Viral sex, levels of selection, and the origin of life.

Several issues in Chao's related paper J. theor. Biol. (1991, 153, 229-246) are revisited. It is argued that mixes of segments from different viral coinfection groups cannot be regarded as sex, unless one is willing to accept that these groups are replicators and individuals. But, because selection in coinfection groups is dynamically analogous to that in trait groups in structured demes, one should also regard these latter groups as replicators. This approach is unacceptable since the groups in question have irregular ploidies, an unfixed number of parents, and no rules analogous to those of meiosis. It is emphasized, however, that the effective presence of neighbour-modulated fitness can ensure dynamical coexistence of covirus segments, even if the equal net reproduction rate within groups is not warranted. It seems that during the origin of coviruses from complete viruses, a higher-level evolutionary unit has become disintegrated, whereas during the origin of life a higher-level unit, the protocell, has emerged from lower-level ones, i.e. unlinked, replicating genes. These two gene-level systems are not homologous, but analogous. Although it is true that the resistance to parasites and the need to avoid a mutational collapse of the genome are likely to have called for some compartmentation in precellular stages of evolution, no clear demonstration, that the proposed mechanisms (the compartmentalized hypercycle and the stochastic corrector model) do in fact solve the error threshold problem, exists. Neither has a plausible mode of protocellular sex been suggested.