Diversity in Living Organisms

Have you ever thought about the multitude of life forms that surround us? Each organism is different from the others to a lesser or greater extent. Consider yourself and a friend; you look similar but differ from a monkey. If we consider a cow and a monkey, the monkey is closer to us than the cow. The Earth is home to millions of species, varying in size from microscopic bacteria to 30-metre long blue whales. To study and understand this vast diversity effectively, we must classify organisms into groups based on their similarities and differences.

In this chapter, we will learn about the basis of classification, the hierarchy of groups, and the major divisions in the plant and animal kingdoms.

1. Basis of Classification

Attempts at classifying living things have been made since ancient times. Aristotle classified animals according to whether they lived on land, in water, or in the air. This is a simple but often misleading way (e.g., corals, whales, octopuses, and starfish all live in water but are very different).

Today, classification is based on interrelated characteristics. Some important criteria for classification are: * Cellular Structure: Is the organism made of prokaryotic or eukaryotic cells? (Eukaryotic cells have a nucleus and allow for complex multi-cellular body designs). * Body Organization: Is the organism unicellular or multicellular? Multicellular organisms have specialized cells for different functions. * Mode of Nutrition: Does the organism produce its own food (autotrophic, like plants) or rely on other organisms for food (heterotrophic, like animals)? * Level of Body Organization: In animals, how are the organs and systems organized?

Classification and Evolution

Charles Darwin first described the idea of evolution in 1859 in his book The Origin of Species. Most life forms that we see today have arisen by an accumulation of changes in body design that allow the organism possessing them to survive better. Organisms with older, simpler body designs are often referred to as "primitive," while those with recent, complex designs are called "advanced."

2. The Hierarchy of Classification Groups

Biologists such as Ernst Haeckel (1894), Robert Whittaker (1969), and Carl Woese (1977) tried to classify all living organisms into broad categories called Kingdoms. Whittaker’s Five-Kingdom Classification is widely accepted.

The five kingdoms are: 1. Monera 2. Protista 3. Fungi 4. Plantae 5. Animalia

To study subgroups, kingdoms are further divided into a hierarchy: * Kingdom * Phylum (for animals) / Division (for plants) * Class * Order * Family * Genus * Species (The basic unit of classification)

2.1 Monera

• Characteristics: These organisms do not have a defined nucleus or organelles (prokaryotic). Some have cell walls, others do not. Mode of nutrition can be autotrophic or heterotrophic.
• Examples: Bacteria, blue-green algae (cyanobacteria), and mycoplasma.

2.2 Protista

• Characteristics: Unicellular, eukaryotic organisms. They use appendages like hair-like cilia or whip-like flagella for movement. Mode of nutrition can be autotrophic or heterotrophic.
• Examples: Unicellular algae, diatoms, protozoans (like Amoeba, Paramecium).

2.3 Fungi

• Characteristics: Heterotrophic, eukaryotic organisms. They are saprophytes (use decaying organic material as food). They have cell walls made of a tough complex sugar called chitin. Many become multicellular at certain stages.
• Examples: Yeast, mushrooms, bread mould (Rhizopus). (Some fungi live in symbiotic relationships with blue-green algae, known as lichens).

3. Plantae (Plant Kingdom)

These are multicellular eukaryotes with cell walls. They are autotrophs and use chlorophyll for photosynthesis. The plant kingdom is divided based on whether the plant body is differentiated, whether it has special tissues for the transport of water (vascular tissues), and whether it bears seeds.

1. Thallophyta: Plants that do not have well-differentiated body design. The plants in this group are commonly called algae. Predominantly aquatic. (e.g., Spirogyra, Ulothrix).
2. Bryophyta: Called the amphibians of the plant kingdom. The plant body is differentiated to form stem and leaf-like structures. However, there is no specialized tissue for the conduction of water (no xylem/phloem). (e.g., Moss/Funaria, Marchantia).
3. Pteridophyta: The plant body is differentiated into roots, stem, and leaves, and has specialized tissue for conduction. They do not produce seeds. (e.g., Ferns, Marsilea).
4. Gymnosperms: Plants with well-differentiated reproductive tissues that make seeds. The seeds are naked (not enclosed in fruits). Usually perennial, evergreen, and woody. (e.g., Pines, Cycas).
5. Angiosperms: These are flowering plants. The seeds develop inside an organ which modifies to become a fruit. Plant embryos in seeds have structures called cotyledons. Based on cotyledons, they are divided into:
   • Monocots: Plants with seeds having a single cotyledon (e.g., wheat, rice).
   • Dicots: Plants with seeds having two cotyledons (e.g., gram, pea).

