Lesson 23 — The Dobzhansky–Muller incompatibility snowball
BIO 202, Spring 2026, draft v0. Hybrid incompatibilities accumulate not linearly but as a snowball. Two diverging lineages produce more incompatibilities the longer they've been apart — faster than linear.
Draft skeleton. Stage scenarios and anchor quotes are in place; the simulator and code panels are not yet wired.
The classic way to categorize speciation barriers: there are two ways to stop reproduction. Stop the wedding, or kill the baby. Stop the wedding — anything that prevents copulation: physical separation, behavioral mismatch, anatomical mismatch. Kill the baby — anything that lets the zygote form but kills it: blastula failure, gastrula failure, dies as a fetus, dies as a newborn before reproducing. Pre-zygotic and post-zygotic, in the fancy version. Just remember: stop the wedding, kill the baby.
— 202_lec26_03
A — Two populations diverging neutrally
Simulate two populations evolving in isolation, each picking up substitutions independently. Each substitution is fine on its own background. The hybrid is also mostly fine — but each new substitution adds a pair of loci that might not work together.
The big thing with species is to what extent these barriers just happen versus to what extent they're actively selected for. This bothers people at first: the vast majority of speciation has absolutely nothing to do with selection. We're not selecting for those differences. Drift is random, mutations are random, and differences accumulate over time. If two things are separated for long enough, when they come back together they often just don't work.
— 202_lec26_04
Each substitution on lineage A creates potential incompatibility with each substitution already accumulated on lineage B. The number of incompatibilities scales as n² where n is the number of substitutions per lineage. Hybrid fitness drops faster than linear.
As soon as I invert a region, I've created a barrier — not between individuals, but between chunks of a chromosome. There can no longer be gene flow between different chromosome chunks. Inversions are the seed of speciation.
— 202_lec25_06
TODO: incompatibility-snowball sim. log-log plot of incompatibilities vs divergence time; slope of 2.
C — Reinforcement closes the gap
Once post-zygotic incompatibility is in place, selection favors pre-zygotic isolation — because hybrids are wasted reproductive effort. Specify the test: when does reinforcement happen, and when does recombination break the linkage between the trait that causes the incompatibility and the trait that signals mate choice?
Reinforcement: the existence of a post-zygotic barrier (subfertile hybrids) creates positive selection for a pre-zygotic barrier (mate preference). You're reinforcing the barriers with each other — making a stronger combined barrier, more quickly independent. But it's a race between selection and recombination. You need subfertile heterozygotes that aren't too bad, and you need a preference, and you need the preference and the reason for subfertility to be linked, so recombination can't unscramble it.
— 202_lec26_05
TODO: reinforcement sim. Selection-vs-recombination race; show when reinforcement succeeds and when it fails.
D — Hybrid fitness across divergence — real data
Compare hybrid fitness as a function of divergence time across taxa. Drosophila, sunflowers, sticklebacks. The snowball pattern (t² scaling) is detectable in real data.
I will either have long-tailed squirrels or I will have short-tailed squirrels. That sounds like a boring world. I want a world where I have long-tailed squirrels and short-tailed squirrels. For that world to be, my medium-tailed squirrels can't breed. I need to stop making them.
— 202_lec27_03
TODO: cross-taxon snowball plot. Fit power-law exponent to each dataset; show clustering around 2. Non-trivial code mod: fit the exponent as a free parameter and ask whether 2 is in the CI.