International A Level Biology / Classification
Classifying living things
Edexcel International A Level Biology (YBI11), chapter 11: Classification, specification point 4.14. Free lesson.
Read the revision notes · International A Level Biology past papers
Classification groups organisms according to how closely related they are, using evidence from both phenotype and genotype. The ranks and binomial names help biologists describe these relationships consistently.
What classification is based on
Classification organises the variety of life into groups based on how closely organisms are related. Biologists compare similarities and differences in phenotype and genotype, so classification is about relationships, not just appearance. Phenotype means observable features, such as anatomy, physiology and behaviour. Genotype means an organism’s DNA, compared using its base sequences.
The eight ranks, from broad to specific
Each classification group sits inside the rank above it. The species is the basic unit, and each higher rank contains a larger collection of organisms. In humans, the ranks are Domain Eukarya, Kingdom Animalia, Phylum Chordata, Class Mammalia, Order Primates, Family Hominidae, Genus Homo and Species sapiens. Eukarya organisms have cells with a nucleus. Chordates have a notochord at some stage of life, a stiff supporting rod along the back that is replaced by the backbone in humans. Mammals have hair and feed their young on milk. Hominidae is the family of great apes, including humans,…
What shared ranks tell you
The ranks show how organisms are related. The more ranks two organisms share, the more closely related they are and the more recent their common ancestor. Humans, lions and tigers all belong to Class Mammalia. Humans split from lions and tigers at the Order: humans are Primates, while lions and tigers are Carnivora. Lions and tigers share Family Felidae and Genus Panthera. They separate at the species level.
How to write a binomial name
A binomial name is the two-part scientific name of a species. It uses the genus name followed by the species name, giving scientists a consistent name across countries. Write the genus with a capital letter and the species name in lower case. Use italics in print. When handwriting, underline both words.
Why appearance alone can mislead
Similar features can evolve separately in unrelated groups that live in similar ways. This means phenotype alone can give a misleading idea of relationships, so biologists also compare DNA base sequences. Dolphins and sharks both have streamlined shapes, but this does not mean they are closely related. Dolphins have lungs, feed their young on milk and have DNA that matches other mammals more closely. Dolphins are mammals and sharks are fish. A spotted hyena looks like a dog, but its DNA base sequences are closer to cats’ than to dogs’. This suggests hyenas share a more recent common…