Selection and the Blond Beach Mouse

1. Selection and the Blond Beach Mouse by Joan SharpDepartment of Biological SciencesSimon Fraser University, BC, Canada

2. Part I: The Adaptive Value of Cryptic Coloration Introducing Dr. Hopi Hoekstra Dr. Hoekstra is Professor of Zoology and Curator of Mammals at Harvard University. She studies the oldfield deer mouse, Peromyscus polionotus, to understand the evolution of its cryptic coloration and behavior.

3. The Oldfield Deer Mouse Peromyscus polionotus

4. The Oldfield Deer Mouse • Oldfield deer mice (Peromyscus polionotus) live in abandoned agricultural fields throughout the SE United States. • They live in loosely packed sandy or clay soils, where they can dig their burrows. • Mainland oldfield deer mice have a dark brown dorsal coat, a light grey belly, and a striped tail.

5. The Beach Mouse The Alabama beach mouse Peromyscus polionotus subspecies ammobates

6. The Beach Mouse • Beach mouse populations of Peromyscus polionotus have colonized sand dune and barrier island habitats, where they live on brilliant white sand with sparse vegetation. • Beach mice look very different from their mainland cousins, with a lightly colored dorsal coat and no pigment on their face, belly, and tail.

7. Eight Subspecies of Beach Mice Eight subspecies of beach mice live on sand dunes on beaches and barrier islands that formed ~6000 years ago along the Gulf Coast of Alabama and Northern Florida and the Atlantic Coast of Florida.

8.  Discuss in your small groups: • What random process was necessary for beach mice to evolve from dark mainland mice? • What non-random process was necessary for beach mice to evolve from dark mainland mice?

9. Evolution of Blond Beach Mice Mutation: A random mutation in one or more of the genes affecting coat coloration caused light coloration in the coat of the mutant mouse. Natural selection: Beach mice with mutations that produced light coat color had higher survival and reproductive success than dark beach mice.

10. Understanding Natural Selection • Biologists ask ultimate questions about why natural selection has favored a particular trait. • Ultimate explanations address the adaptive value of the trait, exploring how the trait increases evolutionary fitness in a specific environment.

11. Understanding Natural Selection • Biologists also ask proximate questions about how a particular trait is expressed. • Proximate explanations address the mechanisms that produce the trait, including its genetic, developmental, or physiological basis.

12. Definitions • Evolutionary fitness measures the relative ability of an individual to produce viable, fertile offspring, relative to other individuals in the population. • An adaptation increases an individual’s evolutionary fitness relative to other individuals in the population.

13. CQ#1: Japanese cranes breed in spring and early summer. Choose the ultimate explanation: • Breeding is most likely to be successful in spring and early summer. • Hormonal changes bring about breeding behaviors. • Breeding is triggered by the effect of increased day length on the birds’ photoreceptors.

14. CQ#1: Japanese cranes breed in spring and early summer. Choose the ultimate explanation: • Breeding is most likely to be successful in spring and early summer. • Hormonal changes bring about breeding behaviors. • Breeding is triggered by the effect of increased day length on the birds’ photoreceptors.

15. CQ#2: Japanese cranes breed in spring and early summer. Choose the proximate explanation: • Breeding is most likely to be successful in spring and early summer. • Increasing day length triggers the release of breeding hormones. • Ample food is available for chicks at this time.

16. CQ#2: Japanese cranes breed in spring and early summer. Choose the proximate explanation: • Breeding is most likely to be successful in spring and early summer. • Increasing day length triggers the release of breeding hormones. • Ample food is available for chicks at this time.

17. Ultimate Questions Hoekstra’s team asked two ultimate questionsabout natural selection for coloration in oldfield deer mice: • Why is blond fur adaptive in beach mice? • Why is dark fur adaptive in mainland mice? Oldfield deer mice have many visual predators, including hawks, owls, and coyotes.

18. Two Complementary Hypotheses Hypothesis 1: Blond beach mice are camouflaged and less vulnerable to predation in a beach habitat. Hypothesis 2: Dark mainland mice are camouflaged and less vulnerable to predation in a mainland grassy field habitat.

19. Work in your small groups to plan an experiment to test these hypotheses, marking sure that you include the following: • Describe your experimental setup. • Independent variable: What will you vary in your experiment? • Dependent variable: What will you measure in your experiment? • What variables will you control or standardize? • How will you provide replication?

20. Experimental Design Can Be Tricky! If you plan an experiment to test predation risk on blond and dark mice in natural beach and field habitats, consider: • The color of mice in a particular habitat may not vary much. • It can be hard (or impossible) to follow an individual mouse and determine its fate.

21. Experimental Design Can Be Tricky! If you plan an experiment to test predation risk on blond and dark mice in enclosures that mimic natural beach and field habitats, consider: • It’s hard to provide a full set of natural predators in an enclosure. • Blond and dark mice may vary in other ways, such as odor, activity level, and behavior. (Do mice try to hide? Escape?)

22. The Experiment • Dr. Hoekstra and her colleagues made models of oldfield deer mice with beach and mainland coloration to assess the adaptive value of camouflaged coloration in nature. • They made 250 soft Plasticine models of crouching Peromyscus polionotus. • Half were painted to look like beach mice. • Half were painted to look like mainland mice.

23. Plasticine Mice

24. The Experiment • The models were set out in 8 linear transects where Peromyscus polionotus lives. • 4 transects were in beach habitats • 4 transects were in mainland habitats • 14 light and 14 dark model mice were randomly set out on open patches 10 m apart along each transect.

