A. Subdisciplines of Evolutionary Biology
B. Perspectives from Evolutionary Biology

Evolutionary biology has two encompassing goals:

To discover the history of life on earth: that is, 1) to determine the ancestor-descendant relationships among all species that have ever lived--their phylogeny; 2) to determine the times at which they originated and became extinct; and 3) to determine the origin of and the rate and course of change in their characteristics.

To understand the causal processes of evolution: that is, to understand 1) the origins of hereditary variations; 2) how various processes act to affect the fate of those variations; 3) the relative importance of the many co-acting processes of change; 4) how rapidly changes occur; 5) how processes such as mutation, natural selection, and genetic drift have given rise to the diverse molecular, anatomical, behavioral, and other characteristics of different organisms; and 6) how populations become different species. Virtually all of biology bears on this vast project of understanding the causes of evolution, and reciprocally, understanding the processes of evolution informs every area of biology.
A. Subdisciplines of Evolutionary Biology

Evolutionary biology includes numerous subdisciplines that differ in their subject matter and methods. Some of the major subdisciplines are:

Behavioral evolution (also called behavioral ecology). Behavioral evolutionists study the evolution of adaptations such as mating systems, courtship behavior, foraging behavior, predator escape mechanisms, and cooperation. Behavioral characteristics evolve in much the same way as structural features. Changes in the neural, hormonal, and developmental mechanisms underlying behavior are also objects of evolutionary study, as are the adaptive differences among species in memory, patterns of learning, and other cognitive processes, some of which are reflected in differences in brain structure. Behavior, physiology, structure, and life history patterns often evolve in concert.

Evolutionary developmental biology. This field seeks to understand evolutionary changes in the processes that translate the genetic information contained in an organism's DNA (its genotype) into its anatomical and other characteristics (its phenotype). In part, it aims to describe how variation at the genetic level results in variation in the characteristics that affect survival and reproduction. Perhaps its greatest significance lies in its potential to reveal the extent to which developmental processes bias, constrain, or facilitate evolution of the phenotype.

Evolutionary ecology. This field looks at how the life histories, diets, and other ecological features of species evolve, how these processes affect the composition and properties of communities and ecosystems, and how species evolve in response to one another. Its salient questions include: How do we account for the evolution of short or long life spans? Why are some species broadly and others narrowly distributed? Do parasites (including microbial pathogens) evolve to be more benign or more virulent as time passes? How do evolutionary changes and evolutionary history affect the number of species in a community, such as a tropical forest or a temperate forest?

Evolutionary genetics. Evolutionary genetics (which includes population genetics) is a central discipline in the study of evolutionary processes. It uses both molecular and classic genetic methods to understand the origin of variation by mutation and recombination. It describes patterns of genetic variation within and among populations and species, and employs both empirical study and mathematical theory to discover how this variation is affected by processes such as genetic drift, gene flow, and natural selection. The mathematical theory of evolutionary genetics is essential for interpreting genetic variation and for predicting evolutionary changes when many factors interact. It also provides a strong foundation for understanding the evolution of special classes of characteristics, such as genome structure and life histories.

Evolutionary paleontology. This field, often called paleobiology, addresses the large-scale evolutionary patterns of the fossil record. It examines the origins and fates of lineages and major groups, evolutionary trends and other changes in anatomy through time, and geographic and temporal variations in diversity throughout the geologic past. It also seeks to understand the physical and biological processes and the unique historical events that have shaped evolution. Paleontological data provide a window on deep time, and thus permit the direct study of problems ranging from the change in the form and distribution of species over millions of years to the evolutionary responses of major groups to both catastrophic and gradual environmental changes. These data also allow calibration of rates for such phenomena as mutations in nucleotide sequences.

Evolutionary physiology and morphology. This broad field looks at how the biochemical, physiological, and anatomical features of organisms provide adaptation to their environments and ways of life, and at the history of these adaptations. It is also beginning to define the limits to adaptation--for such limits may restrict a species' distribution or lead to its extinction. Among the questions studied in this field are: How do the form and the function of a feature change in relation to each other during evolution? How and why are some species tolerant of a broad range, and others of only a narrow range, of environmental factors such as temperature? Is there a diversity of mechanisms by which populations may adapt to a new environment?

