Introduction to Classification

In this tutorial you will be learning about the Linnaean System of Classification used in the biological sciences to describe and categorize all living things.  The focus is on finding out how humans fit within this system.  In addition, you will discover part of the great diversity of life forms and come to understand why some animals are considered to be close to us in their evolutionary history.

How many species are there?

This is not an easy question to answer.  About 1.8 million have been given scientific names.  Nearly 2/3 of these are insects.  Estimates of the total number of living species generally range from 10 to 100 million.  It is likely the actual number is on the order of 13 to 14 million, with most being insects and microscopic life forms in tropical regions.  However, we may never know how many there are because many of them will become extinct before being counted and described.

The tremendous diversity in life today is not new to our planet.  The noted paleontologist Stephen Jay Gould estimated that 99% of all plant and animal species that have existed have already become extinct with most leaving no fossils.  It is also humbling to realize that humans and other large animals are freakishly rare life forms, since 99% of all known animal species are smaller than bumble bees.

Why should we be interested in
learning about the diversity of life?

In order to fully understand our own biological evolution, we need to be aware that humans are animals and that we have close relatives in the animal kingdom.  Grasping the comparative evolutionary distances between different species is important to this understanding.  In addition, it is fun to learn about other kinds of creatures.

When did scientists begin classifying living things?

Before the advent of modern, genetically based evolutionary studies, European and American biology consisted primarily of taxonomy, or classification of organisms into different categories based on their physical characteristics and presumed natural relationship.  The leading naturalists of the 18th and 19th centuries spent their lives identifying and naming newly discovered plants and animals.  However, few of them asked what accounted for the patterns of similarities and differences between the organisms.  This basically non-speculative approach is not surprising since most naturalists two centuries ago held the view that plants and animals (including humans) had been created in their present form and that they have remained unchanged.  As a result, it made no sense to ask how organisms have evolved through time.  Similarly, it was inconceivable that two animals or plants may have had a common ancestor or that extinct species may have been ancestors of modern ones.

  painting of Carolus Linnaeus as a middle aged gentleman
Carolus Linnaeus

One of the most important 18th century naturalists was a Swedish botanist and medical doctor named Karl von Linné.  He wrote 180 books mainly describing plant species in extreme detail.  Since his published writings were mostly in Latin, he is known to the scientific world today as Carolus Linnaeus, which is the Latinized form he chose for his name.

In 1735, Linnaeus published an influential book entitled Systema Naturae in which he outlined his scheme for classifying all known and yet to be discovered organisms according to the greater or lesser extent of their similarities.  This Linnaean system of classification was widely accepted by the early 19th century and is still the basic framework for all taxonomy in the biological sciences today.

The Linnaean system uses two Latin name categories, genus and species, to designate each type of organism.  A genus is a higher level category that includes one or more species under it.  Such a dual level designation is referred to as a binomial nomenclature  or binomen (literally "two names" in Latin).  For example, Linnaeus described humans in his system with the binomen Homo sapiens, or "man who is wise"--Homo is our genus and sapiens is our species.

genus genus
  species     species     species     species  

Linnaeus also created higher, more inclusive classification categories.  For instance, he placed all monkeys and apes along with humans into the order Primates.  His use of the word Primates (from the Latin primus meaning "first") reflects the human centered world view of Western science during the 18th century.  It implied that humans were "created" first.  However, it also indicated that people are animals.

family family
genus genus genus genus
  species     species     species     species     species     species     species     species  
  Photo of Charles Darwin

Charles Darwin

While the form of the Linnaean classification system remains substantially the same, the reasoning behind it has undergone considerable change.   For Linnaeus and his contemporaries, taxonomy served to rationally demonstrate the unchanging order inherent in Biblical creation and was an end in itself.  From this perspective, spending a life dedicated to precisely describing and naming organisms was a religious act because it was revealing the great complexity of life created by God.

This static view of nature was overturned in science by the middle of the 19th century by a small number of radical naturalists, most notably Charles Darwin.  He provided conclusive evidence that evolution of life forms has occurred.  In addition, he proposed natural selection as the mechanism responsible for these changes.

Late in his life, Linnaeus also began to have some doubts about species being unchanging.  Crossbreeding resulting in new varieties of plants suggested to him that life forms could change somewhat.  However, he stopped short of accepting the evolution of one species into another.

Why do we classify living things today?

Since Darwin's time, biological classification has come to be understood as reflecting evolutionary distances and relationships between organisms.  The creatures of our time have had common ancestors in the past.  In a very real sense, they are members of the same family tree.

The great diversity of life is largely a result of branching evolution or adaptive radiation.  This is the diversification of a species into different lines as they adapt to new ecological niches and ultimately evolve into distinct species.  Natural selection is the principal mechanism driving adaptive radiation.

illustration of evolution resulting from adaptive radiation


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