science

Jordan's rule

Jordan's rule is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Jordan's rule rather than just read about it. In short: Jordan's rule (sense 1) is an ecogeographical rule that describes the inverse relationship between water temperature and meristic characteristics in various species of fish. The most commonly observed relationship is that fin ray, vertebrae, or scale numbers increase with decreasing temperature.

Key takeaways

Jordan's rule belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Jordan's rule to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Jordan's rule from memory before moving on to harder problems.

Reference excerpt

Jordan's rule (sense 1) is an ecogeographical rule that describes the inverse relationship between water temperature and meristic characteristics in various species of fish. The most commonly observed relationship is that fin ray, vertebrae, or scale numbers increase with decreasing temperature. The rule is named after David Starr Jordan (1851–1931), the father of American ichthyology. Jordan's law (or rule) (sense 2) is also an ecogeographical rule (named after the same scientist) that states: "‘[g]iven any species in any region, the nearest related species is not likely to be found in the same region nor in a remote region, but in a neighbouring district separated from the first by a barrier of some sort’ This "rule" is frequently violated (see discussion in Fitzpatrick & Turelli 2007), but when patterns are consistent with Jordan's rule (sense 2), this suggests an important role for allopatric speciation in the diversification of the clade in question. Jordan himself wrote: "To this generalization Dr. Allen, in a late number of Science, gives the name of 'Jordan's Law.' The present writer makes no claim to the discovery of this law. The language above quoted is his, but the idea is familiar to all students of geographical distribution and goes back to the master in that field, Moritz Wagner." Thus, Jordan's law is an example of Stigler's law.

See also Allen's rule Allopatric speciation Bergman's rule Jordan's Principle

References

Worked examples

Example 1 — a first encounter with Jordan's rule

Start with the simplest possible case. Write down what Jordan's rule claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Jordan's rule before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Jordan's rule ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Jordan's rule

In research: Jordan's rule appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Jordan's rule in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Jordan's rule is common in secondary-school and first-year university syllabi. It links to neighbouring topics Ecogeographic rules, Ichthyology, so understanding it makes those chapters shorter.
In everyday life: Look for Jordan's rule outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Jordan's rule in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Jordan's rule means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Jordan's rule out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Jordan's rule in simple terms?

Why does Jordan's rule matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Jordan's rule?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Jordan's rule.