Comparative Biology

Under this scientific method, biologists formulate hypotheses, or predictions, from an existing body of knowledge and then test their hypotheses through experiments. Experiments range from simple to complex, and can be performed on a computer, in a laboratory setting, or outdoors. Technological developments during the twentieth century—including high-speed computing, DNA sequencing, and a wide array of visualization techniques— have opened the door to many exciting lines of biological investigation. Biologists are constantly developing new techniques to test increasingly complex questions, and have even designed experiments to re-create natural events such as hurricanes, forest fires, and floods.

The scientific method can be applied to many, but not all, types of scientific inquiry. Experimental methods cannot directly test hypotheses concerning the processes of evolution because these events took place over the course of millions of years, under environmental conditions that are as difficult to define as they would be to recreate. Biologists must therefore rely on a comparative method to deduce how evolutionary events created the patterns of animal diversity that exist today.

One such pattern involves how animals living in similar environments have evolved similarities in particular traits. For centuries, biologists have been interested in how such characteristics adapt the animals to their surroundings and ecological role. A biologist studying the adaptive significance of a morphological trait, such as fur coloration, will look at animals living in similar environments to search for patterns linking this trait with environmental factors such as plant types and density.

A comparative framework can be used while looking at different types of traits, whether they are genetic , morphological , behavioral , or ecological in nature. In setting up a comparison, the biologist must be familiar with the evolutionary relationships of the animals in question. For many groups of organisms, these relationships have been described through a branch of biology called phylogenetics. The product of a phylogenetic analysis is called a phylogeny , which is a hypothesis about relationships between organisms. Phylogenies can be constructed using a combination of genetic, morphological, and behavioral traits. These phylogenies can describe relationships at various levels: gene, species, genus, and so on.

After the biologist selects the level at which she will make a comparison, she uses a number of criteria, or standards, to decide whether the structure to be examined in each organism is homologous . When judging morphological structures to be homologous, criteria may include their position and developmental origin. Function is not a reliable indicator of homology because similar functions may be formed by dissimilar structures (e.g., a bird's wing as opposed to a bat's wing). Such structures would be the result of convergent evolution, and would be called analogous rather than homologous (same function, different structure). The criteria differ for judging homology in other types of traits. For example, judging homology in behavioral traits would require examination of genetic origins and behaviors that may represent a transition between two behaviors that are of interest.

When the biologist maps homologous traits onto the phylogeny and examines the evolutionary relationships between groups sharing similar traits, the patterns revealed may provide clues about how various traits evolved. Biologists may examine, for instance, the correlation, or connection, between the presence of the trait and the environmental or genetic factors that may cause this trait to be expressed. The scientist uses statistical methods to determine whether or not the correlation he has found between the trait of interest and the factors occurred as the result of a random process. If the scientist determines that the patterns were not created randomly, then he concludes that the trait is an adaptation.

A comparative framework is invaluable while studying evolutionary relationships of various animals, and while looking at how traits evolved. However, the comparative method is also useful in cases where the investigator does not need to create a historical, or evolutionary, context. If a biologist is interested in only the function of a particular structure, and not in how it evolved, she may decide to make comparisons of the same structure in more distantly related animals.

For example, a biologist interested in the functions of morphological traits might be interested in how flight structures differ in birds and bats. He knows that wings evolved independently in birds and bats because a published phylogeny indicates that many bird and bat ancestors did not fly. Rather than assume that wings evolved in the most recent common ancestor of bats and birds, and was subsequently lost later on in many groups of reptiles and mammals (the closest living relatives to birds and bats), it is assumed that wings evolved twice. The functional morphologist uses the knowledge that bird and bat wings evolved independently to help direct future research.

see also Adaptation; Biological Evolution.

Judy P. Sheen

Bibliography

Brooks, Daniel R., and Deborah A. McLennan. Phylogeny, Ecology, and Behavior: A Research Program in Comparative Biology. Chicago: University of Chicago Press, 1991.