- Biology has been defined as the study of living things (living organisms). Even though all living things share similar characteristics discussed in the introductory chapter, the living things exhibit a lot of differences. In particular, animals and plants are all living things yet they differ in many aspects. Amongst animals and plants also there exist a lot of differences. There are millions of different plant and animal types exhibiting a range of differences. This created a need for a classification system of living things to make study of the living organisms easier.
- Classification refers to the grouping of living organisms according to their structure.
- In classification, organisms that share a lot of similarities are placed under one group referred to as a taxon (plural= taxa).
- Other than the similarities, grouping of the organisms also takes into account the evolutionary relationships (phylogeny) of the organisms. It is believed that all organisms once had a common ancestor (theory of evolution). During classification, organisms that are believed to have evolved along the same line of evolution are placed in one taxon.
- The scientific study of classification is known as taxonomy. A biologist studying taxonomy is a taxonomist.
- In classifying organisms taxonomists to a great extent rely on the use of external observable features of organisms.
External features of plants used in classification
- The rhizoids as in moss plant
- Fronds in ferns
- The type of root; tap root, adventitious, fibrous, prop, buttress roots.
- Stem presence and type.
- Presence or absence of flowers
- Type of leaves; simple or compound; leaf venation- parallel or net work veined.
- Presence and types of fruits and cones.
External features of animals used in classification
- Tentacles in hydra
- Body covering- feathers, scales, hair or fur
- Shells in snails
- Wings in birds
- Proglotids in tapeworms
- Mammary glands in mammals
- Locomotory structures
- Body pigmentation
Importance of Classification
- Classification systems improve our ability to explain relationships among things.
- Classification helps in identifying living organisms into their correct groups for reference.
- Classification helps in avoiding chaos and confusion when dealing with animals as it arranges the information on organisms in an orderly manner.
- Classification brings together living organisms with similar characteristics together but separate those with different features.
- Taxonomic systems provide relatively stable, unique, and unequivocal names for organisms
- It makes the study of such a wide variety of organisms easy.
- It projects before us a good picture of all life forms at a glance.
- It helps us understand the interrelationship among different groups of organisms.
- It serves as a base for the development of other biological sciences such as biogeography etc.
- Various fields of applied biology such as agriculture, public health and environmental biology depend on classification of pests, disease vectors, pathogens and components of an ecosystem.
Historical background of Classification
- In the past, scientists used to classify organisms based on personal conveniences. They heavily relied on very few observable features. There was no standard classification system as each and every scientist would classify organisms in a way that would suit his intentions.
- In particular, living organisms were simply classified as plants or animals. The plants were also classified as herbs, trees, shrubs. Animals were also grouped into herbivores, carnivores and omnivores.
- Some biologists would also classify plants as:
- Edible or non edible
- Flowering or non-flowering
- The modern classification systems take into account the evolutionary relationships between living organisms. It has overcome the many weaknesses of the artificial (traditional) classification systems.
- From the original parents, arose new groups of organisms that went under structural changes that enabled them to live in different habitats. The structural changes account for the great diversity of living organisms observed today.
- Carolus Linnaeus, a Sweddish biologist is largely credited for his significant contribution to the development of the modern classification system.
Taxonomic Units of Classification
- Taxonomic units of classification refer to the groups or taxa into which organisms are placed as a matter of convenience.
- Grouping of organisms in these groups is based on easily observable characteristics that are common in that group.
- Living organisms which share a lot of characteristics are placed in the same group.
- Each taxonomic unit reflects the position of an organism in relation to the others in the classification scheme.
- In a classification scheme, a hierarchy of groups is recognized and it proceeds from the first largest and highest group, the kingdom to the smallest and lowest unit, the species.
- There are seven taxonomic units of classification.
All living organisms are classified into five major kingdoms:
- Kingdom Monera– This is composed of microscopic unicellular organisms mainly bacteria e.g amoeba.
- Kingdom Protoctista– This kingdom is comprised of members who are microscopic. Though, some are large enough to be seen with the naked eyes. Members of this kingdom include algae and protozoa.
- Kingdom Fungi– Members of this kingdom comprises the mushrooms, toadstools, moulds and yeast.
- Kingdom Plantae– This kingdom comprises the moss plant, ferns, maize plants, hibiscus, meru oak tree etc.
- Kingdom Animalia – Members of this kingdom include the tapeworms, hydra, fishes, human beings, lizards, earthworms etc.
In hierarchy of classification, a kingdom is further divided into several phyla (plural of phylum) or divisions (in plants). Within the phyla or divisions, organisms are further sorted out into groups known as classes based on their similarities and mode of life. Each class is further subdivided into small groups called orders based on structural similarities. Orders subdivide into families which subdivide into genera (plural for genus).Genera are then subdivided into smaller units of classification called the species.
Species is the smallest unit of classification whose members share many similarities and can freely interbreed to give rise to fertile or viable offsprings.
Members of a particular species can, however, exhibit various differences e.g. differences in skin colour or body forms. Within the species, organisms can further be classified based on the differences in colour or forms. In humans, this gives the races, in animals the term used is breed while in plants, variety is preferred. In bacteria, the term strain is used to describe the variant forms.
Members of different but very closely related species can breed but the resulting offspring will be sterile (infertile). In particular, a mule is a sterile offspring between a horse and a donkey. Moving from kingdom to species, it is important to note that the number of organisms in each taxon decreases. The similarities, however, increase as one moves from kingdom to species.
Scientific Naming of Living Organisms
- Scientific naming involves assigning an organism two names in Latin language. The naming system was developed by Carolus Linnaeus in the 18th
- Organisms always have common names and scientific names. Common names are local names by which the organisms are known in the vernacular languages. In particular, a cat is an English name, mbura is a luo name, paka is a Swahili name etc. these names differ across cultures and cannot be used by scientists to communicate across the world. This makes sharing scientific knowledge on organisms very difficult. There was need for a common language and this led to development of scientific language in latin.
- Latin was the preferred language since it was the first language of civilization that was widely spoken at that time. Similarly, latin language is a dead language hence not subjected to a lot of changes. The scientific names are, therefore, static.
- Scientific names are the valid names by which organisms are known all over the world.
- In scientific naming, an organism is assigned a specific name that is unique. The specific name adopts two names. This implies that the specific scientific name of an organism has two names. This double naming system is known as binomial nomenclature.
- In binomial nomenclature, an organism is assigned its genus name and species name.
- Assigning of scientific names to living organisms is governed by a definite set of rules which are internationally recognized and referred to as binomial nomenclature which literally means the rule of double naming system.
Rules of Binomial Nomenclature
Binomial nomenclature requires that:
- The first part of the scientific name is that of the genus name which should begin with a capital letter. The second name is that of species. The species name should be written in small letters e.g.
- Maize- Zea mays
- Lion- Panthera leo
- Leopard- Panthera pardus
- Domestic dog- Canis familiaris
- Human being- Homo sapiens
- When printed in books and other printed works, the scientific names should be printed in italics. However, in handwritten manuscripts and typed works, the genus and species names should be lined separately.
Printed work- Homo sapiens
- The specific name is frequently written with the name of the scientist who first adequately described and named the organism e.g. Balanus balanoides
- Scientists must give a latinised name for a newly described animal or plant species where a Latin name is missing e,g.
Aloe kilifiensis– A type of aloe found in kilifi
Meladogyne kikuyuensis– A nematode found in kikuyu.
Origin of scientific names
Scientific names assigned to organisms can be: