Lesson Plan

Freeing the Elwha (Salmonid Natural History)

Sockeye salmon spawning.

Overall Rating

Add your review
Grade Level:
Sixth Grade-Eighth Grade
Aquatic Studies, Biodiversity, Biology: Animals, Wildlife Biology
One Class Period
Group Size:
Up to 36
National/State Standards:
Washington State Standards
EALR 4: 6-8 LS3A, LS3E, LS2D
EALR 1: Component 1.2 
Social Studies
EALR 5: Component 5.2
EALR 2: Component 2.1


How does biological evolution account for the diversity and distribution of salmonid species?


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 phylogenetic trees. This information can then be used to determine how the phylogenetic tree branched as it did. Genetic isolation, the founder effect, genetic drift, and stochasticity have all played a role in the diversification of the various species, subspecies, stocks, and runs of salmonids in western North America

  • Anadromous- A fish that is born in freshwater, migrates to the ocean to spend its adult life, and returns to freshwater to spawn.
  • Stochastic Event- A random event that can dramatically alter population sizes, selection mechanisms, and genetic isolation. Stochastic events can range from landslides isolating fish populations, droughts and forest fires eliminating habitat and isolating organisms in pockets or available habitat, or hurricanes that randomly kill individuals regardless of their genetic fitness.
  • Phylogenetic Tree- A diagram showing the genetic relationships of biological organisms based on when new species or subspecies differentiated. Historically, they were built using the evaluation of changes in morphology. However, today DNA sequencing is used to evaluate changes in the genetic code.
  • Pluvial Lake- A lake that formed when rainfall inputs were significantly higher than evaporate rates. Often these refer to ancient lakes when climates were different, such as the ice age. An example is Ancient Lake Bonneville, now dried up to become the Great Salt Lake.
  • Pro-glacial Lake- A lake that forms either behind an ice dam formed by a glacier or from the meltwater of a glacier that is dammed by the terminal moraine. An example is Glacial Lake Missoula, where an ice dam blocked the flow of the Clark Fork River in Montana.
  • Genetic Isolation: The absence of genetic exchange between populations or species as a result of geographic separation or of mechanisms that prevent reproduction.
  • Genetic Drift- The random changes in genetic variation related to small isolated populations suffering from stochastic events not necessarily related to fitness or natural selection.
  • Founder Effect- The loss of genetic variation caused by the colonization of a new area by a small population. The limited gene pool means that future offspring can only carry the characteristics of the founders, and may have significantly different characteristics than the species as a whole.
  • Endorheic basin- A depression that water drains into, but has no outlet. The Great Basin is one of the largest endorheic basins in the world. Lakes and ponds are often small scale endorheic basins.


  • Lesson 10- Salmonid Natural History.pptx
  • Speciation Articles.pdf
  • Reflection Journal Pages (Printable handout)
  • Vocabulary Notes (Printable Handout)


Additional Resources



Contains a great analogy between salmon and cows:


For struggling learners:




Stochastic Event
Phylogenetic Tree
Pluvial Lake
Pro-glacial Lake
Genetic Isolation
Genetic Drift
Founder Effect
Endorheic basin

Last updated: February 28, 2015