Theory:

Variations may or may not be successful, or in other words, it is based on the amount of mutation in the particular cell, chromosome or gene.
 
mutation.jpg
Occurrence of variation due to mutation
 
But, whenever some variations occur on the species, all the individuals of the particular species will not survive. But, for a few individuals, the particular variation might help them survive, which ultimately makes them the fittest to the environment, known as a niche.
Niche means a role or position of an individual in the community.
The term niche was first coined by the naturalist Joseph Grinnell in \(1917\) in his paper "The niche relationships of the California Thrasher". To know more about niche Read here.
 
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Mutation leads to variation in plants
 
However, the niche is not stable, and it will change based on environmental factors like temperature, increasing the water level. To some extent, it will drastically change during some catastrophic events, such as a comet hit or meteorite shower.
 
To conclude this, the fittest will survive, and other individuals or the whole species will become extinct.
Reference:
https://upload.wikimedia.org/wikipedia/commons/f/f3/Mutation_and_selection_diagram.svg
https://commons.wikimedia.org/wiki/File:Mendels_peas.png