25. The Experiment • Model mice were checked every 24 hours and attacked models were replaced with identical models. • Predatory attacks were scored by the presence of bite marks, bill marks, dragging of models, or other “injuries.” • The proportion of attacked model mice that matched or mismatched their habitat were calculated (relative to the total number of mice attacked).

26. Work in your small groups to identify the following features of Dr. Hoekstra’s experiment: • What is the independent variable? • What is the dependent variable? • What variable(s) are controlled or standardized? • How is replication provided?

27. What results would you expect if the data support Hypothesis 1? Hypothesis 1: Blond beach mice are camouflaged and less vulnerable to predation in a beach habitat. Make a prediction!

28. Predict the proportion of attacks on dark and light models in a beach habitat Proportion of attacks Dark models Light models

29. What results would you expect if the data support Hypothesis 2? Hypothesis 2:Dark mainland mice are camouflaged and less vulnerable to predation in a mainland grassy field habitat. Make a prediction!

30. Predict the proportion of attacks on dark and light models in a field habitat Proportion of attacks Dark models Light models

31. The Results Predation on dark and light models in beach or field habitats

32. CQ#3: What do these results show? Predation on dark and light models in beach or mainland habitats. • Predation on dark and light models is similar in both habitats. • Camouflaged models suffer no predation in both habitats. • Predation on camouflaged models is reduced relative to non-camouflaged models in both habitats.

33. CQ#3: What do these results show? Predation on dark and light models in beach or mainland habitats. • Predation on dark and light models is similar in both habitats. • Camouflaged models suffer no predation in both habitats. • Predation on camouflaged models is reduced relative to non-camouflaged models in both habitats.

34. Work in your small groups to discuss whether these results allow you to support or reject the two hypotheses. Hypothesis 1: Blond beach mice are camouflaged and less vulnerable to predation in a beach habitat. Hypothesis 2: Dark mainland mice are camouflaged and less vulnerable to predation in a mainland grassy field habitat.

35. Dr. Hoekstra and her colleagues have answered the ultimate question about the evolution of blond coloration in beach mice.

36. Why is blond color adaptive? • Blond coloration is adaptive in beach populations of Peromyscus polionotus because blond mice suffer reduced predation by visual predators. • With increased survival, blond beach mice are more likely to reproduce successfully and have higher evolutionary fitness than dark beach mice.

37. Proximate Questions Hoekstra’s team asked two proximate questionsabout blond coloration in beach mice: • What are the genes that affect coat color in mice? • What mutations in these genes produce blond coat color in beach mice?

38. Part II (Detailed Version) The Genes and Alleles Affecting Coat Color in Mice

39. How is fur coloration determined? • Mammals are unique in having hair or fur. • Each hair grows from a hair follicle, and melanocytes at the base of each follicle produce pigments that are deposited in the growing hair. • In mammals, variation in pigmentation is determined by the distribution and relative amounts of two pigments: pheomelanin and eumelanin.

40. Mammalian Pigments Eumelanin pigments produce dark colored hairs that vary from brown to black, depending on the distribution and number of pigment molecules in each hair. Pheomelanin pigments produce light colored hairs that vary from blond to red. A hair can have a banded pattern if it switches between these pigments as it grows.

41. Genes Affecting Hair Color Many genes affect hair color in mammals, but we’ll consider two primary genes that affect coat color in oldfield deer mice: • Melanocortin-1 receptor (Mc1r) • Agouti

42. Genes Affecting Coat Color in Mice Hopi Hoekstra and her colleagues have identified mutant alleles of these pigment genes that produce blond coloration in beach mice. Dr. Hopi Hoekstra

43. Melanocortin-1 receptor (Mc1r) Gene The product of Mc1r is a transmembrane receptor protein that inserts into the plasma membrane of a melanocyte and signals the cell to produce black/brown eumelanin pigment.

44. Why are Santa Rosa Island beach mice blond? Hopi Hoekstra and her colleagues captured blond beach mice from the sand dunes of Santa Rosa Island, on Florida’s Gulf Coast. They found a point mutation in the Mc1r gene, changing one of the 954 base pairs that make up the gene. Sand dunes of Santa Rosa Island, Florida

45. Why are Santa Rosa Island beach mice blond? Due to a mutation altering a single base pair (CT) in the Mc1r gene, Santa Rosa beach mice have cysteine (a small, uncharged amino acid) at position 65 in the Mc1r transmembrane protein. Field mice have arginine (a large, positively charged amino acid) in position 65.

46. Why are Santa Rosa Island beach mice blond? The Arg65Cys mutation in Santa Rosa Island beach mice reduces the ability of Mc1r to signal the melanocyte to produce black/brown eumelanin. Instead, melanocytes of mutant mice produce pheomelanin, a blond/red pigment.

47. CQ#4: How did the Arg65Cys mutation arise? • The Arg65Cys mutation arose by chance, due to random substitution of one DNA nucleotide for another. • The Arg65Cys mutation arose because beach mice needed blond coloration in order to be camouflaged in their new habitat. • The Arg65Cys mutation arose due to natural selection for blond coloration.

48. CQ#4: How did the Arg65Cys mutation arise? • The Arg65Cys mutation arose by chance, due to random substitution of one DNA nucleotide for another. • The Arg65Cys mutation arose because beach mice needed blond coloration in order to be camouflaged in their new habitat. • The Arg65Cys mutation arose due to natural selection for blond coloration.

49. Gulf Coast Beach Mice Dr. Hoekstra’s team examined the five Gulf Coast subspecies of beach mice, looking for the Arg65Cys mutation in the Mc1r gene.

50. Gulf Coast Beach Mice The Arg65Cys mutation is found in 4 of the 5 beach mouse subspecies found on the Gulf Coast, with varying frequency.