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Case of the
Threespine Stickleback

A Model of Macroevolution

 

by James E. Platt, U. of Denver
Adapted by L. Flammer

DRAFT,
Pending
Review

EVOLUTION

 

Macroevolution

 SYNOPSIS

Students begin by seeking to answer a question: "Why have some freshwater populations of threespine stickleback fish lost their pelvic spines and body armor?" Data and analysis take them into some applied genetics and the Evo-Devo work on regulatory DNA, in which mutations can change where and when a gene is expressed, producing major changes in specific morphology (without fatal effects) on which natural selection can act. This reveals a likely pathway for new species to form in a relatively short time (macroevolution) without the heavy risk that a mutation in a protein-producing gene might bring. Applications to other species are also explored.

 PRINCIPAL CONCEPT

1. Major changes can be triggered relatively safely during development by mutations in regulatory DNA, unlike changes in the mRNA-coding regions of a gene. This is probably a key factor in the evolutionary changes processed through natural selection.

 ASSOCIATED CONCEPTS

1. Traits subject to evolution must have a hereditary component.
2. Evidence of the present can reveal events of the past.
3. Most traits exist for the benefit of the individual rather than for the good of the species.
4. Traits are usually favored by natural selection only when they result in more reproductively successful offspring.

ASSESSABLE OBJECTIVES

   Students will....

1. Recognize a 9:3:3:1 ratio of combination of two traits in a generation as the results of a simple Mendelian dihybrid cross with two traits, each expressing in two forms, and each controlled by single genes with two alleles each, and with each gene showing dominance (two simple Mendelian traits inherited together).
2. Understand how a specific mRNA-binding dye can reveal the expression of a specific gene in a cell or group of cells.
3. Recognize how natural selection can explain different features of a species being expressed in different populations.
4. Recognize what atavisms are, and how regulatory DNA changes can explain the existence of atavisms.
5. Recognize how major evolutionary changes can occur relatively safely through the influence of regulatory DNA.

 MATERIALS

Copy of Teacher Instructions (3-page PDF version of this page)
(Plus Preferred Responses -3 more pages)

 TIME

About one 50 minute period (involves reading, discussing, answering questions; could go an additional period).
STUDENT HANDOUTS The Case of the Three-Spine Stickleback (study packet: 11 pages)

Responses to Questions (questions repeated with spaces to answer: 3 pages)

 

 

TEACHING STRATEGY
& PREPARATION

Prerequisites: Students should have studied Mendelian genetics, including work with dihybrid crosses with two simple Mendelian traits showing dominance. It's also helpful if students have already been introduced to natural selection and/or evolution, and possibly some discussion of microevolution vs. macroevolution.

Additional Resource: The HHMI­Evolution DVD includes useful video clips and animations that could be used with this lesson. Each clip is about 2-3 minutes long. See review and source of this free DVD
Animations include:
Stickleback CT Scan
Pitx1 Expression (excellent demo of how gene regulators work)

Video Clips: (from Lecture 2: Natural Selection of Sticklebacks)
Stickleback ­ Jeopardy
Stickleback Environment (changes in glacial retreat in North America)
Fossil Record of Stickleback Evolution (excellent, showing changes in the fossil record of sticklebacks over 25,000 years, going backwards from about 10,000 years ago)

Glossary (Some terms students should know (or learn in this lesson):
Microevolution: descent with modification; the process by which species change over time as they interact with their environment, producing changes in gene frequencies.
Macroevolution: the process by which all the groups of organisms have been produced, from different species to all the higher taxa. This could be by accumulated microevolution over long periods of time, or by major changes in morphology that are genetically-caused and can lead to new species in a relatively short time
Morphological (morphology): having to do with form or shape.
Precludes: prevents or excludes
Atavisms: structures that sometimes appear in an individual, revealing the presence of usually-hidden or suppressed genes that were expressed in an ancestral form, e.g., the occasional pelvic flipper on a dolphin, or an extra toe on a horse.

STRATEGY: Have students working in teams of 2-4, with one copy of the 11-page Information packet, and a copy of the 3-page Responses packet for each team. One student reads information to the team. When they get to an item to discuss and respond, they do that. Have students rotate turns reading and responding. If you prefer silent reading, you will need enough copies of the information packet for one per student. Note that the teacher "key to preferred responses" is only to serve as a guide to the teacher, never as "THE correct answers." Students will present a variety of responses, but should at least begin to approach those "preferred responses." If they're way off target, you should use "guiding questions" to move the discussion toward more productive understanding along the lines of those "preferred responses."

