Suppose individual robins that have longer beaks have somewhat more offspring than those with shorter beaks, and that this has led to increases in beak length over time. we can say that beak size is associated with the of the robin, and that there is selection operating on beak size.

Individual robins that have longer beaks have somewhat more offspring than those with shorter beaks. This has led to increases in beak length over time. We can say that beak size is associated with the adaptive evolution of the robin, and that there is directional selection operating on beak size.

Explanation:  

Adaptive evolution refers to changes in the organism that lead to the evolution of the species and that are adaptive to determine conditions of the environment. Birds adapt to diverse ecological niches showing specialized beak forms to improve functions such as feeding, mating, defending, competing, and etcetera. These factors play an important role in beak shaping. Natural selection produce adaptive evolution. There are different kinds of natural selection, but in the present example, directional selection is operating.

Directional selection increases in the proportion of individuals with an extreme phenotypic trait, in this case, large beaks. This selection presents more frequently in those cases in which interactions between living organisms and the environment modify in the same direction.

a "theory" a suggestion that is not 100 % accurate, or a figure. some people say its a theory because they of all the passed theory's about one subject. it depends sometimes theory might be accurate and sometimes they are.