Foundations of Modern Biology

There are four unifying principles of biology:

Cell theory. All living organisms are made of at least one cell, the basic unit of function in all organisms. In addition, the core mechanisms and chemistry of all cells in all organisms are similar, and cells emerge only from preexisting cells that multiply through cell division.
Evolution. Through natural selection and genetic drift, a population's inherited traits change from generation to generation.
Gene theory. A living organism's traits are encoded in DNA, the fundamental component of genes. In addition, traits are passed on from one generation to the next by way of these genes. All information flows from the genotype to the phenotype, the observable physical or biochemical characteristics of the organism. Although the phenotype expressed by the gene may adapt to the environment of the organism, that information is not transferred back to the genes. Only through the process of evolution do genes change in response to the environment.
Homeostasis. The physiological processes that allow an organism to maintain its internal environment notwithstanding its external environment.