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A species is a genetically distinct group of organisms, which can interbreed and produce fertile, viable offsprings. External similarity is the common basis for identifying individuals as being members of the same species. Yet a bulldog, a terrier, and a golden retriever are very different in appearance, but they are all dogs because they can interbreed. People can also interbreed with one another, and so can cats, but people cannot interbreed with dogs or cats, nor can these with each other. This shows that individuals of a species are able to interbreed with one another but not with members of other species.

Even if they could interbreed with members of other species they would not be able to produce fertile offsprings. For example, when horses and donkeys mate, they produce mules. However, these mules are infertile. They are unable to breed at all and bear offsprings of their own. Taxonomy is the branch of biology that deals with the identification and naming of living organisms. The Swedish biologist, Carolus Linnaeus (1707 – 1778) established a simple system for classifying and naming organisms. He developed a hierarchy, or in other words a ranking system for classifying organisms, that is the basis for modern taxonomy.

Linnaeu’s classification hierarchy included five levels: kingdom, class, order, genus, and species. Modern scientists have added two more classification levels to this, phylum and family. The current classification system divides all organisms into seven major categories, called taxa (singular: taxon). The categories are as follows: kingdom, phylum, class, order, family, genus and species. The modern classification system is based on increasingly more specific categories. The classification levels become more specific towards the bottom of the hierarchy.

Many organisms belong to the same kingdom. Fewer belong to the same phylum, and even fewer belong to the same class and so on with species being the most specific classification. The categories from most general to most specific are kingdom, phylum, class, order, family, genus, and species. Each level groups together organisms that share more characteristics with each other. To ensure accurate communication of information, biologists have assigned a two-word scientific name to each organism. This is called Binomial Nomenclature.

Nomenclature is a term that refers to a system for naming organisms. In biology the system is “binomial”, meaning that two names are used to specifically identify every organism – genus and species. The binomial system uses a two word Latin name. For example, the binomial name for humans is Homo sapiens. Homo, referring to the genus and sapiens referring to the species. Another example is the binomial name for the red maple tree, which is Acer rubrum. Acer is the Latin name for maple, referring to the genus, and rubrum is the Latin name for red referring to the species.

The genus name refers to the relatively small group of organisms to which a particular type of organism belongs. The species name is usually a Latin description of some important characteristic of the organism. The genus name is always capitalized while the species name is not. This system was designed to eliminate the use of common names and confusion in the scientific world. For example, human beings are known as Homo sapiens. As I have mentioned earlier, Homo refers to the genus while sapiens refers to species. Fossils of other members of our genus such as Homo erectus and Homo habilis have been found.

They have enough similarities to us to be in the same genus, but are different enough to not be the same species. Going further up the ladder, genus Homo is part of the family Hominidae. This family belongs to the order of Primates, as do chimps, gorillas and monkeys. Primates are part of the class Mammalia, the mammals, which belongs to the phylum Chordata, creatures with spinal cords, which is a part of the kingdom Animalia, the highest and most general level. The classifications also tell something about the degree of relation between different organisms.

For example, two animals that belong to the same family and genus are more closely related than two animals that simply belong to the same family. This is one of the main functions of the classification system. Another function of the classification system is to allow biologists to group organisms according to similar characteristics. This organization helps biologists study living organisms and to make predictions about their relationships, including structural information, biochemical information, cytological information, embryological information, behavioral information and fossil information.

When placing an organism into a Taxonomic Category, modern taxonomists may consider all these characteristics. The five kingdoms in Whittaker’s system of classification are the monera, protista, fungi, plantae and animalia. The monera kingdom consists of bacteria and blue-green algae. Bacteria are unicellular prokaryotes; this means that they do not have any membrane-bound organelles such as a nucleus. This separates them a great deal from eukaryotic organisms, such as plants and animals- whose cells contain a nucleus.

Therefore bacteria are placed in their own kingdom know as the monera kingdom. Bacteria can be both autotrophs and heterotrophs and include the disease-causing bacteria. There are a number of ways bacteria can be classified. These include the way they produce energy, their shapes and their staining properties. Bacteria are classified by their physical form and shape. For example spherical shaped bacteria are known as cocci, rod-shaped bacteria are known as bacilli, spiral shaped bacteria are know as spirillum and comma shaped bacteria are known as vibrio.

