This book is an essential read for students of evolutionary processes. Chum salmon, Oncorhynchus keta, are used as an excellent example of the development of intra-specific genetic divergence by temporal isolation. Seasonally distinct reproductive populations are common in many taxa including flowering plants, insects and fishes. By examining the relative contributions of genotype and enironment to phenotypic diversity among seasonally isolated chum stocks the work provides insight into the process of sympatric speciation. Data from field and laboratory work is integrated to clearly show that both disruptive and stabilizing selective forces contribute to phenetic and genetic diersity. This work represents one of the earliest examples of the process of ecological speciation in fishes.