Natural selection is one of evolution’s basic mechanisms. Because of natural selection, individuals best adapted to the environment will have the most reproductive success, producing more offspring than the rest of the population.

Those individuals who are the least adapted to the environment will leave fewer offspring or even die before reproducing themselves. The characteristics of the least adapted individuals will not be inherited by any other member of the population. The population is thus naturally selected over time.

Take a hypothetical population of birds with beaks of variable sizes feeding themselves with different-sized seeds. Those birds with smaller beaks will eat small seeds and produce offspring with smaller beaks. Those with larger beaks can feed themselves with both small and large seeds, and their offspring will have larger beaks.

If a drought reduces the number of small seeds available for several years, birds with larger beaks, living off of larger seeds, are more likely to survive and reproduce. The others will die without reproducing. The descendants of this population will be made up of birds with larger beaks. The population will have naturally selected for itself.