Innovation of conventional spectrometers is of actual technical and economical value. It is also an important way to accelerate the development of spectroscopic instruments. When improving a conventional spectrometer, its dispersion part is pivotal, because it is decisive to the basic performance of the spectrometer. In the present paper, the typical dispersion parts of conventional spectrometers are compared to feature them and find the evolution force among them. The basic characters of the dispersion parts, including spectral range, dispersion power, resolution and throughput, are compared separately and comprehensively by reviewing their decisive factor, formula and typical data. The results not only conclude the feature and the complementariness of the dispersion parts, but also indicate that the trade-off between resolution and throughput is ubiquitous in traditional spectrometers. Further reviewing from this point, the evolution history of traditional spectrometers shows that the conflict between resolution and throughput is an important evolution force. This is a new way to understand the evolution of traditional spectrometers. Moreover, dealing with the trade-off between resolution and throughput correctly will help to analyze and settle the core problem of spectrometers.