- 1 Basis of Classification – Introduction, Importance and Biological System of Classification
One of the interesting Biology Topics is the study of animal behavior and how it is influenced by genetics and the environment.
Basis of Classification – Introduction, Importance and Biological System of Classification
The diversity shown among present-day living organisms is an outcome of the past 3.5 billion years of organic evolution. During this time a great number of new species originated and many more became extinct. Science of paleontology studies evidence of past life, in the form of fossils. With such a huge repository of organisms, it would be almost impossible to study each and every living form separately at an individual level. So, to study the diversity among living organisms in an effective way, we need to arrange various kinds of organisms in an orderly or systematic manner.
The method of arranging organisms into groups or sets on the basis of similarities and differences is called classification. Similarities and differences of organisms show their relationships. In fact, to understand the living world, there is no need to know every detail about all living organisms. Studying only a few representatives from each group would give an idea of all the members of that group.
Importance of Classification
The science of classification is known as taxonomy. It is an important branch of biological science. Classification of living organisms has the following advantages:
- Classification makes the study of a wide variety of organisms easy.
- Classification projects before us a picture of all life forms at a glance.
- Classification is essential to understand the interrelationships among different groups of organisms.
- Classification forms a base for the development of other biological sciences.
For example, biogeography is the study of the geographical distribution of plants and animals totally dependent on the information supplied by classification.
Taxonomy or systematics is the study of the diversity and kind of organisms and the evolutionary relationships among them. It includes the naming and arrangement of organisms into a classification and an examination of their origin and history. The systematic biologist (or taxonomist) tends to deal with information that can be obtained from museum specimens and also from life-history studies, cytology, biochemistry, and physiology of the organisms. The method of systematically arranging the different species of living organisms into closely related groups according to the scheme of likeness and differences is called classification.
Evolution and Classification of Organisms on Earth
Living things are identified and categorized on the basis of their body design in relation to their form and function. Some characteristics are likely to make more wide-ranging changes in body design than others. There is a role of time (for evolution) in this as well. In other words, characteristics that came into existence earlier are likely to be more fundamental than characteristics that have come into existence later.
So, the classification of life forms is closely related to their evolution. What is evolution? Most life forms that we see today have arisen by an accumulation of changes in its body design that allow the organism possessing them to survive better. Charles Darwin (1809-1882) first described this idea of evolution in his book ‘The Origin of Species’, published in 1839.
In 1985, Professor Ernst Mayr of Harvard University argued, that Darwinism should be viewed as five major theories:
1. Perpetual change: This is the basic theory of evolution on which the other theories are based. It states that the living world is neither constant nor perpetually cycling, but it always changes. The properties of organisms undergo transformation across generations, throughout time.
2. Common descent: The second Darwinian theory, “common descent”, states that all forms of life descended from a common ancestor through a branching of lineages.
3. Multiplication of species: Darwin’s third theory states that the evolutionary process produces new species by the splitting and transformation of older ones.
4. Gradualism: Gradualism states that the large differences in anatomical traits that characterize different species originate through the accumulation of many small incremental changes over very long periods of time.
5. Natural selection: Natural selection is Darwin’s most famous theory. It rests on three propositions.
- There is variation among organisms (within populations) for anatomical; behavioral and physiological traits.
- The variation is at least partly heritable so that offspring tend to resemble their parents.
- Organisms with different variant forms leave different numbers of offspring to future generations.
If the idea of evolution is connected to classification, it becomes apparent that some groups or organisms with ancient body designs have not changed much. There are other groups of organisms that have evolved their current body designs, relatively recently. Those in the first group (i.e., with ancient body designs) are commonly referred to as “primitive” or “lower” organisms, while those in the second group (i.e., with recent body designs) are called “advanced” or “higher” organisms. In other words, it can be said that first-formed (ancient, older) organisms are simpler, while younger organisms (later formed, modern) are more complex, i.e., complexity in the design of simpler forms has increased over evolutionary time so that they have ultimately become more complex.
Artificial and Natural System of Classification
Biological classification goes back to Aristotle (384-322 B.C.) and Theophrastus (372-287 B.C.) in Greek times. Early systems of classification were artificial, i.e., they were based on a few arbitrarily chosen criteria such as size, colour, or nature of the organism.
Aristotle classified living organisms based on their nature. He divided animals according to whether they lived on land, sea, or air. Likewise, Theophrastus categorized all known plants on the basis of their form, life span, and habitat. He grouped plants into four categories: trees, Figure 4.1. Carl von Linnaeus, shrubs, undershrubs, and herbs.
A Swedish scientist Carl von Linnaeus (1707 – 1778) published a book in 1735, called Systema Naturae (Classification of Nature). In this book, he classified, described, and named plants, animals, and minerals. He based his classification scheme on the sexual characteristics of living organisms (i.e., the number and arrangement of stamens and carpels in the flower). Thus, his classification was also an artificial one.
In the 19th century, the artificial system of classification was replaced by natural systems of classification. The new system was based on natural affinities between living organisms and indicated the overall similarities and differences between the organisms.
