Theory:

Darwinism or natural selection is based on the following principles:
  1. Overproduction
  2. Struggle for existence
  3. Variations
  4. Survival of the fittest
  5. Origin of species
1.  Overproduction:
 
Living organisms can reproduce and form their progeny in a geometrical way. This leads to an increase in reproductive potential leading to the overproduction of organisms that survive. This concludes that some limiting factors should put a natural limit on the species' existence.
 
2. Struggle for existence:
  
As a result of overproduction, there is an increase in the geometric ratio of population. But the factors such as availability of food and space for the organisms to live does not change. Hence, there is always competition amongst the organisms for these limited resources leading to a struggle for their existence. Only those organisms that struggle and overcome this will survive. This keeps the number of organisms stable in the environment.
The struggle for existence are of three types:
  • Intraspecific struggle: The competition for resources such as food, shelter, breeding places etc., among the organisms of the same species is known as intraspecific struggle.
intraspecific.jpg
Image showing the intraspecific struggle between organisms of same species
  • Interspecific struggle: The competition for resources such as food, shelter, breeding places etc., among the organisms of the two different species living together is known as interspecific struggle. Animals preying on other animals is an example of this.
interspecific.jpg
Image showing the interspecific struggle between organisms of different species
  • Environmental struggle: The struggle faced by the organisms due to environmental factors such as drought, heavy rains, extreme heat or cold, earthquakes and other natural calamities is known as environmental struggle.
Reference:
https://commons.wikimedia.org/wiki/File:Fighting_Flamingos_(6911914548).jpg
https://commons.wikimedia.org/wiki/File:Fighting_Hartebeest.jpg
https://commons.wikimedia.org/wiki/File:Leopard_kill_-_KNP_-_001.jpg