4. Animalia (Animal Kingdom)

These are eukaryotic, multicellular, and heterotrophic organisms. Their cells do not have cell walls. Most animals are mobile. They are classified based on body design and differentiation.

1. Porifera (Sponges): Non-motile animals attached to solid supports. There are holes (pores) all over the body leading to a canal system that helps circulate water, food, and oxygen. Very minimal differentiation. (e.g., Sycon, Spongilla).
2. Coelenterata (Cnidaria): Animals living in water with more body design differentiation. The body has a cavity (coelenteron). The body is made of two layers of cells. (e.g., Hydra, Jellyfish, Corals).
3. Platyhelminthes (Flatworms): Body is bilaterally symmetrical and triploblastic (three layers of cells). No true internal body cavity (coelom). Body is flattened dorsiventrally. (e.g., Planaria, Liver fluke, Tapeworm).
4. Nematoda: Bilaterally symmetrical, triploblastic, and the body is cylindrical. There is a pseudocoelom (false body cavity). Familiar as parasitic worms. (e.g., Ascaris/roundworms).
5. Annelida: Bilaterally symmetrical, triploblastic, with a true body cavity (coelomate). Extensive organ differentiation. The body is segmented. (e.g., Earthworms, Leeches).
6. Arthropoda: The largest group of animals. Bilaterally symmetrical, segmented, and have jointed legs. They possess an open circulatory system. (e.g., Insects, Spiders, Crabs).
7. Mollusca: Bilaterally symmetrical, coelom reduced. Little segmentation. They have an open circulatory system and kidney-like organs. Many have shells. (e.g., Snails, Octopus).
8. Echinodermata: Spiny-skinned animals. Exclusively free-living marine animals. They are triploblastic and coelomate, possessing a unique water-driven tube system for movement. (e.g., Starfish, Sea urchins).
9. Protochordata: Triploblastic, coelomate. Possess a notochord (a long rod-like support structure) at some stage in their lives. (e.g., Balanoglossus).
10. Vertebrata: Possess a true vertebral column (backbone) and internal skeleton. Bilaterally symmetrical, triploblastic, coelomic, and segmented.
    • Pisces (Fishes): Aquatic, exclusively water-living. Cold-blooded, breathe through gills, have scales/plates. Two-chambered heart.
    • Amphibia: Lack scales, have mucus glands. Three-chambered heart. Can live in water and on land. Breathe through gills (larvae) or lungs/skin (adults). (e.g., Frogs, Toads).
    • Reptilia: Cold-blooded, have scales, breathe through lungs. Most have a three-chambered heart (Crocodiles have four). Lay eggs with tough coverings. (e.g., Snakes, Turtles).
    • Aves (Birds): Warm-blooded, four-chambered heart. Two forelimbs modified into wings. Breathe through lungs. Feathers cover the body. (e.g., Crow, Ostrich).
    • Mammalia: Warm-blooded, four-chambered heart. Possess mammary glands for production of milk to nourish their young. Skin has hairs and sweat/oil glands. Most give birth to live young (viviparous). (e.g., Humans, Whales, Rats).

5. Nomenclature

With millions of species, common names create confusion because they vary from place to place and language to language. Binomial Nomenclature was introduced by Carolus Linnaeus. The scientific name of an organism is made of two words: 1. Genus: Begins with a capital letter. 2. Species: Begins with a small letter.

When printed, scientific names are given in italics. When written by hand, the genus name and the species name are underlined separately. Example: Homo sapiens (Human), Panthera tigris (Tiger).

Summary

The diversity of life is systematically organized to make studying biology manageable. Whittaker’s five kingdoms (Monera, Protista, Fungi, Plantae, Animalia) classify life based on cell type, body complexity, and nutrition. Each kingdom is further divided into phyla or divisions, showcasing an evolutionary progression from simple sponges and algae to highly complex mammals and angiosperms. Binomial nomenclature gives every organism a unique, universal scientific name, avoiding the confusion of local names.