This unit is comprised of nine separate lessons [approximately twelve 45-minute class periods] that touch on the topics of variation, selection, and evolution modeling by using examples of mountain sheep, snakes, moths, and finches change over time. As with our other units, students use MEL matrices to link each piece of evidence to one model or another with the aim of determining which model best represents the phenomenon they are trying to explain.

The unit begins with an exploration of variation both within and across species. Variation is the genetic cornerstone of natural selection. Understanding that variation exists, even when we cannot readily see it (variation internal structures etc.), is critically important in order to develop models of which variations confer a survival advantage in a particular environment.

Students then explore changes in populations of mountain sheep, moths, and snakes to explore how traits like bigger horns, colors, and smaller heads confer advantage due to different reasons (e.g. predation, avoiding harmful foods). Using a variety of evidence students develop and evaluate models that explain the variation in trait distribution over time across multiple generations in a population. Students then use these cases to develop a generalized model of natural selection.

Armed with a generalized model of natural selection students attempt to explain new phenomena such as colorfulness in guppy populations in different parts of a stream, bedbugs that are resistant to pesticides, and peacock’s tail displays. Thus students can develop a more robust understanding of natural selection by using their emerging knowledge in new contexts and thereby refining it.

Students are now ready to look at evidence that spans many years and, along with the arrows diagrams, evaluate each piece of evidence to determine how it corresponds to one of two models. The models include the initial variation of the species, selection pressure from environmental factors, generations and offspring, and the final variation of the species.

The unit culminates with a performance assessment involving the case of the ground finch in the Galapagos islands and the changes in the distribution of beaks size in the population before and after a severe drought event.

You can download the nine lessons of this unit in three parts:

Download Evolution Lessons 1-3 (46 MB)

Download Evolution Lessons 4-6 (23 MB)

Download Evolution Lessons 7-9 (23 MB)