Even though bacteria are unicellular, they may not always appear as single cells. Often, they are found in clumps, groups or chains. Bacteria are identified and named by their shapes and the way that they appear in groups. Scientists working in the field of microbiology also classify bacteria according to whether or not they need oxygen to survive. Bacteria that depend on oxygen to live are known as aerobic bacteria. Those that do not require oxygen are know as anaerobic bacteria. Aerobic bacteria are more advanced. They can oxidize things such as glucose and nitrates to obtain their energy.

On the other hand anaerobic bacteria can burn up things such as methane and they can oxidise sulfur molecules and hydrocarbons to get their energy. They may live in mud where there is no oxygen. Bacteria are also classified according to their staining properties. Microbiologists of this high-tech age are still grateful to Danish physician Christian Gram, who invented the gram-staining method in 1884. To gram stain, one must smear a sample of bacteria on a slide, soak it in a violet dye and then treat it with iodine. The slide should then be rinsed with alcohol and counterstained with a pink dye called safranine.

The cell walls of gram-negative bacteria have a very low affinity for the violet stain, which is rinsed out by the alcohol. Once counterstained with safranine, however, the gram-negative bacteria appear bright pink to red. Gram-positive cell walls have a high affinity for the violet stain, and retain it even after they have been rinsed with the alcohol. When the process is complete, they appear dark purple to brown. The difference between the two cell types appears to be in the amount of peptidoglycan in the cell wall. For many years, bacteria were only classified according to their shape and staining properties.

These characters can still be useful in the early stages of identification, but more recent advances in DNA and RNA sequencing can give us a more accurate idea of origins and relationships among these organisms and help us to classify them. The second kingdom is known as kingdom protista. All protists are eukaryotes. They include mostly unicellular eukaryotes but there are also a few samples of multicellular ones. Eukaroyotic cells have nuclei and organelles that are surrounded by membranes. Eukaryotic DNA is contained within a membrane-bound nucleus.

The cells of multicellular protists are not specialized to perform specific functions in the organisms. Protists include organisms such as Euglena, Amoebas and Plasmodia. Some hunt for food like animals, and others can make their own food from sunlight using photosynthesis, just like plants. Protists are mainly classified according to the way they move; generally according to whether they use flagella, cilia or pseupodia. For example Euglena uses flagella to move about. It moves about by spinning. The Paramecium has thousands of hair-like cilia, which it beats in a co-ordinated fashion.

Last but not least the amoeba moves about through the extension of pseudopdia. However, for the pseudopodia to move an amoeba it must be on a surface of some kind such as submerged rocks. The third kingdom is the kingdom of fungi. Fungi are also eukaryotes. They are mostly multicellular but there are also unicellular fungi such as yeast. The cells of fungi have cell walls that contain chitin. Fungi are heterotrophs. Though they may remind us of plants, the absence of a cellulose cell wall and chloroplasts that contain chlorophyll, which prevents them from making their own energy, gives us a clear idea why they are not classified as plants.

Fungi mostly obtain their nutrients by releasing digestive enzymes into a food source. They absorb their food after it has been digested by the enzymes. Fungi act either as decomposers or parasites. They include organisms such as mushrooms, toadstools, moulds and yeast. Fungi are mainly classified according to whether they are saprophytic or parasitic. A saprophyte is an organism, which lives on dead or decaying organic matter. A parasite is an organism that lives on another animal or plant also known as a host. The parasite obtains nourishment from the host without benefiting or killing the host.

However parasites could cause death if they live on a host for a long term and drain it off nourishment. Therefore if a fungus feeds on dead or decaying matter, it is a saprophytic fungus. Examples of saprophytic fungi include bird’s nest and toadstools. A fungus that obtains its nutrition from a living organism is considered to be parasitic. Examples of parasitic fungi include the potato blight. Fungi are also classified according to their fruiting body, which is called a sporangium. They can be classified according to the way they reproduce.

For example some fungi reproduce through spore formation and the dispersal of these spores. Fungi can produce spores either asexually or when two fungal cells fuse and share DNA. Spores are kind of like seeds in that they can be carried with the wind or rain, spreading the fungus to new places. Some species can do both sexual and asexual reproduction, depending on environmental conditions. For example while mushrooms reproduce through the dispersal of their spores, yeast reproduce asexually through binary fission. The fourth kingdom is the plant kingdom.

