At the core of ecological interactions is competition and the principles of evolutionary biology. A combination of random mutations, genetic drift and competition, combined with the work of natural selection, shape population attributes. Competition occurs at all levels of organization from genes to chromosomes, to whole organisms, to family groups, subgroups, populations, communities and even whole ecosystems. Responses span the gamut from aggression to symbiotic cooperation. Each response is judged by natural selection, which continually reshapes gene pools. Ecological principles are universal in all life forms, microbes to whales and humans. Therefore, many biological systems and their respective organisms are studied to uncover new universal principles of ecology. Ultimately, ecological principals result in the formation of models, usually mathematical in nature, that seek to predict outcomes of interactions and change. In the end, the accurate prediction of outcomes via robust models suggests a deep understanding of the underlying principles. Such an understanding is at the core of the science of ecology.