The best description of an organism that is fit for natural selection is that The organism is better able to survive and pass on traits to offspring than competing organisms. Natural selection is the world accepted mechanism to explain the evolution of the species as the process through which the individuals with better abilities to survive have more chances to pass their genes to next generations, leading in this way to more individuals with the same traits, while the less able to survive individuals will die younger. As consequence, after many generations, new species with the adequate traits will populate the land.