Protected domains and the cost of cooperation on weighted networks

RESUMO

Contacts that protect a cooperative group can also make its founding sites difficult for newborn mutants to reach. We study this tension in a donation game with benefit $b$ and cost $c$ on finite weighted networks. Reciprocal contacts determine averaged payoffs and route copying from sources selected globally with a payoff-dependent bias. We compare takeover probabilities of a cooperator and a defector, each introduced singly into a population of the opposite type. Exact linear response about neutral drift and elimination of fast configurations identify protected domains as collective competitors. Under uniform introduction, the sharp weak-selection threshold infima are $b/c=3$ on the four-cycle, $7$ on the five-site path and $1$ on every fixed path with at least six sites. Introduction through newborn offspring eliminates this advantage on the five-site path. On six sites it preserves the infimum one but raises the minimum contact contrast, the largest weight divided by the smallest, needed for threshold $1+\epsilon$ from $656\epsilon^{-2}$ to $4050\epsilon^{-3}$ at leading order. Global polynomial bounds force every six-site design approaching threshold one into the domain hierarchy. Algebraic relations among contact ratios control optimization over all positive weights and establish exact reflection symmetry of the optimizer at sufficiently large contrast. For uniform introduction, the geometry determines absorption and fixation times and tolerance to contact errors. Nonlinear selection narrows the favorable window, making the fixation-probability difference small near the minimum contrast. A sharp variance-time bound quantifies the observation cost when the site- and type-specific terminal fixation probabilities are independently unknown. The same contacts govern collective persistence, access by mutation and the time to resolve the invasion advantage.