The difference between an R-selected species and a K-selected species is that the R-selected species live in variable, unstable habitats, while K-selected species reside in stable environments. R-selected species produce many offspring, but only a few live out their lifespan. K-selected species have a few offspring that have a long life expectancy.
Organisms considered R-selected species tend to be small, reach sexual maturity early in life and reproduce a single time. Mosquitoes are an example of an R-selected species.
The K-selected species are larger in size, reach sexual maturity later in life, reproduce many times during their life span and require more energy to procreate. Whales, giraffes and elephants are examples of K-selected species.