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Presumably a "species" should be an equivalence class, right? So the relation "A and B are of the same species" should be transitive, symmetric, and reflexive.
by benkuykendall 10y ago
Presumably a "species" should be an equivalence class, right? So the relation "A and B are of the same species" should be transitive, symmetric, and reflexive. Symmetry is easy (an organism shares 100% of its genome with itself) as is reflexivity (A shares n% with B iff B shares n% with A). However, transitivity does not follow obviously: for example, A and B could share 98%, B and C could share 98%, but A and C could share 96%. Which species(es) do the three belong to?
- rcthompson 10y agoUnfortunately, speciation is mostly a continuous process, so there's no way to define species by some kind of "degree of similarity/dissimilarity" metric and have it be a transitive relationship. Any species definition that is completely transitive will necessarily have edge cases where two extremely similar organisms fall on different sides of an arbitrary threshold and are declared to belong to two different species. The classic example of non-transitive species relationships from sexually reproducing organisms is a "ring species": https://en.wikipedia.org/wiki/Ring_species https://en.wikipedia.org/wiki/Ring_species
- sn9 10y agoA continuous process and also a human construct that we created to help us reason about how that continuous process works. Every biology major learns about the different species concepts/definitions [0] and why different subfields of biology might prioritize one over another. [0] https://en.wikipedia.org/wiki/Species https://en.wikipedia.org/wiki/Species