The underlying principle of the theory of evolution is natural selection, proposed by Charles Darwin. This principle posits that organisms with favorable traits are more likely to survive and reproduce, passing these traits to future generations. Over time, this process can lead to significant changes in species, adaptive evolution, and even the emergence of new species. The theory emphasizes the importance of variation within populations, as it is the raw material for natural selection to act upon. This foundational concept has fundamentally shaped our understanding of biology and the diversity of life on Earth.