In chapters 2-10 we built up a general abstract model of "livingness", to which we believe all organisms and all cells conform. This model is about the unity of life. It describes what is common to all living things and distinguishes them from the non-living. But an adequate science of life needs to account for diversity as well as unity, and since 1859 this need has been met by the theory of evolution. The explanation of diversity is the primary role of evolutionary theory, though it is not the only role. Evolutionary ideas have been assimilated into all areas of biology, including molecular biology. But we encounter the theory directly when we seek to explain diversity and the occupation of the world's vast range of habitats.
The perspective we adopt in this chapter is quite different from earlier parts of the book. The focus now is on large collections of organisms, not cells and cell constituents. The theory of evolution tells us that organisms change by adapting to alterations in their environment, yet they are still organisms. The theory is itself ever-changing, adapting to advances in knowledge, but at root it is still the same theory.
To paraphrase John Donne, no organism is an island. Organisms exist in breeding populations, not as isolated individuals. Moreover, populations of different species interact: they eat one another, inhabit one another, transport one another, depend on each other's waste products, spread diseases to one another or simply compete for space. In a given geographical area, these interactions constitute an ecosystem. Ecosystems differ, but each comprises a more or less wide diversity of organisms, sometimes hundreds of thousands of species. The individual belongs to a population that is part of an ecosystem, which in turn is part of the biosphere, the part of the planet that houses life.
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