S4 Biology | Classification and Biodiversity prep guide (for tutors)

S4 Biology | Classification and Biodiversity prep guide (for tutors) | Binomial names, keys and Simpson index

2026-11-01

Tutor lesson-prep guides · Secondary Biology

Students think four species means four species, so two samples with four species each are judged equally diverse. This lesson uses one 100-organism sample count to separate how many species there are from how evenly they are split.

Objectives — Students can: (1) state the basis of the five kingdoms and write binomial names correctly (genus capitalised, species lower case, italic or underlined); (2) classify 12 specimens with a dichotomous key and explain the step count in terms of powers of two; (3) distinguish species richness from evenness, compute the Simpson index and compare two communities; (4) name the three levels of biodiversity (genetic, species, ecosystem) and give one concrete conservation argument.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Observation and recording	Two counts of the same quadrat differ: what should you do?	Average them and record the method
Percentages and squares	70 out of 100, squared: what do you get?	0.49
Powers of two (optional)	What is 2 to the power 4?	16, so four steps can separate up to 16 items

02 Lesson flow (50 minutes)

Time	Stage	Tutor does	Student does
0-6 min	Opening question	Two samples have four species each: which is more diverse?	Reason written down
6-12 min	Prerequisite check	Squaring percentages and powers of two	Both correct before moving on
12-22 min	Act 1	Five kingdoms and the rules of binomial names	Three names written
22-32 min	Act 2	Dichotomous keys: one yes/no question per step	12 specimens need 4 steps
32-44 min	Act 3	Richness versus evenness with the Simpson index	D = 0.25 against 0.52
44-48 min	Worked examples	One start-up routine per question type	Students copy the routine
48-50 min	Exit check	Three questions marked on the spot	Answered independently

03 Teaching script (read it out)

Scene 1: Five kingdoms and binomial names: ask what the criterion is

Five kingdoms: Prokaryotae (no nuclear membrane), Protoctista (single-celled eukaryotes), Fungi (cell walls, heterotrophic, absorb), Plantae (photosynthetic, cell walls) and Animalia (multicellular, heterotrophic, motile).
Do not recite the list. Ask students what observable features could be used, then derive the kingdoms from the features. Three rules for binomial names: genus capitalised, species in lower case, whole name italic or underlined. Panthera leo and Panthera pardus share a genus but differ in species, which shows one genus can hold several species. Common names are unreliable, since fish covers several unrelated groups, while the scientific name is universal.

Task: copy Homo sapiens using the three rules and underline the genus.

Scene 2: Dichotomous keys: one yes/no question per step

A key splits into two branches at each step, and each branch asks a single feature with a yes or no answer. For 4 specimens the minimum number of steps is ceil(log2 12) = 4, because 8 < 12 <= 16, and 20 specimens need 5 steps. Knowing this relation stops students guessing the step count, and stops them mixing "backbone present" with "how many legs" in the same step.
Method: list three or four features for every specimen (symmetry, appendages, body covering, respiratory organ), choose the feature that splits the set most evenly as step one, and keep the tree shallow. Then have students rewrite the key while you check only two things: two branches per step, and no specimen left out.

Task: build a three-step key for six shells so that every shell reaches a different endpoint.

Scene 3: Richness and evenness: one sample, two answers

Sample A holds 25, 25, 25, 25 and sample B holds 70, 10, 10, 10, both totalling 100 organisms in four species. Richness is identical, but evenness is not. Simpson index D = sum of (n/N) squared: D(A) = 4 x 0.25 squared = 0.25 and D(B) = 0.49 + 3 x 0.01 = 0.52, so diversity as 1 – D is 0.75 and 0.48.
Be explicit that D is the chance that two randomly drawn organisms belong to the same species, so a larger D means less diverse; many students read it backwards. Close with a park example: one with only sparrows against one split equally between sparrows, pigeons and magpies. Species counts match, but the second food web has alternative routes, so one pest or disease cannot collapse it.

Task: sample C holds 40, 30, 20, 10. Compute D and say whether it is more or less diverse than A (D = 0.30, larger than 0.25, so less diverse).

04 Question types and worked steps

Type	Example	Steps	Answer
Writing a binomial name	Genus capitalised, species lower case, italic	Check the three rules one at a time	Panthera pardus
Dichotomous key	One yes/no question per step	Steps = ceil(log2 n); 12 items need 4	Tree diagram
Richness comparison	Count species only	Ignore how the counts are distributed	A and B both 4
Simpson index	D = sum of (n/N) squared	Ratio first, square second, sum last	D(B) = 0.52

Sanity check (teach this): Both samples total 100 organisms, so they compare directly without any sample-size correction. D(A) = 0.25 means only a quarter of random pairs are the same species, as expected when four species share the sample equally; D(B) = 0.52 is above one half because seven in ten organisms belong to one species. Key check: 4 steps give 2^4 = 16 endpoints, enough for 12 specimens, while three steps allow only eight endpoints and must force two specimens together.

05 Independent practice (3 questions)

Question	Answer	What to watch
Write the scientific name of the Indo-Pacific humpback dolphin (genus Sousa) in correct format	Sousa chinensis, italic	Genus capitalised, species lower case
What is the minimum number of steps in a key for 16 specimens?	4 steps	2 to the power 4 is 16
Sample D holds 50, 30, 20 with N = 100: find the Simpson index	0.38	0.25 + 0.09 + 0.04

06 Common mistakes and what to say

Mistake	Student writes	Say this
Reading D as diversity, so bigger is better	Reversing the direction of the index	“D is the chance two organisms are the same species, so bigger means less variety.”
Capitalising both parts of a binomial name	Treating it like a person name	“Only the genus starts with a capital; the species is always lower case.”
Splitting a key into three branches	Trying to finish the key quickly	“Two branches only: three means two questions glued together and untraceable.”
Calling high richness high diversity	Ignoring evenness	“Four species with seventy percent of one is not the same as four equal quarters.”

07 Homework (paper, section and question numbers)

Secondary Biology chapter index (cells, enzymes, genetics, nutrition, transport, respiration, ecology) — Questions 1, 3 and 5 of the set: binomial names, keys, Simpson index
Secondary prep guides overview (all subjects) — Revise in subject order

08 Exit check (last 5 minutes)

Question	Answer	Decision
Does a larger Simpson index mean more or less diversity?	Less (more dominated)	Wrong means Act 3 again
How many steps does a key for 12 specimens need at minimum?	4 steps	Wrong means Act 2 again
Which part of a binomial name is capitalised?	The genus, first letter only	Two or more wrong means back to Act 1

09 Tutor FAQ

Which year is this for?

S4, the classification and environment parts of HKDSE biology; also a good bridge before the S5 ecology unit.

Must the Simpson index formula be memorised?

Yes: D = sum of (n/N) squared, together with its direction.

What if there are no real specimens?

Use eight picture cards; the skill being trained is one-question-per-step, not handling specimens.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.