Unit 2 Introduction to classification of organisms
Key unit competence: Construct an identification key for collected specimens.
Key ideas
- Classification (taxonomy) is grouping organisms according to their similarities and differences. It makes the huge variety of life easier to study, helps us identify organisms correctly and shows how organisms are related.
- The three main steps are identification (observing features), nomenclature (naming) and classification (placing the organism in groups).
- The taxonomic hierarchy runs from the largest, most varied group to the smallest: Kingdom → Phylum → Class → Order → Family → Genus → Species ("King Philip Came Over For Great Spaghetti"). Some books add Domain above kingdom. Like Rwanda's country → province → district → sector → cell → village, each group fits inside a larger one.
- A species is a group of very similar organisms that can interbreed and produce fertile offspring. A genus is a group of closely related species. A horse and a donkey (same genus Equus) can mate, but their offspring, the mule, is infertile, so they are different species.
- The binomial system (Carl Linnaeus) gives every organism a two-part scientific name: Genus + species, for example Homo sapiens (human), Zea mays (maize), Apis mellifera (honeybee), Commelina africana. The genus starts with a capital letter, the species with a small letter; printed names are in italics, handwritten names are underlined separately. The same name is used all over the world.
- Five kingdoms: Monera (bacteria: single cells, no true nucleus, prokaryotic); Protoctista (Amoeba, Paramecium, algae: mostly single cells with a true nucleus); Fungi (mushrooms, moulds, yeast: no chlorophyll, cell walls of chitin, reproduce by spores, feed on dead or living matter); Plantae (cell walls of cellulose, chlorophyll, make food by photosynthesis); Animalia (multicellular, no cell walls, feed on other organisms, most can move).
- Features used to place organisms in kingdoms: body structure (cells, nucleus, cell wall), mode of nutrition (make food or eat food) and method of reproduction.
- Viruses are not placed in any kingdom: they are not made of cells (just DNA or RNA inside a protein coat), have no metabolism of their own and can reproduce only inside a living host cell. Outside a cell they are inactive and can even be crystallised.
- An identification key helps to name an unknown organism. A dichotomous key ("di" = two) gives pairs of contrasting statements at each step, for example "has wings / has no wings"; each choice leads to another pair or to a name. Use clear, visible features (number of legs, wings, body covering, leaf shape), not size or colour, which vary.
Common misconceptions
- "A spider is an insect." No: insects have six legs and three body parts; spiders have eight legs and two body parts.
- "Mushrooms are plants because they grow in the soil." No: mushrooms have no chlorophyll, cannot photosynthesise, have chitin cell walls and reproduce by spores: they are fungi.
- "In Homo sapiens, both names start with a capital letter." No: only the genus starts with a capital letter: Homo sapiens.
- "A bat is a bird because it flies." No: a bat has fur, gives birth to live young and feeds them on milk: it is a mammal.
Worked examples
Example 1. Put these groups in order from the largest to the smallest: Family, Species, Order, Kingdom, Genus, Class, Phylum.
Solution
Kingdom, Phylum, Class, Order, Family, Genus, Species. The largest group contains the most different organisms; the species contains only one kind of organism.
Example 2. Make a dichotomous key to identify a chicken, a grasshopper, a spider and a snake.
Solution
1a Has legs → go to 2; 1b Has no legs → snake. 2a Has two legs → chicken; 2b Has more than two legs → go to 3. 3a Has six legs → grasshopper; 3b Has eight legs → spider. Each step uses one clear, visible feature with two opposite choices.
Practice questions
State three reasons why classification of organisms is important.
Show answer
It makes studying the variety of organisms easier; it helps to identify organisms correctly; it shows how organisms are related. It also helps scientists communicate using one name.
Common mistake: Writing only "to group organisms" without saying why grouping is useful.
Explain the rules for writing a scientific name, using the name of maize as an example.
Show answer
Two names: genus then species, Zea mays. The genus starts with a capital letter, the species with a small letter; printed in italics, or each name underlined separately when handwritten.
