Ecology is the study of environmental systems. Ecology is not interested only in the components of nature individually but especially in how the parts interact. The subject of ecology is normally divided onto four broad categories: physiological ecology that deals with the response of single species to environmental conditions such as light, temperature. Population ecology focuses on the abundance and distribution of individual species and the factors that cause such distribution. Community ecology deals with the number of species found at given location and their interactions and ecosystems ecology concerned with the structure and function of the entire group of microbes, plants, animals, abiotic environment, and how all the parts interact to generate the whole. Evolutionary ecology, which may operate at any of these levels but most commonly at the physiological or at the population level, that is a rich and dynamic area of ecology focusing on attempting to understand how natural selection developed the structure and function of the organisms and ecosystems at any of these levels.