Experimental versus Evolutionary Sciences
The many questions that people have asked about the animal world since the time of Aristotle can be grouped into two major categories.* The first category seeks to understand the proximate or immediate causes
that underlie the functioning of biological systems at a particular time and place. These include the problems of explaining how animals perform their metabolic, physiological, and behavioral functions at the molecular, cellular, organismal, and even populational levels. For example, how is genetic information expressed to guide the synthesis of proteins? What causes cells to divide to produce new cells? How does population density affect the physiology and behavior of organisms?
The biological sciences that address proximate causes are known as experimental sciences
, and they proceed using the experimental method. This method consists of three steps: (1) predicting how a system being studied will respond to a disturbance, (2) making the disturbance, and then (3) comparing the observed results with the predicted ones. Experimental conditions are repeated to eliminate chance occurrences that might produce erroneous conclusions. Controls—repetitions of the experimental procedure that lack the disturbance— are established to protect against any unperceived factors that may bias the outcome of the experiment. The processes by which animals maintain a body temperature under different environmental conditions, digest their food, migrate to new habitats, or store energy are some additional examples of physiological phenomena that are studied by experiment (See: Support, Protection and Movement
Internal Fluids and Respiration
Digestion and Nutrition
). Subfields of biology that constitute experimental sciences include molecular biology, cell biology, endocrinology, developmental biology, and community ecology. In contrast to questions concerning the proximate causes of biological systems are questions of the ultimate causes that have produced these systems and their distinctive characteristics through evolutionary time. For example, what are the evolutionary factors that caused some birds to acquire complex patterns of seasonal migration between temperate and tropical areas? Why do different species of animals have different numbers of chromosomes in their cells? Why do some animal species maintain complex social systems, whereas the animals of other species are largely solitary?
The biological sciences that address questions of ultimate cause are known as evolutionary sciences
, and they proceed largely using the comparative method
rather than experimentation. Characteristics of molecular biology, cell biology, organismal structure, development, and ecology are compared among related species to identify their patterns of variation. The patterns of similarity and dissimilarity are then used to test hypotheses of relatedness, and thereby to reconstruct the evolutionary tree that relates the species being studied. The evolutionary tree is then used to examine hypotheses of the evolutionary origins of the diverse molecular, cellular, organismal, and populational properties observed in the animal world. Clearly, the evolutionary sciences rely on results of the experimental sciences as a starting point. Evolutionary sciences include comparative biochemistry, molecular evolution, comparative cell biology, comparative anatomy, comparative physiology, and phylogenetic systematics.