Classification and phylogenyAQA A-Level Biology: Revision notes

Section 1

Phylogenetic classification

A phylogenetic classification system attempts to arrange species into groups based on their evolutionary origins and relationships. Species that share a more recent common ancestor are placed closer together.

It uses a hierarchy, in which smaller groups are placed within larger groups, with no overlap. Each group is called a taxon (plural taxa).

Key termsphylogenetic classificationhierarchytaxon

Section 2

The taxonomic hierarchy

One hierarchy comprises, from largest to smallest:

domain, kingdom, phylum, class, order, family, genus, species

Each taxon fits into only one larger taxon. Species in the same genus share more features and a more recent common ancestor than species that share only a family. The more taxa two species share, the more closely related they are.

You do not need to recall different taxonomic systems, such as the three-domain or five-kingdom systems.

Key termsdomaingenus

Exam tip

Remember the order with a mnemonic, for example: Dear King Philip Came Over For Good Soup.

Section 3

Binomial naming

Each species is identified universally by a binomial consisting of the name of its genus and its species, for example Homo sapiens. The genus name has a capital letter, the species name does not, and both are written in italics (or underlined if handwritten).

Binomials are used by scientists in all countries, so the same species is identified whatever the local language or common name.

Key termsbinomial

Common mistake

Writing Homo Sapiens or homo sapiens is incorrect. Use a capital for the genus and a lower case letter for the species.

Section 4

Evidence for evolutionary relationships

Advances in immunology and genome sequencing help to clarify evolutionary relationships between organisms.

Genome sequencing compares the base sequences of DNA. The more similar the sequences, the more closely related the species. Mutations accumulate over time, so species that diverged long ago have more differences.
Immunology compares proteins. Antibodies raised against a protein of one species bind more strongly to the same protein in a closely related species, because related species have similar amino acid sequences.

These methods can show that species that look similar are not closely related, so classification can be checked and revised.

Key termsgenome sequencingimmunology

Section 5

Interpreting data

In a question with data, rank the species by similarity and say what the ranking means. For example, if species B has 98% sequence identity with A and D has 76%, B is more closely related to A. Quote values and calculate, for example differences of 2% and 24%, so D has $24 \div 2 = 12$ times as many differences.

If two independent methods give the same ranking, you can be more confident. A limitation is that one gene or protein may not represent the whole genome.

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Classification and phylogeny

A zoologist is classifying big cats. She records the lion as Panthera leo, the tiger as Panthera tigris and the domestic cat as Felis catus. Each of these three species has been placed in a hierarchy of taxa.
Suggest what the classification tells you about the evolutionary relationships between the lion, the tiger and the domestic cat.2 marks
A zoologist is cataloguing mammals from several countries. The common names of the same animal differ between languages. For example, the grey wolf is called 'wolf' in English, 'Wolf' in German and 'loup' in French, but all scientists refer to it as Canis lupus.
State the genus and the species of the grey wolf, and explain why a species cannot be placed in two different genera in the hierarchy.2 marks
Three species of beetle, P, Q and R, are placed in a phylogenetic classification. P and Q are in the same family but in different genera. R is in the same order as P and Q, but in a different family. A fourth species, S, is placed in the same genus as P.
Of the three species P, Q and R only, use the information to deduce which two are the most closely related and which is the least closely related to the other two, and explain your answer.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).