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Natural Science - Year I

Unit 11: Aristotle and Classification

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Science Web Assignment for Unit 11

This Unit's Homework Page History Lecture Science Lecture Lab Parents' Notes

Science Lecture for Unit 11:
Classification Schemes in Biology

For Class

Studying Life on Earth

We turn now to the study of living things. The most immediate problem that faced early biologists like Aristotle was collecting and sorting through their information. Aristotle started by distinguishing between living and non-living matter. Terrestrial matter has four characteristics, each with its natural motion. Aristotle noted that in most cases, living matter has the ability to propel itself. With that distinction in mind, he proceeded to devise a classification system to help him identify common characteristics among all the specimens that he examined. Biologists still use classification schemes to organize the immense amount of material we have on living organisms. This means that in order to study modern biology, we need to understand some of the problems and limitations of classification and modern classification schemes. There are three major classification approaches. Two of these are based in evolution speculations, and attempt to classify different types of animals by studying their common characters, particularly in DNA sequences, in order to determine common ancestry. The third, cladistics, attempts to group animals by identifying various characteristics, then rating animals and plants by whether or not they possess these characteristics. According to this scheme, organisms with common characteristics can be grouped together, but the groupings does not necessarily imply common ancestry (logically they don't preclude it, either).

Get an overview of the history of the classification system used by most biologists by reading the introduction to the concepts of classification (you need to only read the one link).

Classification Schemes

Now look more closely at some of the problems inherent in creating a good classification scheme. Read The nuts and bolts of classification. (one link) from Miami University.

As you read, think about these questions:

  • What is a classification system? How is it used?
  • Is there only one way to classify a group of objects with several characteristics each?
  • What are some common classification systems that you use every day?
  • What are the different levels of the Linnaean classification system for organisms?
  • How many species have biologists already classified?
  • How many species do some biologists estimate exist?
  • Which group has the most different species?
  • What are the rules of uniqueness, universality, and stability? How does each rule help maintain the usefulness of the classification system?
  • What do we mean by "binomial nomenclature"?
  • What is a taxon?
  • What is a species?
  • What is a genius?
  • What assumptions do those who use evolution make about species that belong to the same genus?
  • Look at the exercise and consider how you would classify the objects in the sample.

Now that we have established the basics for classifying animals and plants, let's look more closely at three major groups. This week, we concentrate on vertebrates. Next week, we will cover invertebrates, and in the following week, we will get to plants. (Obviously, we have left single-celled animals out of this part of the course but that's because we will study them in the context of the 16th century, when hand lenses strong enough to see these life forms were first used.)


In most life science courses, we would start our survey of organisms with the simplest: kingdom Monera, or the bacteria (which are distinguished from all other organisms because they have a special cell membrane and do not have a cell nucleus), and all the other single-celled animals (like algae, amoeba, and paramecia) the Protista. We would then go on to the more complicated Fungi (mushrooms), and then study plants, and finally get to the multicellular organisms with complex systems, the animals. But this approach assumes that we already know all the information and can organize it any way we like.

In this class, we are trying to understand not only the matter of current scientific theory, but the origins of these theories, so we will start with the animals which interested the early biologists: man (we have already mentioned Egyptian surgery and the Greek "father of medicine", Hippocrates) and the larger animals which he hunted or domesticated.

Traditionally, the vertebrate animals have been divided into 5 groups. These groupings are based on structural considerations, such as the complexity of the circulatory system or the nervous system, and on reproduction processes.

Notice that these are arranged in increasing order of complexity. Fish, amphibians, and reptiles are all cold-blooded and generally slow-moving, and have simple circulatory systems (two or three chambered hearts and low blood pressure) which do not need to regulate body temperature or provide increased circulation for long periods of activity. In contrast the warm-blooded vertebrates, birds and mammals, have 4-chambered which not only provide enough oxygenated blood to keep the body temperature stable through a range of external temperatures, but also supply enough blood on demand to support the movement of flying or running long distances.

Look up the five classes of vertebrates in an encyclopedia or use a web resource such as Biology for Kids to find out the basic differences between fish, amphibians, reptiles, birds, and mammals.

Study/Discussion Questions

Further Study On your Own (Optional)