Alternative Strategy: Separate the Information Packet into 4 portions, and hand them out separately, so that students work on one section at a time, and can't peek forward. For example, hand out pp. 1-2, then p. 3, then pp. 4-6, then pp. 7-11. This way, students can work with this as a true mystery case, and it is more like a scientific inquiry. (Thanks to beta-tester Kathy Van Hoeck for this suggestion. It worked well for her.)

 

 

PROCEDURES

Introduce the Stickleback to the class. An excellent way to do this is to get the HHMI­Evolution DVD or VHS tape (free), and show the clips suggested above. Click the HHMI­Evolution DVD to see review and suggestions for use.

Alternatively, project the pictures from the Information packet, read the first two pages, and let them begin working at the top of page 3. Walk around, try to keep all teams at about the same stage, so you can engage all teams in discussion for some degree of informal formative assessment, clarifying and focusing as needed.

ASSESSMENT

See Assessable Objectives for focus of assessment questions.

EXTENSIONS

& VARIATIONS   

Macroevolution: Alive and Well In Sticklebacks, by Dr. James E. Platt (Guest Editorial in The American Biology Teacher of January 2006). Good for an overview article by the author of this lesson - perhaps for an assigned reading following the lesson, with further discussion the next day.

Try our "Blocks & Screws" lesson. This novel exercise directs attention to how evolution consists of the gain and loss of expressed traits, as naturally selected, while their genetic basis may still be retained, resulting in atavisms appearing from time to time.

See other lessons for teaching about Speciation & Macroevolution
And check out the links on our EvoDevo page

Take a look at a delightful PowerPoint presentation developed by teacher Steve Werner about an example of speciation involving odor imprinting.

WHALE EVOLUTION: A recent application of evo-devo to a subject always fascinating to kids:
How ancient whales lost their legs, got sleek and conquered the oceans: an Evo-Devo solution.
This is a beautiful blending of paleontology, developmental morphology and the blossoming field of evolutionary developmental biology, and an excellent example of MILEs: (Multiple Independent Lines of Evidence), confirming the tetrapod origin of whales. A recent study, using porpoise embryos, has revealed how a mutation in the gene for "sonic hedgehog" (shh), a developmental signaling protein necessary for normal limb development, resulted in the loss of the hind limbs of early whales about 35 million years ago. Fossils have shown the gradual reduction in hind limbs prior to that time, over a 15 million year period, but the shh mutation appears to be the final step bringing the full sleekness that we see in cetaceans today. Very clever science.

This 2006 PNAS report, by whale evolution veteran JGM Thewissen, et al., is nicely summarized (with full citation to the original) by PZ Myers on the Pharyngula site. It includes an excellent illustrated cladogram (from the original report) showing the gradual hind limb reduction in ancestral whales, and the corresponding changes in regulatory genes. If you are one of the many teachers using our Becoming Whales and/or Whale Ankles and DNA lessons, be sure to share this latest footnote with your students. If you don't, you should!

 

 

REFERENCES

Cresko, W.A., et al. 2004 Parallel genetic basis for repeated evolution of armor loss in Alaskan threespine stickleback populations. Proc. Nat. Acad. Sci. U.S.A. 101:6050-6055, Fig. 4, p. 6053.

Carroll, S.B., Grenier and Weatherbee. 2005, From DNA to Diversity. Figure 4.6, page 112. Blackwell Publishing, and Howard Hughes Medical Institute DVD. 2006. Evolution - constant change and common threads

Shapiro et al. 2006. Corrigendum: Genetic and developmental basis of evolutionary pelvic reduction in threespine sticklebacks. Nature 439:1041

Szeto et al., 1999, Genes and Development. 13:484-494

US Geological Survey (manatee photo)

Skulls Unlimited Newsletter, 2006 (manatee skeleton)

Thewissen, J.G.M., et al 2009. From Land to Water: the Origin of Whales, Dolphins, and Porpoises. Evolution: Education and Outreach Online, Volume 2, Number 2 / June, 2009. Special Issue: Transitional Fossils, June 05, 2009 pp 272-288. Fig. 24 (From Taiji Whale Museum, Japan.)

 ATTRIBUTION

Some of the ideas in this lesson may have been adapted from earlier, unacknowledged sources without our knowledge. If the reader believes this to be the case, please let us know, and appropriate corrections will be made. Thanks.

1. Original Source: James Platt, Ph.D., University of Denver, Dept. of Biological Sciences. Email jplatt@du.edu

2. Modified by: Larry Flammer, 10 October 2007.

3. Revised July 2009


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