Lesson 25 — Deciding one species or two — and what would change your mind
BIO 202, Spring 2026, draft v0. Species is a hypothesis about future independence. Different cases test the hypothesis differently. There is no one right definition.
Draft skeleton. Stage scenarios and anchor quotes are in place; the simulator and code panels are not yet wired.
Species is a hypothesis. When you say two things are different species, what you're really saying is: I hypothesize that these two things will not influence each other in the future. When you frame it as a hypothesis of independence, a lot of the really difficult marginal cases become much more obvious. The way you'd really want to test it is have a time machine and check whether they're interbreeding 100 years from now. Until then it's a model — how good are the data that you buy that prediction.
— 202_lec25_01
A — Two populations, you make the call
Show the student data on a marginal pair — overlapping but distinguishable in phenotype, weakly interfertile in the lab, geographically distinct. Ask: one species or two? Whichever they pick, ask what observation would flip their answer.
Looking for an answer is a terrible way to learn anything. Knowing a bunch of definitions for species does not help you understand a concept. Working your way through something hard, to understand the nuance, even if you arrive at a different answer than somebody else does. If you and Abby agree on the same five forces but weight them differently, you can productively discuss. If you just are focused on a definition and you meet someone with a different definition, you just disagree and you're stuck.
— 202_lec23_01
TODO: one-or-two decision UI. Show data; let student commit; require a free-text "what would change my mind" before unlocking.
B — The ring of warblers
A breeds with B, B with C, C with D, D with E. But E and A cannot. There is no clean answer. The species concept that works for one part of the ring doesn't work for the whole. Walk through greenish warblers, Larus gulls, Ensatina salamanders.
A breeds with B. B breeds with C. C breeds with D. D breeds with E. But E and A cannot breed with each other, and where they overlap, they don't. This is the greenish warbler complex around the Himalayan mountains. The Larus gulls do this around the North Pole. The Ensatina salamanders do it around the Central Valley in California. The "can they breed" definition of species breaks here.
— 202_lec23_02
TODO: ring-species walkthrough. Pairwise interfertility map; one-vs-multiple species count under different definitions.
C — Ontology vs epistemology
Separate the definition of what species are from the method of figuring out whether two things are one. Different methods are right for different cases. The ontological commitment — that species are independent lineages — is stable; the epistemological method varies.
Ontology is understanding what things are. Epistemology is how we know what they are. It's the difference between knowledge and method. People conflate them. If you take too hard a stance on the epistemological way you figure out if something is a species — gene flow, reproductive isolation — you murky up your ontological definition. Define what species are (independent lineages); use whatever method fits the case (gene flow for living sexual organisms; morphology for fossils; ecology for asexuals).
— 202_lec25_02
TODO: ontology-vs-method matrix. Five cases × three methods; show that the right method depends on the case.
D — Five hard cases
Decide one species or two for: a ring species, Rhagoletis flies (host-race speciation), F2 breakdown in sticklebacks, color polymorphism in chrysanthemums, sympatric apple/hawthorn fly populations. For each, name the observation that would flip your call. (Scaffold S20 already drills this; this lesson formalizes it.)
Before apples were here, the Hawthorn flies had a bunch of different ways to find hawthorns. All the alleles for which compound they were smelling were neutrally moving around. Apples come over. Now some of those alleles actually lead the flies to apples instead of hawthorns. The flies that go to apples mate with other flies on apples and lay their eggs on apples. The flies that go to hawthorns mate with each other on hawthorns. They've inbred into two lineages. Their FST is dropping. The behavioral preference, formerly neutral, has just severed the population.
— 202_lec25_03
TODO: five-case decision panel. Connect to S20 scaffold. Non-trivial code mod: student picks a real species-status dispute from current literature and writes a brief defending one side.