1.19: On the road to biological literacy: what are the major themes in biology.

Contemplating unity and diversity. An entomologist organizes and categorizes specimens.

Biology is, literally, the study of life. But what is life? And how is it distinguished from non-life? Spend a few moments trying to define exactly what life is and you’ll realize that it is not easily described with a simple definition. A useful approach is to consider the characteristics shared by all living organisms and living systems:

• A complex, ordered organization consisting of one or more cells. Cells carry out the functions necessary for life.
• The use and transformation of energy to perform work. Organisms can perform many reactions and activities, by acquiring, using, and transforming energy.
• Sensitivity and responsiveness to the external environment. Living organisms are able to respond to stimuli—such as light, moisture, or another organism.
• Regulation and homeostasis. Organisms are able to maintain relatively constant internal conditions that may differ from the external environment.
• Growth, development, and reproduction. Organisms can grow and develop, and they carry information relating to these and other processes that they can pass on to offspring.
• Evolutionary adaptation leading to descent with modification over time. Populations have the capacity to change over time. As a consequence of organisms’ ability to reproduce and of evolutionary change, populations may become better adapted to their environments.

In this guide to biology, as we explore the many facets of biology and its relevance to life in the modern world, you will find two central and unifying themes recurring throughout.

Hierarchical organization. Life is organized on many levels within individual organisms, including atoms, cells, tissues, and organs. And in the larger world, organisms themselves are organized into many levels: populations, communities, and ecosystems within the biosphere.

The power of evolution. Evolution, the change in genetic characteristics of a population over time, accounts for the diversity of organisms and the unity among them.

These central unifying themes connect the diverse topics in biology, which include the chemical, cellular, and energetic foundations of life; the genetics, evolution, and behavior of individuals; the staggering diversity of life and the unity underlying it; and ecology, the environment, and the links between organisms and the world they inhabit. Let’s continue our exploration of life!