Complex sentence: In general, the fitness increment of any one gene depends in a complicated way on many other genes; but, because of the way that the science of population genetics was developed, evolutionary scientists tend to think of epistasis as the exception to the rule.
Split into simpler sentences: In general, the fitness increment of any one gene depends in a complicated way on many other genes. But because of the way that the science of population genetics was developed, evolutionary scientists tend to think of epistasis as the exception to the rule.