According to Darwin, in the struggle for survival, those individuals whose traits best enable them to survive and reproduce will, on average, leave the largest number of offspring that in turn survive to reproduce. This is the basic idea behind his theory of:

This is the basic idea behind his theory of Natural selection

Explanation:

The theory of Natural selection was demonstrate in Darwin's book known as 'Origin of species' which was written in 1859

In this book, the process of change of an organism over time is described and also mentioned about their physical as well as behavioral changes.

Natural selection means the survival and reproduction of individuals by changing the phenotypes.

Natural selection is a process by which the nature accepts those organisms that can change their phenotype in order to survive with changing environment and eliminate those phenotypes that are not favorable for their survival.