Darwin argued that natural selection was the main engine of evolutionary change, and it remains the best explanation for adaptation, the way an organism's features fit the demands of its environment: the shape of a beak, the camouflage of a moth, the flight muscles of a hawk. But the modern understanding of evolution, built on population genetics that did not exist in Darwin's lifetime, recognizes that not every change in a population's genetic makeup is the work of selection.
The clearest challenge to a purely selectionist picture came from the Japanese geneticist Motoo Kimura, who proposed in 1968 what is now called the neutral theory of molecular evolution. Kimura argued, and molecular data have since supported, that most changes at the level of DNA sequence are neither helpful nor harmful to the organism carrying them, and spread through a population or vanish from it by simple chance, a process called genetic drift, rather than because selection favored or disfavored them. This is not a challenge to natural selection as the explanation for adaptation, which Kimura never disputed, but a claim about which changes selection is actually responsible for at the molecular level, a distinction that would have been impossible to draw before DNA sequencing existed.
The current, still developing consensus in evolutionary biology is that selection and drift operate together, with their relative importance varying by the size of the population, the part of the genome in question, and how strongly a given trait affects survival and reproduction. Some biologists, working under the banner of an extended evolutionary synthesis, further argue that factors such as how an organism develops from embryo to adult, and even behavior itself, can shape the direction evolution takes in ways the mid twentieth century's modern synthesis did not fully capture. These are live debates within evolutionary biology rather than challenges to whether evolution happened, a distinction a nonspecialist reader can easily lose in headlines that flatten selection is not the whole story into Darwin was wrong.