to form zygote, which is also known as fertilization. For ease of studying, the events of sexual reproduction are classified into three stages. The three stages of sexual reproduction are :
1. Pre-fertilization events : events occurring before fertilization.
3. Post-fertilization events : events occurring after fertilization.
These are the three stages in sexual reproduction.
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