Classification of Living Beings

Think of a library with thousands of books thrown in one heap. To find one book you would search the whole heap. Put them on shelves by subject, and you can find any book quickly, and even guess what a new book is about from its shelf. Classifying living things works the same way. It:

1. helps us understand evolution: how today’s organisms came from earlier ones;
2. lets us study millions of organisms in a short time, by learning about a few groups instead of every single one;
3. makes it easy to give each organism a scientific name;
4. shows the natural relationships between living things: who is a close relative of whom;
5. makes the study of living things easier and more scientific.

The Swedish naturalist Carolus Linnaeus (1707–1778) did more than anyone before him to sort living things. He divided them into two kingdoms, the plant kingdom and the animal kingdom, and he made a system for naming them. Because he was the first to study the classification and naming of living things in such detail, he is called the father of modern taxonomy (taxonomy is the science of classifying and naming organisms).

Two names for every species

A buffalo is bhainsi in Nepali, bhains in Hindi and buffalo in English. Which name should a scientist in Japan use? Linnaeus gave every kind of organism one name of two Latin words, the same in every language:

• the first word is the genus (the generic name);
• the second word is the species (the specific name).

This way of naming is called the binomial system of nomenclature (bi = two, nomen = name). The rules for writing a scientific name:

1. The genus begins with a capital letter; the species with a small letter: Homo sapiens, not Homo Sapiens.
2. The name is printed in italics. When written by hand, each word is underlined separately: Homo sapiens.

Common name	Generic name	Specific name
Mustard	Brassica	campestris
Pea	Pisum	sativum
Maize	Zea	mays
Rice (paddy)	Oryza	sativa
Potato	Solanum	tuberosum
Tiger	Panthera	tigris
Lion	Panthera	leo
Leopard	Panthera	pardus
Human	Homo	sapiens
Cat	Felis	catus
Dog	Canis	familiaris
Cow	Bos	taurus

Genus and species

A species is a group of similar organisms that can breed with each other and have young that can also breed. All the people in the world are one species, Homo sapiens. The species is the smallest unit of classification.

A genus is a group of species that are very alike. It comes just below the family and just above the species. The genus Canis includes the dog, the jackal and the wolf; the genus Panthera includes the tiger, the lion and the leopard. That is why the tiger and the lion share their first name, Panthera.

Groups inside groups

Living things are classified in a ladder of groups, from the biggest to the smallest:

Kingdom → Phylum (or Division, for plants) → Class → Order → Family → Genus → Species

Each of these levels is called a taxon (plural taxa). Here are the mustard plant and the human, step by step:

Follow the human down the ladder:

• Kingdom Animalia holds every animal: worms, insects, fish, birds, monkeys and us.
• Phylum Chordata keeps only animals with a notochord (a stiff rod along the back) at some stage of life; nearly all of them are vertebrates, animals with a backbone.
• Class Mammalia keeps only the animals whose mothers feed their young on milk from mammary glands, and which have hair.
• Order Primates keeps lemurs, monkeys, apes (gibbons and the great apes) and humans, with grasping hands and forward-facing eyes.
• Family Hominidae keeps the great apes: orangutans, gorillas, chimpanzees and humans.
• Genus Homo has only one living species today: us.
• Species sapiens: you.

So the higher the taxon, the more kinds of organisms it includes; the lower the taxon, the fewer, and the more they share. Two organisms in the same genus are close relatives; two that share only a kingdom are distant ones. The ladder shows how every lower taxon is part of the one above it.

Different scientists have suggested different systems. For about 200 years, Linnaeus’s two-kingdom system (plants and animals) was used, but it has these shortcomings:

1. Prokaryotes (cells without a proper nucleus, like bacteria) and eukaryotes (cells with a nucleus) are not kept apart.
2. Unicellular and multicellular organisms are not separated.
3. Photosynthetic algae and non-photosynthetic fungi are put together as plants.
4. Some organisms fit in neither kingdom. Euglena has chlorophyll like a plant but swims and can take in food like an animal.

To fix these shortcomings, the American ecologist Robert H. Whittaker proposed the five-kingdom system in 1969. It is more scientific, and it is the system most used in schools today.

Whittaker sorted every living thing by asking a few questions, one after another:

1. What kind of cell? Organisms with prokaryotic cells go into kingdom Monera. All the rest are eukaryotes.
2. One cell or many? Unicellular eukaryotes go into kingdom Protista.
3. Is there a cell wall? Multicellular organisms without a cell wall are kingdom Animalia.
4. How do they get food? Multicellular organisms with a cell wall that make their own food (autotrophs) are kingdom Plantae; those that absorb food made by others (heterotrophs) are kingdom Fungi.

Their role in the ecosystem matches: plants are producers, animals are consumers, and fungi are decomposers, breaking down dead things. Fungi and animals are both heterotrophs, but fungi pour digestive juices onto their food, digest it outside their bodies and absorb it through their cell walls, while animals take food in and digest it inside.

