Deciphering how natural selection emerges from demographic differences among genotypes, and reciprocally how evolution affects population dynamics, is key to understanding population responses to environment stress, especially in non-trivial ecological scenarios. Many microbes can trigger programmed cell death (PCD) in response to stress, causing massive population decline followed by rapid rebound. To understand how selection may operate on this trait, we exposed monocultures and mixtures of two closely related strains of the microalga Dunalliela salina that do or do not induce PCD to multiple cycles of hyper-osmotic shock, and tracked demography and selection throughout. Population dynamics in mixtures and monocultures were largely consistent, suggesting that strong ecological interactions do not drive selection on PCD during the decline-rebound phase. Instead, long-term coexistence was largely explained by density-dependent competition near the stationary phase, regardless of the large frequency fluctuations at each cycle induced by the decline-rebound dynamics.