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Table 2 Four cases developed to support the teaching of evolution across the biology curriculum

From: Integrative cases for teaching evolution

a. Mouse Fur Color - evolution of light fur in beach mice b. Pea taste - evolution of the wrinkled pea from ancestral (wild type) round pea shape
The beach mouse, Peromyscus polionotus, lives in the southeastern US. The fur color of subspecies varies from dark to very light brown and is correlated with the color of the sand on which they live. Activities presented in this case show the relationships among depredation, substrate color and fur color. Peromyscus fur color is closely associated with the MC1R protein, which stimulates the synthesis of the pigment, eumelanin. A single nucleotide mutation in the mc1r gene results in a different form of the protein which impacts fur color. The biosynthesis of eumelanin in the melanocyte is graphically presented. Recent studies have shown correlations between the mc1r alleles and the coat color phenotypes in populations of beach mice. Data from the work of Kaufman (1974) on owl hunting of mice in different backgrounds, correlations of background “brightness” and fur color (Belk and Smith 1996) and predation studies of Peromyscus (Vignieri et al.2010) are included in this case. These studies provided basic information for the development of a game, where light or dark mice live on either a light or dark background and are subjected to predation. A second, timed, game allows the player to find light mice in various background colors. Additional data on allele frequencies and distribution of fur color alleles (e.g., Hoekstra et al.2006) are included in the case. Every college student has heard the story of Mendel’s peas (Pisum sativum), first domesticated (an example of artificial selection) 10,000 BCE. Historically, this organism served as the introduction to basic transmission genetics in both secondary and post-secondary biology classes. Modern evidence shows that an insertion of 800 nucleotides in the gene for starch branching enzyme (SBE1) renders that enzyme non-functional in the homozygous recessive (rr) condition. The biochemical pathway in pea cells with both functional and non-functional SBE1 proteins is shown. Because sugars are not converted into complex starches, these peas (rr genotype) accumulate sugars and are sweeter. Ancient peoples liked the sweeter phenotype, applying intense selection, which led to fixation of the r allele in populations of cultivated peas. This intense selection is the basis for the Selective Farmer Game, where the player can see the results of selection (or not) on the proportion of alleles (R, r) in a field of peas over time.
c. Monkey opsins - evolution of trichromatic vision in Old World monkeys d. Clam toxin - evolution of toxin resistance in clams
The adaptive advantages of both dichromatic and trichromatic monkeys are presented in this case, using experimental data and activities that illustrate their foraging behavior. The phylogenetic relationships, as well as biogeographical information, among di- and trichromatic monkeys are well known; thus, the approximate time of the development of trichromatic vision can be inferred. Most students are aware that red/green color vision is sex-linked in primates; a thorough discussion of the molecular and transmission genetics of these traits is presented. The differences between the three types of primate genes underlying color perception and the resulting proteins are discussed. Gene duplication and historical nucleotide changes in the red/green opsins are considered, as well as the structure and function of opsin proteins and cone cells. This case includes data regarding food selection and pattern recognition in di- and trichromats (Smith et al.2003; Caine and Mundy 2000; and Saito et al.2005). These data were the inspiration for the development of two games: 1) finding colored cereal as a di- or trichromat in a field; and 2) pattern recognition as either a di- or trichromat in variously colored backgrounds. Mya arenaria is a species of soft shell clams, native to the eastern North American coast, where dinoflagellate “blooms” kill coastal creatures. The agent of death is saxitoxin, a potent neurotoxin. This toxin binds to voltage-gated sodium channels, preventing the required flow of sodium ions across the membrane for conduction of nerve impulses. The cell biology module of this case has a complete discussion of the cell biology and biomechanics of propagation of the action potential and how that is altered in the presence of a neurotoxin. Paralysis is the typical response to saxitoxin. In soft shell clams, a single nucleotide substitution in the gene coding for these channel proteins prevents binding of saxitoxin, ensuring normal nerve conduction even if the toxin is present (Bricelj et al.2005). This is good for the clam, but bad for its predators, who can be poisoned by saxitoxin accumulation in clams. Studies have shown (Connell et al.2007) that resistant clams with the mutated sodium channel are found where dinoflagellate blooms are more likely to occur.