Final answer:
The main tenet of Darwin's theory of evolution by natural selection is that species evolve over time through variations in traits; advantageous traits increase an organism's likelihood of survival and reproduction, leading to those traits becoming more prevalent in the species.
Step-by-step explanation:
The main tenet supporting Charles Darwin's theory of evolution by natural selection is that all life has evolved from a common ancestor and that this evolution occurs through the process of natural selection. Darwin proposed that in a given population, organisms have variations and those with favorable traits are more likely to survive and reproduce. Over time, these advantageous traits become more common within the population, leading to evolutionary changes.
Darwin's theory was controversial due to its challenge to the then-prevailing notion of divine creation, but it eventually laid the groundwork for modern biological science by providing a systematic explanation for the adaptation and diversity of species. His idea came from his observations made during his voyage aboard the HMS Beagle, especially significant findings in the Galápagos Islands, and from studying works by earlier scholars such as Lamarck, Lyell, and Malthus. Darwin's theory emphasized that the environment plays a critical role in determining which traits are favorable, and as the environment changes, different traits may become advantageous.
In summary, evolution by natural selection is a core principle of biology that explains the unity and diversity of life by the differential survival and reproduction of individuals with certain heritable traits.