by Dr. Gary Rosenberg
- What is a Mollusk?
- Molluscan classes
- Naming Names
- Establishing Relationships
- Origins of Diversity
- Classification of the Mollusca
Three closely related terms -- classification, systematics and taxonomy -- are important for understanding how biological groups are recognized and named.
Classification is the distribution of objects into groups. In biology, the objects are organisms (individuals), populations, species, or groups of higher rank.
Systematics is the science of classifying biological diversity. Biologists use various methods for estimating how closely species are related to each other, including comparative anatomy, genetics, and paleontology (the study of fossil organisms). The groups reflect biologists' best estimate of the relationships of the organisms.
Taxonomy is the naming of biological groups. These groups are called "taxa." The singular of "taxa" is "taxon." In the hierarchy of names above, each of the examples is a taxon.
To biologists, similarity per se is not what allows recognition of groups, because not all similarity indicates relatedness. For example, whales and sharks both have fins, but many other features of their anatomy lead us to believe that they evolved fins independently. Biologists attempt to identify similarities due to descent from a common ancestor.
Organisms can have primitive similarity because of distant ancestry, or derived similarity because of immediate ancestry. Derived similarities in particular allow identification of biological groups. For example, both humans and chimpanzees have four limbs and hair, and they nurse their young, but these features do not tell us that humans and chimps are closely related, because all mammals have these features. They are primitive characteristics that reflect distant ancestry. These same features are derived similarities in relation to other groups. Thus having four limbs is a derived feature that separates tetrapods (mammals, birds, amphibians, reptiles and some fish) from all other organisms. Hair is a derived feature that separates mammals from other tetrapods. This means that the immediate common ancestor of all mammals had hair, and no other organism did.
The pattern of derived similarities is used to infer the relationships of the taxa, and is presented in a branching diagram called a cladogram (clados is Greek for branch), which is an evolutionary tree. It can be very difficult to determine whether a given similarity is convergent (as in fish and whale fins), primitive, or derived. This means that it can be very difficult to find the correct tree showing the relationships of a group of organisms. Biologists are continually refining their ideas of the relationships of organisms and, as a result, groups are often redefined or undergo name changes as our knowledge increases.
There is always a temptation to take evolutionary trees as representing progress. People often refer to one organism as primitive and another as advanced, but an organism is never entirely primitive -- it will have some primitive features and some derived ones. The broadest definition of evolution is simply "change over time." Evolution does not necessarily mean progress. A species that is restricted to caves might become blind -- this is change, but if it is progress, it is progress in only a local sense.
The above material has been adapted from Dr. Rosenberg's The Encyclopedia of Seashells, published by Robert Halt, Ltd., London, 1992.