Which of the following best describes natural selection?

Natural selection is best described as a process by which organisms that are better adapted to their environment tend to survive and reproduce more successfully than those that are less well-adapted. This concept emphasizes the role of environmental pressures in shaping the traits of a population over time. As certain traits confer advantages in specific environments, individuals possessing these advantageous traits are more likely to survive, reproduce, and pass those traits on to the next generation.

This means that natural selection is not a random process; it is guided by the specific challenges and resources present in the environment that determine which traits are beneficial. Over generations, these beneficial traits can become more common within a population, leading to evolutionary changes.

The other options do not capture the essence of natural selection accurately. For example, random changes in allele frequencies refer to genetic drift, which is a different mechanism of evolution, while the introduction of new genes relates more to gene flow. The theory of extinction by catastrophes does not involve adaptation or survival but rather focuses on sudden events that can lead to the demise of species. Thus, the correct answer encapsulates the adaptive nature of natural selection effectively.