The r/K-selection theory was proposed by:
MacArthur and Wilson introduced the r/K selection framework while developing ideas about population growth and island biogeography. The labels come from the intrinsic growth rate, r, and carrying capacity, K, in the logistic model. Later ecologists refined or criticized the simple two-category scheme, but it remains useful as an introductory way to organize correlated life-history traits. Survivorship and reproductive schedules are connected through allocation. An organism has finite energy for growth, maintenance, defense, and reproduction; investment in one function constrains the others. Natural selection does not pursue a universal ideal. It favors combinations that leave more surviving descendants under the mortality risks and resource patterns experienced by a population. A useful check is to ask what happens at the biological extremes and whether the proposed mechanism still makes sense. Interpreting the example at the appropriate population scale keeps the causal mechanism distinct from a simple correlation or an absolute rule.
Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 12