Lewis/Life - Teaching Techniques

Chapter 20 - Teaching Techniques

A. General Tips
This is an interesting chapter and your students will basically like it. They will be interested in some of the pockets of genetic anomalies mentioned here.
Generally, your best bet is to present this material in outline form. The information flows quite nicely in a logical, sequential fashion.
What do you want to do about Hardy-Weinberg equilibrium? How much emphasis are you going to place on the mathematical mechanics of the equation? Lewis gives a good, solid explanation; unfortunately some of your students may panic. If you are going to concentrate on the mathematics, be certain you know exactly how the algebra works before getting into the lecture. For additional tips, see the notes below specific to Section III. Also, the answer to the To Think About question # 4 may help.
William Castle (an American) came up with the genetic/mathematical relationship found in the Hardy-Weinberg equation about the same time as Hardy and Weinberg. Some people think Castle's name should be included in the official name. You might ask your students to research this problem and propose a response.
You might wish to copy and distribute to your students the Overview of Chapter Objectives flowchart found at the beginning of this Instructor's Manual Chapter.
B. Possible Approaches to the Chapter
The material in this chapter -- actually in the whole unit -- lends itself well to the annotated outline. Such an outline could be assigned to be handed in before you lecture on the material. That way the students would have a perception of the sequence of events under consideration and the lecture itself could be used to focus on a conceptual or developmental understanding of the information (rather than on a straight chronology).
I suggest you have your students make several spontaneous lists, as mentioned below. Often this material, as interesting as it is, comes with some built-in misconceptions. By asking to students to jot down certain points as you are about to work on them, they will see where their own weaknesses are and you will be able to direct your lecture accordingly. The misconceptions will not be blown out of proportion.
C. Hints Related to Specific Chapter Topics
- I. Recall what was said about hybrids in Chapter 18 of this Instructor's Manual. You may wish to challenge some students to research this Bullwinkle problem. Does this type of "inappropriate overture" occur often in nature? Has any research been done to figure out what triggers these misguided romantic pinings?
- III. A simple way to introduce the Hardy-Weinberg equation is to start with a simple monohybrid cross. Assume both parents are heterozygous. Set up the square. Assume four offspring, one for each box. How many alleles altogether? Eight. How many are dominant? Four. How many recessive? Four. First box, 2/8 or 25% of population is homozygous dominant. Second and third boxes combined, 4/8 or 50% of population is heterozygous. Fourth box, 2/8 or 25% of population is homozygous recessive.
  The dominant is p, the recessive is q. p + q = 1; 0.5 + 0.5 = 1.0. These are just the alleles.
  Squaring both sides: p2 + 2pq + q2 = 1
  The sides are squared because each individual has two alleles. The p2 represents two copies of the dominant allele; pq represents the heterozygote; the q2 represents two copies of the recessive allele.
  (0.5)2 + (2 * 0.5 * 0.5) + (0.5)2 = 1
  After you have established the validity of the idea, you can alter your frequencies -- as Lewis does on page 405 -- and continue.
  Students often forget that TWO alleles are involved for each individual.
- IV.A. Note the reference to the Y chromosome and refer back to the To Think About question # 5 in Chapter 19.
  Look at Biology in Action 20.1. Consider the Additional Topics question # 3 in Chapter 18.
- IV.C. Suggest students think of some examples of (potential) genetic drift in their own families. Almost any distinctive trait, such as a birthmark or an irregularly shaped earlobe, will do.
- IV.E.1. Directional selection is quite logical. Students may be amazed at disruptive selection. Ask if disruptive selection leads to speciation; remind them that both alleles are present in the population.
- IV.E.2. Balanced polymorphism is probably one of the most interesting topics in this chapter. Some of the students will have heard of the sickle cell / malaria connection. Very few will realize that there are numerous examples of balanced polymorphism.
  Discuss the cystic fibrosis/cholera connection. Be prepared for some interesting questions. You may have to refer ahead to the opening vignette in Chapter 22. Keep the causative explanation simple. In addition to the toxin opening the channels, the additional osmotic force, the tendency toward equilibrium, causes water to move out of supposedly unaffected cells and toward cells that are losing fluid because of the toxin.
- V. Before beginning this section, ask the students to jot down ways in which they think speciation could occur. Tell them to be specific with regards to reproduction. Once they see that reproductive isolation is the key to speciation, the delineation of the forms of reproductive isolation becomes quite obvious. If you did not assign an annotated outline for this chapter, consider outlining this section on the board or overhead to give the students a sense of ideological continuity.
- VI. Questions contrasting gradualism and punctuated equilibrium usually arise. The definitions are not particularly difficult, but many students have difficulty relating those definitions to specific evolutionary examples. Also, there is some disagreement as to what constitutes a quantum leap. Know in advance exactly what you want your students to remember about this. Both extremes exist and sometimes arguments arise.
- VII. If you haven't done so previously, ask your students to write down what percentage of species that have ever existed are extinct today. Ask them also to compose a list of ways in which species become extinct. Most of them will give a rather low percentage and most will stress human intervention. Human intervention is not the major cause of extinction throughout geological history.
  Use the board or the overhead to list causes of extinction. Try to correlate this with geologic history.
  I suggest you stress Table 20.3 and that you compare and contrast the information presented there with the modern concepts of extinction. Recall that over 99% of all species that ever lived are extinct. Part of the problem is our inability to conceptualize millions and billions of years.
  Ask the students to compare the differences between mass extinctions and local extinctions. This may be more difficult than it seems.