Final answer:
The refinement of Darwin's theory has shown that evolution can only be measured or seen across generations within a population.
Step-by-step explanation:
The refinement of Darwin's theory has shown that evolution can only be measured or seen across generations within a population. Darwin's theory of evolution by natural selection explains how species change over time. Through the process of natural selection, individuals with traits that are more favorable for their environment are more likely to survive and reproduce, passing those traits to the next generation. This leads to changes in the characteristics of a population over time.