The big idea: There are far more endangered species than money, land and time to save them all.
So conservationists must choose which species to protect first — they prioritise.
The EDGE programme is one way of choosing. It rates each species on two things at once:
- How evolutionarily distinct it is (how unusual its place on the tree of life). - How globally endangered it is (how close it is to extinction).
| The two letters in 'EDGE' | What it measures |
|---|---|
| ED — Evolutionarily Distinct | Has few close living relatives — a unique, long branch on the tree of life |
| GE — Globally Endangered | At high risk of extinction worldwide (e.g. very few individuals left) |
What 'EDGE' stands for: E·D·G·E = Evolutionarily Distinct and Globally Endangered.
A species scores highly only when it is BOTH — strange on the tree of life AND close to dying out.
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An evolutionarily distinct species has very few close living relatives. Its branch on the evolutionary tree is long and lonely, so it carries a large share of unique evolutionary history and unique genes.
If that species goes extinct, that whole branch — and everything special about it — is lost forever, because no similar species can replace it.
- Biodiversity
- The variety of life — the range of different species (and of genes and ecosystems) in an area.
- Conservation
- Protecting species and habitats so that biodiversity is maintained for the future.
- Evolutionarily distinct
- Having very few close living relatives — a long, separate branch on the tree of life, carrying unique evolutionary history.
- Globally endangered
- At high risk of extinction across the whole world (e.g. very small or fast-falling population).
- EDGE species
- A species that is BOTH highly evolutionarily distinct AND globally endangered — so it is given high priority for conservation.
EDGE combines the two scores. A species that is distinct but common is not urgent — it is not about to disappear. A species that is endangered but has many close relatives is a smaller loss to the tree of life, because similar species remain.
The top priority is a species that is high on both — distinct AND endangered. Saving it protects the most unique, irreplaceable part of biodiversity.
| Example species | Evolutionarily distinct? | Globally endangered? | EDGE priority |
|---|---|---|---|
| A lone-branch reptile near extinction | Yes — few relatives | Yes — very rare | HIGH |
| A common bird with many close relatives | No — many relatives | No — abundant | LOW |
| A rare frog with many similar species | No — many relatives | Yes — rare | MEDIUM |
Why distinctness matters: Saving an evolutionarily distinct species protects a unique branch of the tree of life and its unique genes.
If it dies out, no other species can replace what is lost — so its disappearance costs biodiversity far more than losing a species with many close relatives.
An evolutionarily distinct species sits alone on a long branch — a high EDGE score.
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How this is tested: On Paper 1A (multiple choice) you are asked to identify why a species is prioritised by EDGE — the answer is that it is both evolutionarily distinct AND globally endangered.
On Paper 2 a short Describe question (about 2 marks) asks you to describe the purpose of the EDGE of Existence programme.
IB-style question — the purpose of EDGE
Describe the purpose of the EDGE of Existence conservation programme. [2]
How to score both marks
- Say what EDGE chooses between. It is a way of prioritising which species to conserve when resources are limited — it ranks species rather than treating them all the same.
- Give the two criteria it uses. It targets species that are evolutionarily distinct (few close relatives / a unique branch of the tree of life) and globally endangered (at high risk of extinction).
- Answer the command term ('describe'). Purpose: to direct conservation effort to the species whose loss would remove the most unique, irreplaceable evolutionary history — those that are both distinct and endangered.
Final answer
The purpose of EDGE is to prioritise conservation: it ranks and targets species that are both evolutionarily distinct (few close relatives, a unique branch of the tree of life) and globally endangered (at high risk of extinction), so that limited effort protects the most irreplaceable biodiversity.
The two marks the examiner wants: Mark 1: EDGE is for prioritising / choosing which species to conserve.
Mark 2: it targets species that are evolutionarily distinct AND globally endangered.
A common mistake is naming only one of the two criteria — you need both.