Classification and cladistics

A group consisting of a **common ancestor AND all of its descendants** — a **monophyletic** group / a complete branch of the tree of life.

Tracing back to **one common ancestor**, with **all** of that ancestor's descendants included — another word for a clade.

**Natural** = grouped by real **common ancestry** (so each group is a clade). **Artificial** = grouped by convenient **surface features**, which can lump unrelated species together.

A feature **inherited from a shared common ancestor**. **Useful** — it groups genuine relatives.

A similar feature that evolved **independently** by **convergent evolution**. **Misleading** — it reflects lifestyle, not ancestry, so it would group unrelated species.

A **shared derived (homologous) trait** — one that first appeared in a common ancestor and was passed to **all** of its descendants.

Their streamlined shape is **analogous** (convergent evolution for fast swimming), not inherited from a recent shared ancestor. A dolphin is a **mammal**, a shark a **fish**.

A branching **tree-diagram** showing the **most probable evolutionary relationships** among groups (clades), based on the best current evidence.

A **common ancestor** — the point where one ancestral lineage **diverged (split)** into two lineages.

Find the pair whose branches meet at the **most recent node** (nearest the tips) — they share the **most recent common ancestor**.

The **deepest (oldest) node**, on the far left — the **common ancestor of every group** on the tree.

Their branches meet only at a **deep node (near the root)**, so their **common ancestor is much older** — they diverged earlier.

It shows the **most probable** relationships from the **evidence available** (now mainly DNA). New evidence can lead to it being **revised**.

Data from **molecular sequences** — the **base sequence** of a gene or the **amino-acid sequence** of a protein — used to work out how closely species are related.

A **gene's base sequence** (e.g. the **rRNA** gene) and a **protein's amino-acid sequence** (e.g. **haemoglobin**).

They **diverged longer ago** (more time for mutations to accumulate) and are **more distantly related**.

A **more recent** split — the species are **more closely related**.

Because **neutral mutations** accumulate at a **roughly constant rate**, the **number of sequence differences** can **estimate the time** since two species shared a common ancestor.

**Strength:** it is objective/quantitative and works even **without fossils**. **Caution:** rates **vary between genes and lineages**, so it must be **calibrated against fossils**.

A group made of **one common ancestor and ALL of its descendants** — nothing left out, nothing unrelated added in.

It describes a **valid clade** — an **ancestor plus all its descendants**. Cladistics aims to make every named group monophyletic.

When **molecular/cladistic (DNA) evidence conflicts** with the traditional **morphology-based** grouping, showing the old group is **not a clade**.

**DNA** showed its members did **not share a single common ancestor** (it was not a clade), so it was **split and reorganised** into several smaller true clades.

Cladistics places birds **inside the dinosaur/reptile lineage** (closest to crocodiles); a 'reptile' group without birds is **paraphyletic**, so to be a clade it must **include birds**.

If **DNA** shows its true closest relatives are in a **different genus**, it is **moved (and renamed)** so the genus stays a **monophyletic clade**.

That it is **provisional and evidence-led** — the **current best hypothesis** of relationships, open to revision when better molecular data arrives.

Unrelated species can **look alike** (convergent evolution) and close relatives can **look very different**, so a looks-based group may mix lineages or split real ones.