All plants are multicellular eukaryotes. They make their own energy through photosynthesis. Therefore they are phototrophs. Plant cells have chloroplasts, which contain chlorophyll, which is essential for photosynthesis. The cells of a plant have cell walls, made of cellulose. Plants cells are specialized for different functions such as photosynthesis, the transport of material and support. Kingdom plantae includes mosses and liverworts, which are the simplest forms of plants, ferns, conifers otherwise known as gymnosperms and flowering plants also known as angiosperms.

Members of the kingdom plantae can be classified according to the type of plant it is. For example according to whether it is an angiosperm or a moss or another type of plant. There are a variety of plant forms. Although the vast majority of species live on land, there are some aquatic species. Plants range in size from the small moss plants to the giant sequoias. Mosses and liverworts, which are also, know as bryophytes are found in moist, shaded areas. They have simplistic transport mechanisms and are anchored by root like structures called rhizoids.

Mosses and liverworts do not have real roots, stems or transport systems and because they lack true vascular tissue, they are incapable of efficient conduction of water. It is this lack of transport, which keeps them small. Food and water diffuses between cells. Ferns are simple vascular plants. They have a transport system and a primitive stem and roots. Like the moss, ferns are also spore producers. Generally they both reproduce asexually through the dispersal of spores. Ferns do not have cuticles covering their leaves therefore they lose a lot of water and are easily dehydrated just like mosses.

As a result of this ferns do not grow in places that receive direct sunlight. Unlike ferns conifers have a waxy cuticle covering their needle-like leaves. As a result of the cuticle and the reduced surface area of the leaves, which are need-like, conifers are much less prone to dehydration. They can grow in harsh environments. Conifers reproduce sexually through cones and they use wind pollination. Angiosperms are the most successful form of plant life on earth. They live in every habitat where plants may be found, from the hot, dry desert, to the arctic Tundra.

There are more species of angiosperms than any other plant group. Compared to the other forms of plants angiosperms have a much more complex transport system. They have a well-develop root, stem and leaf structures with vascular tissues for conduction of water and food. They have a waxy cuticle covered by cutin and stomata that can close to prevent excessive loss of water. Angiosperms reproduce sexually. They produce seeds, which are enclosed in a fruit. Angiosperms spread their pollens through animals such as insects and their seeds through animals such as birds, which eat the fruits.

Angiosperms have mechanisms that enable highly effective seed dispersal. The fifth and last kingdom is the kingdom animalia. Animals are multicellular, eukaryotes. They cannot make their own food. They have to consume either plants, which make their own energy, or other animals to obtain energy. Therefore they are heterotrophs. Animal cells do not have cell walls or chloroplasts. Most animals have separate systems for support and locomotion, digestion, excretion, nervous, endocrine, respiratory, circulatory, and reproduction. Most members of the animal kingdom can move from place to place.

However, some are permanently attached to surfaces such as sponges and barnacles. There are a lot of criteria used to classify animals. The main method used to classify them is according to whether they are vertebrates or invertebrates. This means that they are classified according to weather there is a backbone present. Invertebrates neither have a backbone nor a nervous system. They include animals such as sponges, jellyfish, annelids, mollusks and arthropods. Sponges are the simplest form of animals. Annelids include earthworms; mollusks include clams, oysters, and slugs.

They usually have a hard shell. The arthropods are the most highly evolved invertebrates. They include insects, crustaces such as crabs and lobsters and arachnids such as spiders. They have a hard exoskeleton. The invertebrates are classified among themselves according to several criteria such as body symmetry (weather they have radial or bilateral symmetry), presence body segmentation, cephalization (the concentration of important nerves at the end/head of an animal) and type of coelom (type of gut). The vertebrates have a backbone and a nervous system.

All vertebrates have a heart and a closed circulation system. Vertebrates include fish, amphibians, reptiles, birds and mammals. Mammals, which include humans, are the most highly evolved forms of animals. Some characteristics, which define mammals, include the presence of fur, sweat glands, and mammary glands. Mammals have an average body temperature of 37 degrees Celsius. Except for exceptions such as the duck billed platypus, which lays eggs, most mammals give birth to their young. Reproduction method is also an import method of classifying animals. Fish have scales.

They have no limbs and they move around by swimming. They lay their eggs. Amphibians live in both water and land. They cannot dwell to far from places such as ponds, lakes and so on. They have to lay their eggs in the water. Most reptiles also have scales. They also lay eggs. They do not give birth to their young. Birds have feathers. They have wings and most of them can moves about by flying. They have a body temperature of approximately 42 degrees Celsius. This is because their metabolism needs to work extremely fast in order for them to fly. Birds also lay eggs.

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