Experimental fitness landscapes to understand the molecular evolution of RNA-based life

In evolutionary biology, the relationship between genotype and Darwinian fitness is known as a fitness landscape. These landscapes underlie natural selection, so understanding them would greatly improve quantitative prediction of evolutionary outcomes, guiding the development of synthetic living systems. However, the structure of fitness landscapes is essentially unknown. Our ability to experimentally probe these landscapes is physically limited by the number of different sequences that can be identified. This number has increased dramatically in the last several years, leading to qualitatively new investigations. Several approaches to illuminate fitness landscapes are possible, ranging from tight focus on a single peak to random speckling or even comprehensive coverage of an entire landscape. We discuss recent experimental studies of fitness landscapes, with a special focus on functional RNA, an important system for both synthetic cells and the origin of life.