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Charles Darwin primarily focused on the concept of natural selection in his writings, especially in his seminal work, "On the Origin of Species." Natural selection is a fundamental mechanism in the theory of evolution, illustrating how organisms that are better adapted to their environments tend to survive and reproduce more than those that are less well-adapted. This process leads to the gradual evolution of species over time.

Darwin described how variations in traits among individuals in a population can affect their survival and reproductive success. He emphasized the role of competition, predation, and environmental pressures in shaping these traits. By establishing natural selection as a key driving force in evolution, Darwin provided insights that transformed our understanding of biological diversity and adaptation.

While the other choices such as genetic engineering, species classification, and environmental change are relevant to biology, they do not capture the main focus of Darwin’s contributions. Genetic engineering pertains to modern techniques for altering DNA, species classification relates to the categorization of organisms which also came later as a formal taxonomy, and environmental change pertains to ecosystems rather than the mechanisms of evolution that Darwin studied. Thus, natural selection stands out as the central theme of Darwin’s work.