Explain that classification systems aim to reflect evolutionary relationships.

1.2 Concept and Uses of Classification Systems

Objective

Explain that classification systems aim to reflect evolutionary relationships.

Core ideas (syllabus 1.2)

• Organisms are placed into groups that share common features.
• A species is a group of organisms that can in nature interbreed and produce fertile offspring.
• Each species has a two‑part scientific name (binomial nomenclature): Genus species, written in italics with the genus capitalised.
• A simple dichotomous key can be used to identify organisms by answering a series of “either/or” questions.

Why do we classify living things?

• To organise the immense diversity of organisms.
• To provide a common language for scientists worldwide.
• To predict characteristics of organisms based on the group they belong to.
• To illustrate how species are related through common ancestry.

Definition of species

A species is a group of organisms that can in nature interbreed and produce fertile offspring.
Example: Homo sapiens (modern humans) – all humans can mate with one another and have fertile children.

Binomial nomenclature

The scientific name of every species consists of two parts:

1. Genus – capitalised (e.g., Felis).
2. Specific epithet – lower‑case (e.g., catus).

Both parts are italicised (or underlined when handwritten). Examples:

• Felis catus – domestic cat
• Quercus robur – English oak
• Escherichia coli – a common bacterium

Constructing a dichotomous key (action‑oriented)

To build a useful key you should:

1. Choose a set of contrasting, observable characters (e.g., presence/absence of a backbone, type of covering).
2. Arrange the characters from the most general (e.g., vertebrate vs. invertebrate) to the most specific (e.g., type of flower).
3. Write each pair of alternatives as a clear “either/or” statement, assigning a number to each step.
4. Test the key with several specimens and revise any ambiguous statements.

Using a dichotomous key – sample activity (local examples)

Follow the numbered statements; choose the option that best describes the organism and proceed to the indicated next step.

1. Does the organism have a backbone (vertebrate) or not?
   Yes → 2 | No → 5
2. Does it have feathers?
   Yes → 3 | No → 4
3. Is it capable of flight?
   Yes → Passer domesticus (House sparrow) | No → Struthio camelus (Ostrich)
4. Does it have hair or fur?
   Yes → Canis lupus familiaris (Domestic dog) | No → Rana temporaria (Common frog)
5. Is it a plant?
   Yes → 6 | No → 7
6. Does it have flowers?
   Yes → Rosa rubiginosa (Sweet briar) | No → Polypodium vulgare (Common polypody fern)
7. Is it an invertebrate animal with an exoskeleton?
   Yes → Carabus nemoralis (Ground beetle) | No → Octopus vulgaris (Common octopus)

Historical approaches

Early classification systems grouped organisms according to easily observed traits such as size, shape, colour, or habitat. These schemes did not consider evolutionary history and often placed unrelated organisms together.

Modern classification – reflecting evolutionary relationships

Today’s systems aim to group organisms that share a **recent common ancestor**. This phylogenetic (evolutionary) approach is based on three main lines of evidence:

• Shared derived characteristics (synapomorphies).
• Genetic similarity revealed by DNA‑base sequencing.
• Fossil evidence that calibrates the timing of divergence.

Key features of a phylogenetic system

1. Groups are defined by synapomorphies rather than superficial similarity.
2. Taxa are arranged in a hierarchy that mirrors the branching pattern of evolution.
3. Names reflect evolutionary lineage; closely related taxa have more recent common ancestors.
4. DNA‑sequence similarity is explicitly used: organisms that share a recent ancestor have more similar DNA.

Taxonomic hierarchy (Linnaean system)

Phylogenetic trees – visualising evolutionary relationships

A phylogenetic tree (cladogram) shows how groups diverge from common ancestors. The length of branches may represent:

• Time (chronogram) – measured in millions of years.
• Amount of evolutionary change (phylogram) – measured by genetic differences.

How classification reflects evolution

• Organisms placed in the same taxon share a more recent common ancestor than those in different taxa.
• DNA sequencing provides quantitative evidence of genetic similarity, confirming evolutionary links.
• Fossil records help to calibrate when divergence events occurred, allowing trees to be scaled in time.

Summary

Classification systems are not arbitrary lists; they are scientific tools that organise life according to evolutionary history. By grouping organisms that share common ancestors—and by using DNA evidence, fossil data, and shared derived traits—scientists can predict traits, understand biodiversity, and trace the tree of life.