## Problem 147 — rank:274

**Title:** Prezygotic versus postzygotic isolation

**Added binding assumptions (LaTeX):**

```latex
Normalize stage success to matched compatible-control probabilities $p_i^0>0$ before interpreting a barrier as reproductive isolation. The ratios $r_i=p_i/p_i^0$ need not lie below one, so the normalized barrier can be negative for a cross outperforming its control; it is an index, not a failure probability. Products use stage-conditional probabilities from the same cross ensemble. Unnormalized total reproductive failure is a separate observable.
```

**Direct edits:** 2. **Fresh external checks:** None; supplied sources retained as provenance.