Final answer:
The observation supporting natural selection is that beneficial traits become more common in a population due to their survival and reproductive advantages, leading to species evolution.
Step-by-step explanation:
The observation that supports the principle of natural selection is that "Trait variants that help organisms survive to reproduce become more common in a population over time." This is because natural selection posits that organisms with traits that confer a survival and reproductive advantage are more likely to pass those traits on to subsequent generations. Over time, these advantageous traits become more prevalent in the population, leading to an evolution of the species.
Three key requisites for natural selection to operate are variation among individuals within a population, differential survival and reproduction associated with possession of certain traits, and heritability of those traits from parents to offspring. As a result, species adapt to their environments, and those with beneficial heritable traits tend to flourish and propagate these traits within the population.