Common mistake: Underlining both words with one continuous line or starting the species with a capital.
Define species and genus.
Show answer
A species is a group of similar organisms that can interbreed to produce fertile offspring. A genus is a group of closely related species.
Common mistake: Forgetting the word "fertile" in the definition of species.
Give two features of each kingdom: Monera, Fungi and Plantae.
Show answer
Monera: single cells, no true nucleus. Fungi: no chlorophyll, reproduce by spores, chitin cell walls. Plantae: cellulose cell walls, chlorophyll, photosynthesis.
Common mistake: Saying that fungi make their own food.
Give two reasons why viruses are not placed in any kingdom.
Show answer
They are not made of cells (only DNA or RNA in a protein coat); they have no metabolism and can reproduce only inside living host cells.
Common mistake: Saying viruses are bacteria.
Construct a dichotomous key to separate a bean plant, a mushroom, a cow and a hen.
Show answer
1a Green, makes food → bean plant; 1b Not green → 2. 2a Does not move, grows from spores → mushroom; 2b Moves around → 3. 3a Has feathers and two legs → hen; 3b Has fur and four legs → cow.
Common mistake: Using statements that are not opposite, or that more than one organism fits.
A leopard is classified as Panthera pardus. Name the genus and the species and explain what the name tells us.
Show answer
Genus Panthera, species pardus. It shows the leopard is closely related to other Panthera species such as the lion (Panthera leo).
Common mistake: Calling "pardus" the genus.
Match the hierarchy of classification with Rwanda's administrative units and explain the idea.
Show answer
Kingdom like the country, phylum like a province, class like a district, order like a sector, family like a cell, genus like a village, species like a household. Smaller groups fit inside larger ones and contain fewer, more similar members.
Common mistake: Putting the order upside down (species as the largest group).
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Who developed the binomial system of naming organisms? a. Kingdom 2. Organisms that cannot make their own food and must feed on others are: b. heterotrophs 3. Amoeba and Paramecium belong to the kingdom: c. Protoctista 4. Which group contains the largest number of different organisms? d. Carl Linnaeus Show answer
1 → d (Carl Linnaeus) · 2 → b (heterotrophs) · 3 → c (Protoctista) · 4 → a (Kingdom)
- Linnaeus gave every organism a two-part Latin name.
- Animals and fungi are heterotrophs.
- They are single cells with a true nucleus.
- Kingdom is the largest and most varied group.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
Protoctista · genus · two contrasting statements · kingdom- An organism is a single cell with a nucleus and chloroplasts, living in a pond. It belongs to .
- A group of closely related species forms a .
- A dichotomous key gives, at each step .
Show answer
1: Protoctista · 2: genus · 3: two contrasting statements
- Single cell with a true nucleus: a unicellular alga, a protoctist.
- Several species make a genus; several genera make a family.
- "Di" means two.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Yeast is an example of: a. Homo sapiens 2. Which kingdom contains organisms with cell walls made of chitin that reproduce by spores? b. classification 3. Grouping organisms according to their similarities and differences is called: c. Fungi 4. Which is the correct way to write the scientific name of a human? d. a unicellular fungus Show answer
1 → d (a unicellular fungus) · 2 → c (Fungi) · 3 → b (classification) · 4 → a (Homo sapiens)
- Yeast is a single-celled fungus used in baking and brewing.
- Fungi such as mushrooms, moulds and yeast.
- Classification (taxonomy) places organisms in groups.
- Genus with a capital letter, species with a small letter, in italics when printed.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
step 3 · interbreed and produce fertile offspring · insect · genus- In a key, step 1 reads "Has wings → go to 2 / Has no wings → go to 3". Step 2 reads "Feathers → bird / Transparent wings → insect". A housefly ends at .
- In the scientific name Zea mays, the word "Zea" is the .
- Members of the same species can .
Show answer
1: insect · 2: genus · 3: interbreed and produce fertile offspring
- The housefly has wings (step 2) that are transparent.
- The first name is the genus; the second is the species.
- Fertile offspring is the test of a species.