Taxonomy is the study of scientific classification, in particular the classification of living organisms according to their natural relationships. Taxonomy's first father was the philosopher Aristotle (384-322 BC), sometimes called the "father of science." It was Aristotle who first introduced the two key concepts of taxonomy as we practice it today: classification of oranisms by type and binomial definition.
Aristotle was the first to attempt to classify all the kinds of animals in his History of Animals (Historia Animalium in Latin). He grouped the types of creatures according to their similarities: animals with blood and animals without blood, animals that live on water and animals that live on land. Aristotle's view of life was hierarchical. He assumed that creatures could be grouped in order from lowest to highest, with the human species being the highest. Subsequent commentators on Aristotle interpreted this as a "ladder of nature" (scala naturae) or a "Great Chain of Being," but these were not Aristotle's terms. His system of classification was not evolutionary, and the various species on the ladder had no specific genetic relationship to each other. Aristotle regarded the essence of species as fixed and unchanging, and this view persisted for the next two thousand years.
His other innovation was binomial definition. "Binomial" means "two names," and according to this system each kind of organism can be defined by the two names of its "genus and difference." The word "genus" comes from the Greek root for "birth," and among its meanings are "family" and "race." Aristotle's notion of definition was to place every object in a family and then to differentiate it from the other members of that family by some unique characteristic. He defined humans, for example, as the "rational animal." This, according to Aristotelian thought, defines the essence of what it is to be human, as opposed to such pseudo-definitions as "featherless biped."
But what Aristotle did not do was methodically use binomial definition in his system of biological classification. This innovation had to await the development of modern science after the Rennaissance.
Aristotle's influence was profound and long-lasting. Much of his work has not survived to the present day, so that we don't know the details of his study of plants, but his student Theophrastus (372-287 BC) continued it, becoming known as the "father of botany." He is believed to have planted the first botanical garden on the grounds of Aristotle's Lyceum. Most of the text of his two botanical works, On Plants (De Historia Plantarum) and The Causes of Plants (De Causis Plantarum) still exists, although only in Latin translations. The first describes the anatomy of plants and classifies them into trees, shrubs, herbaceous perennials, and herbs. The second work discusses their propagation and growth and served in part as a practical guide to farmers and gardeners. However, he introduced no new principles of classification.
After Aristotle, there was little innovation in the fields of the biological sciences until the 16th century AD. At this time, voyages of exploration were beginning to discover plants and animals new to Europeans, which excited the interest of natural philosophers, as scientists were then called. There was great interest in naming these new species and fitting them into the existing classifications, and this in turn led to new systems of classification. Many of the botanists of this period were also physicians, who were interested in the use of plants for producing medicines.
Andrea Cesalpino (1519-1603) was an Italian physician who created one of the first new systems of classifying plants since the time of Aristotle. He was a professor of materia medica, the study of the preparation of medicines from plants, at the University of Pisa, and was also in charge of the university's botanical garden. There, he wrote a series of works titled On Plants (De Plantis), detailing his system of classification. While his work was in large part based on the work of Aristotle and his successors, his innovation in basing his system of classifying plants on the basis of the structure of their fruits and seeds influenced subsequent scientists such as Linnaeus.
One botanist who was influenced by Cesalpino was Gaspard Bauhin (1560-1620), a Swiss physician and anatomist. In his 1623 Illustrated Exposition of Plants (Pinax Theatri Botanica), he described about six thousand species and gave them names based on their "natural affinities," grouping them into genus and species. He was thus the first scientist to use binomial nomenclature in classification of species, anticipating the work of Linnaeus.
By the time Carl (Carolus) Linnaeus (1707-1778) was born, there were many systems of botanical classification in use, with new plants constantly being discovered and named. This, in fact, was the problem — there were too many inconsistent systems, and the same plant might have several different scientific names, according to different methods of classification.
During his childhood, Linnaeus was so fond of collecting plants that he was known as "the little botanist." He later became a physician, as so many other early taxonomists did, but returned to botany as his primary study.
He published his most innovative work as a young man in 1735. The System of Nature (Systema Naturae) is notable for an overall framework of classification that organized all plants and animals from the level of kingdoms all the way down to species. The full subtitle of its tenth edition was: System of nature through the three kingdoms of nature, according to classes, orders, genera and species, with characteristics, differences, synonyms, places. This system of classification, although greatly modified, is essentially the one we use today.
Linnaeus followed this work with The Genera of Plants and The Species of Plants, setting out a system of plant classification based on the structure of flower parts, in which he was influenced by Cesalpino. This method, in which plants were grouped together according to the number of stamens in their flowers, for example, was not accurate, but it was easy to use and thus readily adapted by scientists who were continually discovering more new varieties of plants. Linnaeus himself undertook much work in the field, and he was even more influential through his students, whom he sent around the world to gather specimens.
His major works went through a great deal of revision in his lifetime, eliminating errors and coming closer to the system that was eventually adopted by taxonomists worldwide. His methods of classifying plants have been completely superseded by a deeper scientific understanding. Originally, Linnaeus had only used binomial nomenclature to classify plants, but he later extended this system to include animals and even minerals. There were also errors, subsequently corrected. At first, for example, he had placed the whales among the fishes, but later moved them into the mammals. He was also the first taxonomist to place humans among the primates (or Anthropomorpha) and to give them the binomen Homo sapiens.
If Linnaeus is now considered the father of taxonomy, his success rested on the work of his predecessors. He was the first, in his System of Nature, to combine a hierarchical system of classification from kingdom to species with the method of binomial nomenclature, using it consistently to identify every species of both plants and animals then known to him.
While he continued throughout his lifetime to revise and expand this great work, so his successors have continued to revise the principles of taxonomy, now according to genetic principles, informed by the analysis of DNA. So it always is with science: we stand on the shoulders of our predecessors, always reaching higher.
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. Thousands more are added to the list every year. Over the last half century, scientific estimates of the total number of living specieshave ranged from 3 to 100 million. The most recent methodical survey indicates that it is likely to beclose to 9 million, with 6.5 million of them living on the land and 2.2 million in the oceans. Tropical forests and deep ocean areas very likely hold the highest number of still unknown species. However, we may never know how many there are because it is probable thatmost will become extinct before being discovered 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, given that 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 interesting 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 nonspeculative 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.
One of the most important 18th century naturalists was a Swedish botanist and medical doctor named Karl von Linn�. 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 modern humans in his system with the binomen Homo sapiens, or "man who is wise". Homo is our genus and sapiens is our 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.
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.
Copyright � 1998-2012 by Dennis O'Neil. All rights reserved.