Why organism have different mode of fertilization


Fertilization is the process of fusion of male and female gamete to form zygote. It is of two types on the basis of location of fusion of gametes:
  1. Internal fertilisation is the process in which fusion of gametes (egg and sperm)takes place inside the body of female. For example, in humans fertilisation is internal.
  2. ​External fertilisation is the process in which fusion of gametes (egg and sperm) takes place outside the female's body. The male releases sperm cells onto the eggs which are laid external to the female's body and fusion takes place outside the body. For example, frogs.

Depending on which organism finds it more advantageous to use internal fertilisation or external fertilisation, different organisms have different modes of fertilisation. 

Organisms exhibiting external fertilisation releases more number of gametes into the surrounding to increase the chance of fertilisation. Because the fertilisation is external there is less possibility of fusion of gametes in the outside environment and if somehow fertilisation carried out successfully there will be chances that the externally fertilised eggs and young ones are at greater risk from predators. So, to ensure the chances of survival, these organisms releases large number of gametes to the surrounding.  

Internal fertilization is generally considered as more advantageous, as they have more chances of survival since they happen to be safe from damage by external factors in environment such as predators, physical factors etc. They do not get affected by the proximity of distance between eggs and sperms as sperm cells are released directly into the tracts of females. The foetus grows in protected environment (bodies of females).


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