This study investigates the impact of predation on the evolution of life history traits in Trinidadian guppies (Poecilia reticulata). The research explores how differences in predation pressure among guppy populations influence their life history patterns, such as age of maturity, reproductive effort, and offspring size. The study compares guppies from three different predator regimes: Crenicichla (predominantly adult guppies), Rivulus (predominantly juvenile guppies), and Aequidens (low predation on all size classes). Key findings include significant differences in life history parameters among the populations. Guppies in Crenicichla sites mature at a smaller size, have higher reproductive allotment (RA), produce more and smaller offspring, and reproduce more frequently than those in Rivulus or Aequidens sites. Guppies in Aequidens sites show intermediate life history traits compared to the other two. These differences are consistent with theoretical predictions about how predation influences life history evolution. The study also presents evidence that predation is the primary selective force shaping these life history patterns. This is supported by within-stream comparisons, an introduction experiment, and a laboratory selection experiment. These experiments show that guppies exposed to different predator regimes exhibit distinct life history traits, suggesting that predation directly influences evolutionary changes in guppies. The results indicate that predation pressure leads to rapid evolutionary changes in guppy life history traits, with differences persisting across seasons. The study highlights the role of predation in shaping life history evolution, demonstrating that predation is a key factor in the divergence of life history traits among guppy populations. The findings support the idea that predation is a major selective force in the evolution of life history traits in guppies.This study investigates the impact of predation on the evolution of life history traits in Trinidadian guppies (Poecilia reticulata). The research explores how differences in predation pressure among guppy populations influence their life history patterns, such as age of maturity, reproductive effort, and offspring size. The study compares guppies from three different predator regimes: Crenicichla (predominantly adult guppies), Rivulus (predominantly juvenile guppies), and Aequidens (low predation on all size classes). Key findings include significant differences in life history parameters among the populations. Guppies in Crenicichla sites mature at a smaller size, have higher reproductive allotment (RA), produce more and smaller offspring, and reproduce more frequently than those in Rivulus or Aequidens sites. Guppies in Aequidens sites show intermediate life history traits compared to the other two. These differences are consistent with theoretical predictions about how predation influences life history evolution. The study also presents evidence that predation is the primary selective force shaping these life history patterns. This is supported by within-stream comparisons, an introduction experiment, and a laboratory selection experiment. These experiments show that guppies exposed to different predator regimes exhibit distinct life history traits, suggesting that predation directly influences evolutionary changes in guppies. The results indicate that predation pressure leads to rapid evolutionary changes in guppy life history traits, with differences persisting across seasons. The study highlights the role of predation in shaping life history evolution, demonstrating that predation is a key factor in the divergence of life history traits among guppy populations. The findings support the idea that predation is a major selective force in the evolution of life history traits in guppies.