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Biodiversity and classification: Class 11 Biology
Meaning of biodiversity, the need for classification, the taxonomic hierarchy, binomial nomenclature, the five kingdom system and three domains, and the main features of the major groups of organisms.
Key points
Biodiversity is the variety of living things on Earth. It includes the variety of species, the genetic differences within a species and the variety of ecosystems. Over 1.5 million species have been named, and many more are believed to remain unknown.
Classification is the grouping of organisms according to similarities. It makes the study of so many organisms easier, shows evolutionary relationships and helps scientists identify new species. The branch of biology that deals with classification and naming is taxonomy. Carolus Linnaeus laid the foundation of modern taxonomy.
The taxonomic hierarchy, from largest group to smallest:
- Domain, kingdom, phylum (division in plants), class, order, family, genus, species.
- A memory phrase is: Dear King Philip Came Over For Good Soup.
- Each level contains the groups below it. Organisms in one species are alike in structure and can breed to give fertile offspring.
Binomial nomenclature gives every species a two part Latin name. The first word is the genus and starts with a capital letter, and the second word is the species and starts with a small letter. The name is written in italics, or underlined when handwritten. Examples are Homo sapiens for humans, Panthera leo for lion and Mangifera indica for mango. The system is universal, so scientists in every country use the same name.
Classification systems have changed over time. Aristotle grouped living things as plants and animals. Whittaker in 1969 proposed the five kingdom system: Monera, Protista, Fungi, Plantae and Animalia. Later, Carl Woese divided life into three domains on the basis of ribosomal RNA: Bacteria, Archaea and Eukarya.
The five kingdoms and their main features:
- Monera: prokaryotes with no nucleus, mostly unicellular. Examples are bacteria and cyanobacteria.
- Protista: unicellular eukaryotes such as Amoeba, Paramecium and Euglena.
- Fungi: eukaryotes with cell walls of chitin that feed by absorbing digested food. They are heterotrophs, such as mushrooms, yeast and moulds.
- Plantae: multicellular eukaryotes with cellulose walls and chloroplasts, which make food by photosynthesis.
- Animalia: multicellular eukaryotes with no cell wall, which take in food and mostly move.
Plants are divided into bryophytes (mosses and liverworts, with no vascular tissue), pteridophytes (ferns, with vascular tissue and spores), gymnosperms (naked seeds, such as pine) and angiosperms (flowering plants with seeds enclosed in fruits). Angiosperms are further split into monocots with one seed leaf and dicots with two seed leaves.
Animals are classified by body plan. Porifera (sponges) have pores and no organs. Cnidaria (jellyfish, Hydra) have stinging cells and radial symmetry. Platyhelminthes (flatworms) have flat bodies, and Nematoda (roundworms) have round bodies. Annelida (earthworms) have segmented bodies. Arthropoda have jointed legs and an exoskeleton. Mollusca have soft bodies and often a shell. Echinodermata (starfish) have spiny skin. Chordata have a notochord.
The chordates include fish, amphibians, reptiles, birds and mammals. Fish have gills and fins. Amphibians live on land and in water and have moist skin. Reptiles have dry scaly skin. Birds have feathers and lay eggs. Mammals have hair and mammary glands, and most give birth to young.
Dichotomous keys are tools to identify an organism. They are made of paired statements. At each step the user chooses the statement that fits the specimen until a name is reached. Biodiversity is threatened by habitat loss, pollution and overhunting, and conserving it protects ecosystems that supply food, medicines and clean air.
Study tip: Learn the hierarchy with a memory phrase and write one example of the full classification, such as the human from domain to species. Make a one page table of the phyla with a key feature and an example for each. When asked to name a species, always use the correct format.
Definitions and flashcards
Say the meaning first, then open the card to check.
Taxonomy
The science of naming, describing and classifying organisms.
Species
A group of organisms that can interbreed to produce fertile offspring.
Genus
A group of closely related species.
Binomial nomenclature
The two word system of naming species, genus then species.
Taxon
Any group in the classification hierarchy.
Domain
The highest level of classification, with three: Bacteria, Archaea and Eukarya.
Dichotomous key
A tool for identifying organisms using paired choices.
Chordate
An animal that has a notochord at some stage of life.
Biodiversity
The variety of life, including species, genes and ecosystems.
Short questions with model answers
These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.
What is binomial nomenclature? Give one example. (2 marks)
It is the system in which each species has a two word Latin name made of the genus and the species. For example, the human is Homo sapiens.
Why do we classify living organisms? (2 marks)
Classification makes the study of the huge number of organisms easier. It shows their relationships and common ancestry, helps in identifying them and gives each a universal name.
List the levels of the taxonomic hierarchy in order. (3 marks)
Domain, kingdom, phylum, class, order, family, genus and species. The hierarchy goes from the largest group with many different organisms to the species, where organisms are very similar.
Differentiate between monocots and dicots. (3 marks)
A monocot seed has one cotyledon and its leaves have parallel veins, and its flower parts are in threes. A dicot seed has two cotyledons, its leaves have net veins and its flower parts are in fours or fives. Wheat is a monocot and gram is a dicot.
Describe the five kingdom system of classification. (5 marks)
Whittaker proposed the system in 1969 and it uses cell type, body organisation and mode of nutrition. Monera contains prokaryotes such as bacteria. Protista contains mostly unicellular eukaryotes such as Amoeba. Fungi are eukaryotes with chitin cell walls that absorb food. Plantae are multicellular producers with cellulose walls and chloroplasts. Animalia are multicellular consumers with no cell walls. The system was later developed into three domains because archaea differ greatly from true bacteria.
State the main features of mammals. (3 marks)
Mammals have hair or fur, mammary glands that produce milk, a four chambered heart and a diaphragm. They are warm blooded, and most give birth to live young.
Common mistakes
- Writing scientific names without italics or underline, or starting the species word with a capital letter.
- Putting the taxonomic levels in the wrong order, or mixing species and genus.
- Treating viruses as one of the five kingdoms. They are not placed in any kingdom.
- Saying all fungi and plants are the same because both have cell walls. Fungi have chitin and plants have cellulose.
- Calling all water animals fish. Whales are mammals.