  Evolutionary adaptation is often likened to climbing a hill or peak. While
this process is simple for fitness landscapes where mutations are independent,
the interaction between mutations (epistasis) as well as mutations at loci that
affect more than one trait (pleiotropy) are crucial in complex and realistic
fitness landscapes. We investigate the impact of epistasis and pleiotropy on
adaptive evolution by studying the evolution of a population of asexual haploid
organisms (haplotypes) in a model of N interacting loci, where each locus
interacts with K other loci. We use a quantitative measure of the magnitude of
epistatic interactions between substitutions, and find that it is an increasing
function of K. When haplotypes adapt at high mutation rates, more epistatic
pairs of substitutions are observed on the line of descent than expected. The
highest fitness is attained in landscapes with an intermediate amount of
ruggedness that balance the higher fitness potential of interacting genes with
their concomitant decreased evolvability. Our findings imply that the synergism
between loci that interact epistatically is crucial for evolving genetic
modules with high fitness, while too much ruggedness stalls the adaptive
process.
