Hickinbotham, Simon John, Susan Stepney, and Paulien Hogeweg. “Nothing in evolution makes sense except in the light of parasites.” bioRxiv (2021).
URL
The emergence of parasites in evolving replicating systems appears to be inevitable. Parasites emerge readily in models and laboratory experiments of the hypothesised earliest replicating systems: the RNA world. Phylogenetic reconstructions also suggest very early evolution of viruses and other parasitic mobile genetic elements in our biosphere. The evolution of such parasites would lead to extinction unless prevented by compartmentalisation or spatial pattern formation, and the emergence of multilevel selection. Today and apparently since the earliest times, many intricate defence and counter-defence strategies have evolved. Here we bring together for the first time automata chemistry models and spatial RNA world models, to study the emergence of parasites and the evolving complexity to cope with the parasites. Our system is initialised with a hand-designed program string that copies other program strings one character at a time, with a small chance of point mutation. Almost immediately, short parasites arise; these are copied more quickly, and so have an evolutionary advantage. Spatial pattern formation, in the form of chaotic waves of replicators followed by parasites, can prevent extinction. The replicators also become shorter, and so are replicated faster. They evolve a mechanism to slow down replication, which reduces the difference of replication rate of replicators and parasites. They also evolve explicit mechanisms to discriminate copies of self from parasites; these mechanisms become increasingly complex. Replicators speciate into lineages and can become longer, despite the fitness cost that entails. We do not see a classical co-evolutionary arms-race of a replicator and a parasite lineage: instead new parasite species continually arise from mutated replicators, rather than from evolving parasite lineages. Finally we note that evolution itself evolves, for example by effectively increasing point mutation rates, and by generating novel emergent mutational operators. The inevitable emergence of parasites in replicator systems drives the evolution of complex replicators and complex ecosystems with high population density. Even in the absence of parasites, the evolved replicators outperform the initial replicator and the early short replicators. Modelling replication as an active computational process opens up many degrees of freedom that are exploited not only to meet environmental challenges, but also to modify the evolutionary process itself.