What does this equation mean?

Graded against biological evolution, several parts of this history are exact rather than merely suggestive. Descent is literal: design documents, safety-case arguments, and component specifications pass from Nautilus to Shippingport to the commercial PWR fleet as physical inheritance, not metaphorical resemblance. Variation across national programs — American, French, Soviet, British, Canadian, Chinese — produced genuinely distinct lineages under geographic and institutional isolation, exactly as allopatric speciation produces distinct populations under geographic separation. Selection by markets, regulators, and accidents is observably real and, in the case of TMI, Chernobyl, and Fukushima, dated and measurable. And the founder-effect and lock-in mechanism Cowan documents is not just analogous to a general economic idea — it is the same model. W. Brian Arthur’s 1989 formalization of competing technologies under increasing returns describes exactly this dynamic as a generalized Pólya urn: two technologies accumulate adopters, and the probability the next adopter chooses technology A over B is proportional to how many adopters A already has, so that $P(\text{next adopter} = A \mid n_A, n_B) = \frac{n_A + a}{n_A + n_B + a + b}$. where $n_A$ and $n_B$ are cumulative past adoptions and a, b are small initial weights representing early, largely arbitrary advantages [ 2 ] . Once $n_A$ pulls ahead by chance or by an unrelated advantage — a submarine program’s development budget, say — the process is self-reinforcing regardless of whether A is the objectively superior technology, and it can lock in permanently long before the alternative gets a fair trial. That is Cowan’s finding about light water stated in the exact mathematical language Arthur built for the general phenomenon, not a loose parallel drawn after the fact [ 1 ] [ 2 ] .