Ecology is the study of the relationship between an organism and its environment. Seeds, and fruits containing seeds, may be dispersed away from the parent plant by wind, water, or animals, which reduce seed predation as well as competition between offspring. If a seed germinates in a favorable location, the developing seedling faces many selective pressures in its environment, both living and nonliving factors. Plants live in two worlds at the same time; their roots are in the soil and their shoots are in the air. Because they are anchored by their roots in one place, plants are considered indicators of the environment in which they grow. We can tell a lot about the environment of a plant simply by looking at the plant and its structures.

Individual plants of the same kind are part of a population, and different populations of organisms are part of a community. A community and its abiotic factors are called an ecosystem. Food chains interconnect to form a food web within a community, both of which describe who eats whom. Plants are the producers of a community, supplying the energy and minerals needed by other organisms in the ecosystem. Bacteria and fungi are decomposers, and animals are consumers, including herbivores that eat plants or carnivores that eat animals. Plants interact with many organisms in the community including herbivores, competitors, and parasites, all of which have negative effects on the plants. Plants also have mutualistic relationships with other organisms in which both organisms benefit from the interaction. Flowering plants and their pollinators, and seed plants and the animals that disperse them, are examples of mutualistic relationships.

The essence of natural selection is that individuals best adapted to their environment produce the most offspring and pass on their genes to the next generation. Organisms produce more offspring than needed to replace themselves and more than environmental resources can support. Most offspring die before maturity, and those with genes that confer the best adaptations to their environment reproduce the most. Natural selection functions at the population level, and a population, not an individual, is the smallest unit that can evolve. Populations evolve in the context of their environment, which confers success or failure to individuals with the best-adapted characteristics.

Direct evidence for natural selection comes from experimentation and from comparisons with artificial selection. Natural selection is one of the main driving forces for evolution. Direct evidence exists for several of the principles of natural selection: the overproduction of seeds indicates that populations can increase dramatically; a small fraction of seeds grows into reproductively mature plants; different individuals in a population have variable, genetically controlled features; plants that are better adapted to a habitat have more reproductive success in that habitat than plants that are not so well adapted to it.