The term "survival of the fittest" is most closely associated with:

"Survival of the fittest" is a phrase that encapsulates the concept of natural selection, which is a fundamental mechanism of evolution proposed by Charles Darwin. It describes the process through which organisms that are better adapted to their environment tend to survive and reproduce more successfully than those less suited to their environment. This idea emphasizes that fitness involves not just physical strength, but also the ability to find resources, avoid predators, and successfully reproduce in given environmental conditions.

Natural selection operates based on variations within populations. Individuals with advantageous traits are more likely to pass those traits on to their offspring, gradually leading to evolutionary changes over generations. This concept explains how species adapt to their environments over time and helps to clarify the dynamics of ecosystems.

In contrast, the other options refer to different concepts related to biology and evolution. Evolution by extinction refers to species becoming extinct rather than adapting or evolving; adaptive radiation describes the process where organisms diversify rapidly into a wide variety of forms to adapt to different environments; and artificial selection involves human intervention in breeding traits for desired characteristics. Each of these concepts is important in the study of biology but does not encapsulate the meaning behind "survival of the fittest" as effectively as natural selection does.