What combination of factors both natural and man-made is necessary for healthy river restoration and how does this enhance the sustainability of natural and human communities? Objective How does biological evolution account for the diversity and distribution of salmonid species? Background This lesson focuses on some of the natural history and stochastic events that have influenced the current distributions of salmonid fish in western North America. The relatedness of salmonid species and some of the events of the past, of which the ice age is the most important, can be examined using phylogen Duration: 60 Minutes.