With this chapter, we begin our study of ecology, a study of the relationships among organisms and their environments. You have already learned about the chemistry of life, the biology of cells, patterns of heredity, molecular genetics, and how evolution occurs and creates species diversity. Now it is time to see how species actually act in the real world, how they interact with each other and with their environments. Organisms can be grouped into tiered levels of organization: populations, communities, ecosystems, and biomes. We start with populations (the first tier), all the individuals of a particular species living in a particular area. Populations are not static. Individuals in the population reproduce, move to new areas or die, and the population changes size. The size and growth of a population are influenced by interactions with other populations and by environmental factors, such as carrying capacity and catastrophic climatic events. All these complex interactions result in each population having a characteristic size, dispersion pattern, growth rate, mortality rate, and survivorship curve. Some populations have been selected for fast and large reproductive efforts but live a relatively short time, while other populations have been selected for slower reproductive rates with longer life spans. There are trade-offs between quantity and quality. While we talk about populations, however, do not forget that each population is made up of individuals and that each individual has its own particular genetic endowment, which determines its phenotype. And natural selection acts on the individual phenotype.