Evolution: the erratic path towards complexity

A central goal of evolutionary biology is to explain the origin of complex organs — the ribosomal machinery that translates the genetic code, the immune system that accurately distinguishes self from non-self, eyes that can resolve precise images, and so on. Although we understand in broad outline how such extraordinary systems can evolve by natural selection, we know very little about the actual steps involved, and can hardly begin to answer general questions about the evolution of complexity. For example, how much time is required for some particular structure to evolve? In a recent paper, Lenski et al. [1] give an intriguing example of how “digital organisms” can evolve. Their work suggests many lines of research, which might shed new light on an old problem. Complex systems — systems whose function requires many interdependent parts — are vanishingly unlikely to arise purely by chance. Darwin’s explanation of their origin is that natural selection establishes a series of variants, each of which increases fitness. This is an efficient way of sifting through an enormous number of possibilities, provided there is a sequence of ever-increasing fitness that leads to the desired feature. To use Sewall Wright’s metaphor, there must be a path uphill on the “adaptive landscape”.