Jordan's rule

Jordan's rule is an ecogeographical rule named after the father of American ichthyology David Starr Jordan describes the inverse relationship between water temperature and meristic characteristics. The most common found relationship is for fin ray, vertebrae, or scale numbers to increase with decreasing temperature.^[1]

References

↑ McDowall, R. M. (March 2008). "Jordan’s and other ecogeographical rules, and the vertebral number in fishes". Journal of Biogeography. 35 (3): 501–508. doi:10.1111/j.1365-2699.2007.01823.x.

Biological rules
Rules	Allen's rule Shorter appendages in colder climates Bergmann's rule Larger bodies in colder climates Cope's rule Bodies get larger over time Dollo's law Evolution is irreversible Fahrenholz's rule Host and parasite phylogenies become congruent Foster's rule (Insular gigantism, Insular dwarfism) Small species get larger, large species smaller, after colonizing islands Gause's law Complete competitors cannot coexist Gloger's rule Lighter coloration in colder, drier climates Haldane's rule Hybrid sexes that are absent, rare, or sterile, are heterogamic Hennig's progression rule In cladistics, the most primitive species are found in earliest, central, part of group's area Jordan's rule Inverse relationship between water temperature and no. of fin rays, vertebrae Lack's principle Birds lay only as many eggs as they can provide food for Rapoport's rule Latitudinal range increases with latitude Rensch's rule Sexual size dimorphism increases with size when males are larger, decreases with size when females are larger Rosa's rule Groups evolve from character variation in primitive species to a fixed character state in advanced ones Thorson's rule No. of eggs of benthic marine invertebrates decreases with latitude Van Valen's law Probability of extinction of a group is constant over time von Baer's laws Embryos start from a common form and develop into increasingly specialised forms Williston's law Parts in an organism become reduced in number and specialized in function
Related	Countergradient variation Where genetics opposes environment as a factor Gigantothermy Large ectothermic animals more easily maintain constant body temperature

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