The Complexity Ratchet: Stronger than Selection, Stronger than Evolvability, Weaker than Robustness

Area of Science:

• Evolutionary biology
• Computational biology
• Theoretical biology

Background:

• The evolution of biological complexity is a long-standing question in evolutionary biology.
• Existing theories often rely on natural selection to explain increasing complexity.
• Alternative mechanisms, such as non-selective processes, are also considered.

Purpose of the Study:

• To investigate the existence of a 'complexity ratchet' in digital organisms.
• To determine the role of selection and non-selective effects in the evolution of complexity.
• To understand the long-term dynamics of complexity and its relationship with fitness.

Main Methods:

• Utilizing the in silico experimental evolution platform Aevol.
• Evolving populations of digital organisms under varying environmental conditions.
• Analyzing the fitness, robustness, and evolvability of simple versus complex organisms.

Main Results:

• Organisms predominantly evolved complexity, even when it conferred lower fitness and offered no robustness or evolvability advantage.
• Selection was found to be necessary for complexity to evolve, ruling out purely non-selective explanations.
• Complex organisms rarely reverted to simplicity, instead accumulating further complexity with slow fitness gains.

Conclusions:

• A complexity ratchet, driven by negative epistasis, appears to govern the evolution of complexity.
• This ratchet prevents simplification, as mutations favoring simplicity become deleterious after complex mutations are fixed.
• Robustness, rather than selection, can overcome the constraints imposed by the complexity ratchet.