Carolus Linnaeus (Carl von Linne) was bom on May 23, 1707, in Rashult, Sweden, and was a doctor by profession. He was also a naturalist and botanist attached to the University of Uppsala. He died on January 1778. He had a great talent for collecting and classifying objects, especially flowers. At the age of 22, he published his first paper on the sexuality of plants. While serving as a personal physician of a banker George Clifford, he studied the diversity of plants in his employer’s garden.
Later he published 14 papers and also brought about three important books. The first book Systema Naturae was released in 1735 and from it all fundamental taxonomical research has taken off. He used morphology, i.e., the comparative study of organismal forms. His system of classification was a simple scheme for arranging plants so as to be able to identify them again. In 10th edition of this book (1 758), the binomial system was applied to the animals.
Linnaeus introduced the binomial system of biological nomenclature, in which organisms are classified as species, and grouped into genera, rather than concise Latin descriptions, the methods used earlier. His system facilitated the identification and cataloging of plants and animals and proved very popular, though it was based on superficial features rather than evolutionary relationships and so was unnatural.
His second book, Genera Plantarum was published in 1737, in which descriptions of several genera were given. His third book, Species Plantarum (1753) in two volumes is still the official starting point for botanical nomenclature, and many of his names are still used. Because his knowledge of animals was limited, his lower categories, such as genera, were very broad and included animals that are very distantly related. Much of his classification has been drastically altered now, but the basic principles in his scheme are still followed.
Biological Systems of Classification & Branches of Taxonomy
Living organisms have been classified variously according to different criteria, however, the following two systems are most in use:
Two-kingdom System of Classification:
The differences between plants and animals are very clear. Plants are stationary (fixed), trap the radiant energy of the sun to make their food, and some plants even grow indefinitely. Animals generally move, eat plants or animals as food, and their bodies stop growing after attaining a certain size. In fact, plant cells are surrounded by a cell wall and contain chloroplasts to prepare their own food (photosynthesis). Animal cells neither have a cell wall nor chloroplasts. Because of these basic differences, plants and animals are divided into two groups – The plant kingdom and the Animal kingdom. This system of classification was first suggested by Carolus Linnaeus in 1758.
Later taxonomic studies indicated that certain organisms did not strictly fit either under the Plant kingdom or the Animal kingdom. Accordingly, a German Zoologist, E.H. Haeckel (1866) raised a third kingdom, Protista, for unicellular organisms. When significant differences became evident among the protists, an American ecologist, Robert H. Whittaker (1959) proposed a fourth kingdom, Monera, for the bacteria which are prokaryotes, and fifth, kingdom Fungi for the fungi which lack chlorophyll, and obtain their food through absorption.
Here it is clear that the classification of organisms is a progressive (ongoing) process. As more and more data on life forms become available, classification becomes more and more refined. In this chapter, we will follow a two-kingdom system of classification.
Five-kingdom System of Classification:
Whittaker (1959) has classified living organisms into the following five kingdoms (= broad categories):
- Kingdom Monera (Prokaryotic bacteria and blue-green algae).
- Kingdom Protista (Unicellular eukaryotic organisms-protozoans, fungi, and algae).
- Kingdom Fungi (Multinucleate higher fungi).
- Kingdom Plantae (Multicellular green plants and advanced algae).
- Kingdom Animalia (Multicellular animals).
Whittaker based his classification scheme on the following three levels of organization: 1. The prokaryotic versus eukaryotic structure of cells, 2. The unicellular versus multicellular or multinucleate organisation, and 3. The three different modes of nutrition: are photosynthesis (plants), absorption from the environment (fungi), and ingestion (animals). Carl Woese (1977) further divided Kingdom Monera into Archaebacteria (or Archaea) and Eubacteria (or Bacteria).
Modern Scheme of Five-Kingdom Classification
In 1982, Margulis and Schwartz revised the five-kingdom classification. It includes one prokaryotic kingdom – the Prokaryotae and four eukaryotic kingdoms the Protocista, the Fungi, the Plantae, and the Animalia. This scheme is widely accepted. The eukaryotes can be regarded as belonging to a super kingdom the Eukaryotae.
The kingdom Protoctista is a collection of all the eukaryotic organisms that do not fit neatly into the other three eukaryotic kingdoms. Many of these organisms are unicellular. The Protocista contains eukaryotes that are generally regarded as being identical or similar to early plants (Algae), early animals (Protozoa) and early fungi (Oomycota or water molds). The kingdom also includes a group known as slime molds which have spores like fungi but can creep slowly over surfaces and are therefore motile like the animals.
One group of ‘organisms’ that does not fit neatly into any classification scheme is viruses. Viruses are extremely small particles consisting of only a piece of genetic material (PNA or RNA) in a protective coat of protein. They do not have a cellular structure, unlike all other organisms, and can only reproduce by invading living cells.
The smallest organisms are often collectively known as microorganisms. These include archaea and bacteria (prokaryotes), viruses, fungi and protists. These groupings are useful for practical reasons because the techniques used in their study are often similar. For example, a microscope is needed to view them and sterile (i.e., aseptic) techniques are needed to culture them. The study of microorganisms forms a branch of biology, celled microbiology.