Draft skeleton. Stage scenarios and anchor quotes are in place; the simulator and code panels are not yet wired.
Speciation traits, over time, become more common among species. This is selection not on the individual but on the whole species — on the act of speciating. It can be opposite of selection at the individual level. If I inherit sexual selection, my descendant species are more likely to speciate than one that inherits lower sexual selection. Lineages that speciate fast become more numerous on the planet — even if individuals within them aren't more fit.
— 202_lec29_01
A — Many species with identical individual fitnesses
Set up many lineages, each with identical individual-level fitness. No selection on individuals. But lineages can split (speciate) or terminate (go extinct). Some lineages split faster than others.
Speciation is how we save and sequester diversity. By separating populations such that I can't drift them to fixation or select them to fixation, I just get two different things. It's how we go from a homogeneous world of weird chemotrophs in the ancient oceans to a world where we have trees and grass and humans. Evolution destroys diversity. Speciation preserves it.
— 202_lec25_05
TODO: birth-death tree sim. Lineages with identical individual fitness but heritable speciation rates; watch the tree skew toward high-speciation lineages.
B — Add heritable speciation traits
Some lineages inherit sexual selection (or small geographic range, or specialist-feeding ecology — any trait that promotes splitting). Watch the descendant species count climb for those lineages, while individuals within them are no fitter than anyone else.
Two species arrive on a remote island: a platypus and a fairy wren. Platypus has weak sexual selection. Fairy wrens have strong sexual selection — they cheat constantly despite monogamous parenting. Same starting population. Same ecological capacity. Time passes. The platypus stays one species. The fairy wrens have speciated, maybe twice — three species now. Same total number of birds, but split across three. Were the new species better off as individuals? No — smaller effective populations, less efficient selection. But the lineage made more species. The trait of being-easy-to-speciate evolved its way to dominance.
— 202_lec29_02
TODO: platypus-vs-fairy-wren sim. Same starting population, different speciation rates; species count over time.
C — The diagnostic at the lineage level
Run the Price ratio with lineages as groups. cov(W_lineage, Z_lineage) measures species-trait differences in lineage diversification. Within-lineage cov(w_individual, z_individual) is what the rest of the course already taught. The diagnostic answers "is selection acting above the individual?"
The purpose of a species definition is to understand the partitioning of variation — the generation of diversity on the planet. If I'm evolving independently just because I'm a bird on an island or a bunch of mice in a box, I'm evolving independently, but not for a reason that matters. What we care about is a reason that matters. Speciation that survives recontact — that's the thing that creates permanent new differences on the planet.
— 202_lec27_02
TODO: lineage-level Price ratio. Show cases where it dominates the individual-level term (the lineage is the unit) and cases where it doesn't.
D — Bird wing shape and speciation rate
Across bird clades, wing shape predicts speciation rate: pointy wings (good for long-distance flight) make fewer species; rounded wings (poor dispersers) make more. Why? Because dispersal homogenizes; poor dispersers stay in place and accumulate isolation. The trait that promotes lineage proliferation is the trait that fails the individual-level fitness test for "flies well." Two opposite signals at two levels.
Pointy wings make fewer species. Rounded wings make more. Wing shape vs speciation. *(Compressed paraphrase of the AVONET / Sheard et al. finding the corpus references.)*
— 461_lec(wing-speciation, exact ID pending)
TODO: AVONET wing-shape vs speciation-rate analysis. Use data/clean/avonet_birds.csv; estimate per-clade speciation rate; .R export. Non-trivial code mod: re-run with a different trait (body mass, beak length) and ask whether the lineage-level signal flips when the individual-level signal does.