Organisms are sorted into groups by the features they share. A species is defined by breeding, every species has a two-part scientific name, and a dichotomous key identifies an organism one pair of choices at a time. Extended: classification aims to show evolutionary relationships, and DNA base sequences reveal them.
Organisms are classified into groups by the features that they share. A species is a group of organisms that can reproduce to produce fertile offspring. The binomial system is an internationally agreed system in which the scientific name has two parts: the genus then the species, as in Panthera leo. A dichotomous key identifies an organism through pairs of statements about features you can see. Extended: classification aims to reflect evolutionary relationships, and organisms that share a more recent ancestor have more similar DNA base sequences.
Fig. 1: species in the same genus share more features than species in different families
Section 01
Species and scientific names.
Cross each pair of animals in the simulator, then try to breed their offspring. Then click two species in the tree to see how closely related they are.
Classification: organisms are placed in groups by the features that they share. Members of a small group, such as a genus, share more features than members of a large group, such as a kingdom.
Species: a group of organisms that can reproduce to produce fertile offspring. A Great Dane and a Chihuahua look very different, but their puppies are fertile, so they are the same species, Canis familiaris.
Not the same species: a horse and a donkey can mate, but the mule is almost always sterile. Horses (Equus caballus) and donkeys (Equus asinus) are different species in the same genus. A lion and a tiger can produce a liger, but male ligers are sterile.
Binomial system: an internationally agreed two-part name showing the genus and the species. The genus starts with a capital letter, the species with a small letter, and the name is printed in italics (underlined when handwritten).
Why it helps: common names differ between languages and regions, but the scientific name is the same for scientists everywhere.
Examiner trap
A species is not defined by appearance. Animals that look alike can be separate species, and animals that look very different, like dog breeds, can be one species. The test is whether they produce fertile offspring. Offspring alone is not enough: a mule is offspring, but it is sterile.
Section 02
Following a dichotomous key.
Look at the mystery organism (drag to turn it), answer each question with (a) or (b) and follow the key until it names the organism. Press New organism and repeat until you can do it without mistakes.
Step
Statement a
Statement b
1
jointed legs and a hard exoskeleton: go to 2
a backbone: go to 5
2
more than five pairs of legs: centipede
five pairs of legs or fewer: go to 3
3
has antennae: go to 4
no antennae: spider
4
wings and three pairs of legs: bee
no wings, more than three pairs of legs: crab
5
has fur: dog
no fur: go to 6
6
has feathers: bird
no feathers: go to 7
7
has fins: fish
no fins: go to 8
8
moist, smooth skin: frog
dry, scaly skin: lizard
Constructing a key: list the organisms, choose one visible feature that splits them into two groups, and keep splitting each group until every organism is on its own. A key for n organisms needs n − 1 pairs of statements.
Good statements are a pair of opposites about the same feature (“has wings” and “has no wings”), so every organism fits exactly one.
Examiner trap
Base every statement on a feature you can see and that does not change. “Is large”, “is brown” or “lives in a pond” lose marks: size and colour vary between individuals and with age, and habitat is not a feature of the organism.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
Which description of a species is correct?
Quick check+10 XP
In the simulator a Great Dane is crossed with a Chihuahua. What does the result show?
Quick check+10 XP
How should the scientific name of the lion be written?
Quick check+10 XP
In the key in the simulator, which pair of statements separates the spider from the crab?
Section 03
Extended: classification and DNA.
Extended content. Choose a species to compare with, look at how many bases differ, then press Join the closest pair until the tree is complete.
Aim of classification: classification systems aim to reflect evolutionary relationships, so that organisms in the same group share a common ancestor.
DNA as evidence: the sequences of bases in DNA are used as a means of classification. Mutations build up in DNA over time, so the longer ago two groups shared an ancestor, the more their base sequences differ.
In the simulator: human and chimpanzee sequences are 90% the same, human and gorilla 85%, human and mouse 70%, and the chicken only 58 to 65% the same as the others. So humans and chimpanzees share the most recent common ancestor, and the chicken is the most distantly related.
Why DNA helps: body features can mislead (a whale looks like a fish), but DNA comparisons show the true relationships.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which statement about the binomial system is correct?
Exam style+20 XP
A student writes a key to separate four leaves. Which pair of statements is most suitable for the first step?
Exam style+20 XP
Extended: why are DNA base sequences used to classify organisms?
Exam style+20 XP
Extended: part of a gene is 90% the same in species P and Q, 85% the same in P and R, and 70% the same in P and S. Which species shares the most recent common ancestor with P?
Skill unlocked
Classification and dichotomous keys, mastered.
This skill is now recorded on your profile. Keep the chain going.