Classifying organisms by common ancestry (clades)
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Question
Define a clade.
Answer
A group consisting of a **common ancestor AND all of its descendants** — a **monophyletic** group / a complete branch of the tree of life.
Question
What does 'monophyletic' mean?
Answer
Tracing back to **one common ancestor**, with **all** of that ancestor's descendants included — another word for a clade.
Question
Natural vs artificial classification?
Answer
**Natural** = grouped by real **common ancestry** (so each group is a clade). **Artificial** = grouped by convenient **surface features**, which can lump unrelated species together.
Question
Homologous trait — what is it, and is it useful for classification?
Answer
A feature **inherited from a shared common ancestor**. **Useful** — it groups genuine relatives.
Question
Analogous trait — what is it, and why is it misleading?
Answer
A similar feature that evolved **independently** by **convergent evolution**. **Misleading** — it reflects lifestyle, not ancestry, so it would group unrelated species.
Question
What kind of trait defines a clade in cladistics?
Answer
A **shared derived (homologous) trait** — one that first appeared in a common ancestor and was passed to **all** of its descendants.
Question
Why aren't a dolphin and a shark in the same clade despite their similar shape?
Answer
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**.
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Topic 1.7 hub
Classification and cladistics
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