The single most important concept and theme of On the Origin of Species is natural selection. This concept (along with sexual selection, which is of secondary importance) drives Darwin’s theory of evolution, which he generally referred to as natural selection theory.

Natural selection is the process by which species morph into varieties and new species over thousands or millions of years. Any population of organisms by chance occasionally produces individuals with physical or instinctual modifications that make these individuals diverge just slightly from their neighbors. If a divergent characteristic proves advantageous to an individual in the struggle for life, that individual is much more likely to survive and reproduce. Subsequently, its offspring likely have the same divergent characteristic(s) and, similarly, have the same advantage in the struggle for life. Over time, these organisms produce a new variety of species. If enough of these traits, or adaptations, are “selected for,” then eventually a new species arises. Natural selection is the process by which useful traits are selected for and reiterated through generations of successful organisms.

One of the key features of natural selection is its reliance on slow, cumulative changes—that is, a series of minute modifications.