Human evolution. Many evolutionary biologists draw on the conceptual subdisciplines of evolutionary biology to study particular groups of organisms. Of these groups, one is especially notable: the genus Homo. The many anthropologists and biologists who take human evolution as their subject use principles, concepts, methods, and information from evolutionary systematics, paleontology, genetics, ecology, animal behavior--the full panoply of evolutionary disciplines. Other researchers study genetic variation and the processes that affect it in contemporary human populations (a subject intimately related to other areas of human genetics, such as medical genetics). Still others work in the controversial area of human behavior and psychology.

Molecular evolution. Developing hand in hand with the spectacular advance of molecular biology, this field investigates the history and causes of evolutionary changes in the nucleotide sequences of genes (DNA), the structure and number of genes, their physical organization on chromosomes, and many other molecular phenomena. This field also provides tools for investigating numerous questions about the evolution of organisms, ranging from phylogenetic relationships among species to mating patterns within populations.

Systematics. Systematists distinguish and name species, infer phylogenetic relationships among species, and classify species on the basis of their evolutionary relationships. Systematists have contributed greatly to our understanding of variation and the nature of species. Their special knowledge of particular groups of organisms is indispensable both for inferring the history of evolution and for understanding the detailed workings of evolutionary processes, since each group of organisms presents special, fascinating, and often important questions. Moreover, systematists' knowledge often has unexpected uses. Knowledge of the systematics and biological characteristics of deer mice became invaluable when the novel hantavirus, harbored by these mice, caused fatalities in the United States. Likewise, plants that are related to a species in which a pharmacologically useful compound has been found are likely to contain similar compounds.

B. Perspectives from Evolutionary Biology

Biological disciplines such as molecular biology and physiology ask "how" questions: How do organisms and their parts work? Evolutionary biology adds "why" questions: Why do specific organisms have particular features rather than others? Thus, while much of biology addresses the proximate causation of observed phenomena, evolutionary biology addresses ultimate causation. Answers to questions about ultimate causation might include "because this species inherited the feature from its distant ancestors," or "because a history of natural selection favored this feature over others." That a human embryo has gill slits can be understood only in light of their inheritance from early vertebrate ancestors; that we walk upright can be understood as an adaptation, a trait favored by natural selection in our more recent ancestors. In emphasizing history, we must, at the same time, recognize that evolution is an active, ongoing process that affects humans and all other living organisms.

The study of evolution entails several perspectives that have made important conceptual contributions to biology.

Chance and necessity. A fundamental principle of evolutionary science is that living systems owe their properties to an interplay between stochastic (random) events and deterministic (consistent, predictable) processes. Random mutations, asteroid impacts, and other such events have greatly influenced the course of species' evolution. Therefore, evolutionary biologists have developed probabilistic theories that describe the likelihood of various evolutionary trajectories. An important corollary of random events is historical contingency. Although some adaptations to environmental factors are reasonably predictable, other characteristics of organisms are the consequence of "historical accidents" that launched evolution along one path rather than others. The modifications of the forelimbs for flight, for example, are very different in birds, bats, and pterodactyls, presumably because different mutations presented natural selection with different options in these lineages.

Variation. Whereas physiologists may view variation as undesirable "noise" or experimental error that obscures a "true" value, variation is the all-important object of study for most evolutionary biologists. Probably no lesson from evolutionary biology is more important than the realization that there are no Platonic "essences," or fixed, "true," "normal" properties. Almost every character is somewhat different among the individuals of a population. Evolutionary biologists' emphasis on variation has borne methodological fruit--namely, statistical methods, such as analysis of variance and path analysis, that are widely used in other fields. The evolutionary perspective on variation also has implications for how we think about "normality" and "abnormality," and about differences in human characteristics. Awareness of variation within populations is a powerful antidote to racism and stereotyping of ethnic and other groups.

Biological diversity. Evolutionary biologists are not only intrigued by the diversity of life, but are also keenly aware of the contributions to biology that come from studying diverse organisms. To be sure, immense advances in biology have come from in-depth studies of "model" organisms" such as yeasts, corn, rats, the bacterium Escherichia coli, and the fruit fly Drosophila melanogaster; indeed, many evolutionary biologists study these model organisms. However, without examining other species, we cannot know how widely applicable the principles revealed by these model systems are--and, in fact, we know that many such principles apply only with modification, or not at all, to vast numbers of other species. Gene regulation, for example, was first elucidated in bacteria, but is very different in eukaryotes. We need to study diverse organisms in order to learn about physiological adaptations to water shortage in desert plants (including potential crops), the mechanisms by which parasites combat their hosts' immune systems, or the evolution of social behavior, communication, or learning in animals such as primates. Different organisms present different biological questions, and some species are more suitable than others for addressing each question.