Competition is central to the theory of natural selection. why does competition occur?

Competition occurs basically because of the limited supply of items. For example, two animals compete for the same food basically because there is no enough food for the two of them. Had there been enough, they will just go on their ways. The same also is happening for the shelter, territory, or even partners. 

It seems that you have missed the necessary options for us to answer this question, but anyway, here is the answer. Competition is central to the theory of natural selection. Competition occurs because organisms typically produce too many offspring, and resources are limited. Hope this answers your question.

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