The top row indicates interactions between two genes that are either additive ("a"), show positive epistasis ("b") or reciprocal sign epistasis ("c"). Below are fitness landscapes which display greater and greater levels of global epistasis between large numbers of genes. Purely additive interactions lead to a single smooth peak ("d"), as increasing numbers of genes exhibit epistasis, the landscape becomes more rugged ("e") and when all genes interact epistatically the landscape becomes so rugged that mutations have seemingly random effects ("f").