The unit of evolution is population, not individual. 

Because evolution occurs in a group of species and not individually.

Hope it helps :)

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Unit of evolution is population, not the individual
because evolution cannot happen only in one individual if it happen in only one individual we do not call it as evolution  
evolution should occur in may individuals or population only then we can say that evolution has taken place
if only one individual evolves there is no guarantee that it will continue
only if it occurs in large no of population the new evolved species can continue to live on earth