Using Dichotomous Keys
AdvancedA dichotomous key offers two mutually exclusive choices at each step; pick one and move on or reach an answer. Step by step, any organism can be identified.
- A dichotomous key offers two mutually exclusive choices at each step to guide identification.
- Compare exactly one feature per step.
- Options must be mutually exclusive so every organism has a single path.
文體小知識：科普知識文章用生活實例解釋科學概念，先看現象再講原理，讀完就能解釋日常見到的東西。
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1You meet an unfamiliar bird in the field — how do you find out what it is? Scientists reach for a key: the dichotomous key. At each step it offers exactly two choices — feathers or no feathers? Fins or no fins? Pick one, and the key leads you onward or straight to the answer.
2The power of a key comes from halving: each step cuts the possibilities in two. Four animals take two or three steps; eight species need only three choices. Two rules make it work: compare one feature per step, and make the two options mutually exclusive — no organism may fit both.
3Designing a key is thinking in reverse: survey all your subjects, find a feature that splits the whole group in two, then find fresh features for each half and keep splitting. Always trial-run the result — if some organism has no path or two paths, rewrite that step with a better feature.
4Museums and field guides worldwide rely on exactly this tool. Next lesson we carry our classifying and keying eyes back home — to see how rich Hong Kong's biodiversity really is.
1Summary 重點摘要
A dichotomous key offers two mutually exclusive choices at each step; pick one and move on or reach an answer. Step by step, any organism can be identified.
2Vocabulary cards Vocabulary
A tool that identifies organisms step by step, offering two mutually exclusive choices at each step.
A pair of parallel statements (a and b) in a key, of which only one can apply.
The key feature used to tell organisms apart, e.g. feathers or the number of wings.
The path taken after a choice, leading either to the next step or to the answer.
The process of finding out what an organism is by using a key.
Two options that cannot both apply — one must be chosen; this is what makes a key work.
A common first step in animal keys: 'backbone or not?', splitting animals into two large groups at once.
Testing a finished key on real users to confirm every organism has exactly one path.
3Step by step Steps
- What a key looks likeA dichotomous key is like a questionnaire of paired choices: each line offers a mutually exclusive pair, say 1a has feathers, 1b no feathers. Choose 'has feathers' and the answer 'bird' appears; choose 'no feathers' and the instruction sends you to step 2 to keep comparing. Working two choices at a time, the key leads an unfamiliar organism step by step to its correct name.
- One feature per stepEach pair in a key describes a single feature: step one asks only 'feathers?', without slipping in colour or size as well. Comparing one feature at a time keeps the thinking clear; mixing several makes both options fit at once and no choice possible. The rule looks simple, yet it is the key to a key — designers and users alike must follow it strictly.
- Choices must be mutually exclusiveEach pair must be mutually exclusive, meaning any organism can match only one option: 'can fly' with 'cannot fly' works as a pair; 'can fly' with 'is black' does not, because a black crow satisfies both and leaves the user stranded. When designing, ask of each pair whether some creature could satisfy both — if so, swap in a feature that divides cleanly.
- Worked example: four animalsTo identify a sparrow, dog, goldfish and snake: step 1 asks about feathers — feathers give the sparrow, none moves on to step 2; step 2 asks about fur — fur gives the dog, none moves to step 3; step 3 asks whether the body is scaled and limbless or finned — the former is the snake, the latter the goldfish. Three steps place all four animals, showing how efficient keying is.
- Designing a keyDesign a key from the broad to the fine: to key out shark, dolphin, squid and jellyfish, first find a feature that splits the four in two — backbone or not separates the first pair from the second; then give each half a new feature, distinguishing shark from dolphin by breathing organs and squid from jellyfish by body symmetry. Every step obeying the two rules of exclusivity and single features makes the key tight and usable.
- Checking and usingA finished key must be checked: let classmates run through it, and if any organism can take two paths, or reaches a step with no path at all, that step's feature must be replaced until every organism has exactly one route. Scientists describing a new species likewise key it out first, confirming it matches no known species before naming it. A good key usually takes several rounds of revision before it is final.
4Diagrams Diagrams
5Worked example Example
Example: design a dichotomous key for pigeon, dog, snake and goldfish.
- Step 1: 1a body covered with feathers → pigeon; 1b no feathers → go to 2.
- Step 2: 2a body has fur → dog; 2b no fur → go to 3.
- Step 3: 3a no limbs, body covered with scales → snake; 3b has fins and lives in water → goldfish.
- Check: each animal follows exactly one path, none overlapping.
- Answer: Feathers = pigeon, fur = dog, limbless with scales = snake, fins = goldfish — two choices per step, narrowing steadily.
6Common mistakes Mistakes
7Quick check Quick check
Q1How many options does each step of a dichotomous key offer?
Q2What should you do first when using a key?
Q3Can 'has wings' and 'is black' serve as one couplet? Why?
Q4With a balanced key for 8 species, at least how many choices identify one species?
Q5After testing, one organism can take two paths. What should you do?
8Recap Recap
- A dichotomous key offers two mutually exclusive choices at each step to guide identification.
- Compare exactly one feature per step.
- Options must be mutually exclusive so every organism has a single path.
- When designing, first find a feature that splits the whole group in two, then continue within halves.
- Always trial-run the finished key and swap features where it fails.