Basis of classification	Kingdom
1.Type of cell: prokaryotic	Monera
2.Structure of the body: eukaryotic and unicellular	Protista
3.Cell wall present; autotrophic; producer	Plantae
4.Cell wall present; heterotrophic; decomposer	Fungi
5.No cell wall; heterotrophic; consumer	Animalia

So the bases of the five-kingdom system are the structure of the cell, the structure of the body, the presence of a cell wall, the mode of nutrition and the role in the ecosystem.

1. Kingdom Monera

Kingdom Monera contains the prokaryotic microorganisms: the bacteria. Their features:

1. They are prokaryotes: single cells (sometimes in chains or clusters).
2. The cell has no proper nucleus; its genetic material, DNA (deoxyribonucleic acid), lies loose in the cytoplasm.
3. Many can survive very harsh conditions: heat, cold, dryness, salt.
4. By their food habits they may be autotrophic (making their own food), parasitic (living on other living things), saprophytic (feeding on dead matter) or symbiotic (living together with another organism, both gaining).

Monera are of three types:

a. Archaebacteria (archae = ancient) are thought to be among the oldest forms of life. They live in extreme places: hot springs, very salty lakes, marshes and the stomachs of cattle. They have no chlorophyll. Some make their food from chemicals (not from light); others feed on dead matter. Examples: Pyrolobus, which lives around hot vents on the sea floor in water hotter than 100 °C (the great pressure there stops it boiling), Hyperthermus, and the methane makers of marshes.

b. Eubacteria (eu = true) are the true bacteria, the ones we usually mean by “bacteria”. Their cell wall is made of a substance called peptidoglycan, and many move with whip-like flagella. Examples: Rhizobium, which lives in the root nodules of beans and peas and makes nitrogen fertilizer for them; Lactobacillus, which turns milk into curd; Clostridium; Azotobacter.

c. Cyanobacteria, also called blue-green algae (BGA), contain chlorophyll and make their own food by photosynthesis: they are autotrophic. Examples: Nostoc and Anabaena, which grow in rice fields and add nitrogen to the soil, and Spirulina, sold as a health food.

2. Kingdom Protista

Kingdom Protista contains the unicellular eukaryotes. Their features:

1. They are eukaryotes: their cells have a proper nucleus.
2. Most are one cell, with all its organelles inside a cell membrane.
3. They move with pseudopodia (“false feet”, bulges of the cell), cilia (many short hairs) or flagella (one or two long whips).
4. Some are parasites, taking food from a host; the tiny animal-like ones that drift in water (zooplankton) are heterotrophs.
5. They reproduce asexually (by splitting) and sexually.
6. Examples: Amoeba, Euglena, Paramecium, Plasmodium (the malaria parasite).

Protists are of three kinds:

Kind	What they are like	Examples
Animal-like protists	cells like animal cells; take in food	Amoeba, Paramecium, Plasmodium
Plant-like protists	contain chlorophyll and make food by photosynthesis	Euglena, diatoms
Fungus-like protists	feed like fungi, on dead and rotting matter	slime molds, water molds

3. Kingdom Fungi

Kingdom Fungi contains mushrooms, yeasts and molds such as Mucor, which grows on old bread. Their features:

1. They have no chlorophyll. Most are multicellular (mushroom, Mucor), but some are unicellular (yeast).
2. Most are saprotrophic: they absorb food from dead and decaying matter.
3. The body is a thallus: it is not divided into root, stem and leaves.
4. The body is made of mycelium, a network of fine threads called hyphae.
5. They reproduce asexually (by spores and budding) and sexually.
6. They store food as glycogen, as animals do (plants store starch). Their cell walls are made of chitin, the same material as an insect’s hard outer skeleton.
7. Yeast is used in fermentation: to make bread dough rise and to make alcohol.
8. Examples: Mucor, yeast, mushroom, Penicillium.

You will study one fungus, the mushroom, in detail in the next unit: Mushroom.

By their mode of nutrition, fungi are of three types:

1. Saprotrophic fungi absorb food from dead and decaying organic matter. Examples: Mucor, mushroom, Penicillium.
2. Parasitic fungi absorb food from living organisms and harm them. Examples: Puccinia, which causes rust disease in wheat; Taphrina, which causes leaf curl in peach.
3. Symbiotic fungi live in a close partnership with another organism, both gaining. A lichen is a fungus and an alga living together: the fungus gives shelter and holds water and minerals; the alga makes food (carbohydrates) by photosynthesis and shares it.

4 and 5. Kingdoms Plantae and Animalia

The last two kingdoms are the ones you know best:

• Kingdom Plantae: multicellular eukaryotes with a cell wall of cellulose and chlorophyll; they make their own food (autotrophs, producers). Examples: moss, fern, pine, rice, rhododendron.
• Kingdom Animalia: multicellular eukaryotes with no cell wall and no chlorophyll; they take in food (heterotrophs, consumers). Examples: earthworm, butterfly, fish, frog, monkey, human.

1. Choose the correct answer

2. Give reasons

3. Write the differences between

4. Answer the following questions