A graded SNV series whose zero-mutation prototype is found in market animals and which progresses toward the human epidemic genotype indicates the virus was actively evolving within civets at the market, consistent with market amplification/adaptation rather than a long-established stable civet reservoir.
Reasoning
The 27 SNVs partition sequences into an ordered ladder (0 → 2-7 → 16-22 → 25-27 mutations). The least-evolved prototype (0 SNVs) is recovered from market animals, and intermediate genotypes (2-7 SNVs) are shared between market civets and an early human patient, while fully human-epidemic strains carry the most mutations. An ordered accumulation of substitutions within the market population, with intermediates linking animal and human sequences, is the signature of ongoing microevolution/adaptation in the host being sampled - here civets at the market. A stable long-term reservoir would not be expected to yield a graded, still-diversifying series culminating in the human-infecting genotype. This supports civets being an amplifying host in which the virus adapts toward human infectivity, and that this was occurring at the market that seeded human cases.
Verdict — approved (checked)
Reconstruction. Premises: the 27 SNVs sort sequences into an ordered ladder (0 → 2-7 → 16-22 → 25-27); the 0-SNV prototype is in market animals; 2-7-SNV intermediates are shared between market civets and an early human case; the most-mutated genotypes are the human epidemic strains. Load-bearing step: an ordered, still-diversifying substitution series sampled within a host population, polarised from an animal-borne least-derived form toward the human genotype, is the signature of ongoing microevolution in that population rather than sampling from a fixed long-established reservoir.
Evaluation. Conditional on the premises the step holds for the hedged conclusion actually stated (“indicates”/“consistent with”, not “proves”). The polarity — least-derived form in animals, most-derived in humans — establishes the animal→human direction, and the presence of multiple co-circulating market-civet genotypes shows active diversification in that population, which a stable long-term reservoir (expected to present standing diversity without a graded ladder culminating in the human strain being freshly sampled) would not produce. Candidate undercutting defeater: the ladder could reflect a transmission chain whose mutations partly accumulated in humans rather than within civets. This trims the strongest reading (“adapting within civets specifically”) but not the stated conclusion that civets were an amplifying host at the market rather than the stable reservoir — which the animal-side prototype and shared intermediates carry regardless. Approved; traced from the SNV ladder, author-blind.