Using the in silico experimental evolution platform Aevol, we have tested the existence of a complexity ratchet by evolving populations of digital organisms under environmental conditions in which simple organisms can very well thrive and reproduce. We observed that in most simulations, organisms become complex although such organisms are a lot less fit than simple ones and have no robustness or evolvability advantage. This excludes selection from the set of possible explanations for the evolution of complexity. However, complementary experiments showed that selection is nevertheless necessary for complexity to evolve, also excluding non-selective effects. Analyzing the long-term fate of complex organisms, we showed that complex organisms almost never switch back to simplicity despite the potential fitness benefit. On the contrary, they consistently accumulate complexity in the long term, meanwhile slowly increasing their fitness but never overtaking that of simple organisms. This suggests the existence of a complexity ratchet powered by negative epistasis: Mutations leading to simple solutions, which are favorable at the beginning of the simulation, become deleterious after other mutations—leading to complex solutions—have been fixed. This also suggests that this complexity ratchet cannot be beaten by selection, but that it can be overthrown by robustness because of the constraints it imposes on the coding capacity of the genome.

中文翻译：

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复杂性棘轮：强于选择，强于可进化性，弱于鲁棒性

使用计算机实验进化平台 Aevol，我们通过在简单生物可以很好地繁衍和繁殖的环境条件下进化数字生物种群来测试复杂棘轮的存在。我们观察到，在大多数模拟中，生物体变得复杂，尽管这些生物体比简单生物体的适应性差得多，并且没有稳健性或可进化性优势。这排除了从复杂性演变的一组可能解释中进行选择。然而，补充实验表明，选择对于复杂性的发展仍然是必要的，也不包括非选择性效应。分析复杂生物体的长期命运，我们表明尽管具有潜在的适应性益处，复杂生物体几乎永远不会变回简单。相反，从长远来看，它们不断地积累复杂性，同时缓慢地增加它们的适应性，但永远不会超过简单生物体的适应性。这表明存在由负上位性驱动的复杂棘轮：导致简单解的突变在模拟开始时是有利的，在其他导致复杂解的突变被修复后变得有害。这也表明这种复杂性棘轮不能被选择打败，但它可以被鲁棒性推翻，因为它对基因组的编码能力施加了限制。导致简单解决方案的突变在模拟开始时是有利的，在其他突变（导致复杂解决方案）被修复后变得有害。这也表明这种复杂性棘轮不能被选择打败，但它可以被鲁棒性推翻，因为它对基因组的编码能力施加了限制。导致简单解决方案的突变在模拟开始时是有利的，在其他突变（导致复杂解决方案）被修复后变得有害。这也表明这种复杂性棘轮不能被选择打败，但它可以被鲁棒性推翻，因为它对基因组的编码能力施加了限制。