Natural selection acts on ______ but populations ______.

Natural selection is a fundamental mechanism of evolution that operates on individuals within a population. Individual organisms exhibit variations in traits, some of which may confer advantages for survival and reproduction in a given environment. Those individuals with beneficial traits are more likely to survive and reproduce, passing on those advantageous traits to their offspring. Over time, this process leads to changes in the population, allowing it to evolve as advantageous traits become more common.

The concept of populations evolving underscores that natural selection and other evolutionary forces act on the collective traits of a population rather than individuals autonomously. This distinction is essential in understanding how evolution occurs; while individuals experience selection, the genetic makeup of the population changes as a result of these selective pressures.

By contrasting the dynamic processes acting on individuals with the resultant changes seen in populations, it highlights the relationship between selection and evolution, making this the appropriate answer to the question posed.