Are Humans Still Evolving?

Are Humans Still Evolving? A NATURAL SELECTION DISCUSSION LESSON

MARTIN SHIELDS

S

tudents harbor widespread misconceptions about the mechanisms of organic evolution. The problem is especially apparent when they discuss the evolution of their own species – year after year I hear from my students that Homo sapiens will soon evolve to lose the pinky toe and become more intelligent. Such misconceptions are not effectively addressed by pedagogy that teaches Darwin’s theory via memorization of facts and vocabulary words. This lesson attempts to better develop sound comprehension of natural selection theory by prompting students to use its concepts to explain the evolutionary status of humans. I have used the following lesson successfully with all ability levels in first and second high school biology courses. I expect that the lesson would be useful in certain college biology courses as well. MARTIN SHIELDS is a biology teacher at Pascack Hills High School, Montvale, NJ 07645; e-mail: [email protected].

The Question This activity centers on the question: Are humans still evolving? Of course there is little doubt that human populations currently undergo microevolutionary changes in allele frequencies due to natural selection and other factors. Anthropologist Meredith Small (1999) writes, “… some people live and some people die, and some people pass on more genes than others. Therefore, there is a change in the human gene pool over time.” Some advanced students will realize this immediately. Nevertheless, humans behave differently enough from other species to make the question interesting to consider and to discuss. And, even in an Advanced Placement Biology classroom, many students will initially focus on the popular misconceptions that they have encountered repeatedly in the past. Moreover, the purpose of this activity is not to expeditiously arrive at a definitive answer to the question EVOLVING HUMANS 21

posed. Rather, the lesson’s value lies in student application, explication, and discussion of selection theory. The question is simply a tool to stimulate analysis and student debate on the accurate transfer of evolution’s concepts to a case study species.

Introductory vs. Advanced Classes To my introductory 9th and 10th grade classes, the question of current human evolution translates to: Are humans changing over time due to natural selection? I often offer this version to them as a clarification of the original question. In some ability levels of Introductory Biology, I do not specifically teach the concepts (or at least not the terms) microevolution, macroevolution, or allele frequency. In these classes, the lesson is nevertheless effective at advancing understanding of evolutionary biology. The focus in these classes centers simply on whether or not certain traits are favored by natural selection and passed on more often than others. In Honors Level Introductory Biology and Advanced Placement classes, my students learn the more complex aspects of population genetics before this lesson. In these courses, the students tend to subdivide the question themselves into: Are humans still undergoing microevolution and are they still undergoing macroevolution? Microevolution refers to the relatively small changes that occur within a species over a relatively small number of generations. Macroevolution refers to large change, such as the origin of a new species (speciation) that usually occurs over a much longer time period (Greenberg, 2001).

The Lesson Sequence I begin with students writing one to two page essays or journal entries on the central question, “Are humans still evolving?” This first step is critical in promoting individual idea development. I stress to the students that they will not be graded according to the side of the issue they choose to argue. It is important to avoid restricting students at this point through fear of getting the wrong answer. I want to draw the students out, to coax the entrenched misconceptions to the surface. After I have read their essays, we have a class discussion. The students and I clear desks from the room center and we arrange ourselves in a circle. While this may seem unnecessary or incidental I have found the quality of student-to-student interaction vastly improves when I am not physically positioned as a focal point. The group discussion usually fills a 44-minute class period.

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I begin the discussion by choosing a student to express his/her ideas on the topic. Then I call on an adjacent student, followed by a series of nearby students. I call on students in the beginning, rather than soliciting volunteers, to prevent the discussion from moving immediately into debate. I have found it fruitful to begin by eliciting a pool of ideas (some valid, some not) that can subsequently be critiqued by the group. In this initial phase, I provide little feedback and I refrain from correcting inaccuracies. I want other students to do that later. Also, at this point, I don’t want to inhibit students. Eventually, I open the discussion to volunteer contributors and I begin to pose questions. I usually pick a recurring misconception, repeat it, and solicit opinions on its validity. The discussion heats up and it is easy to simply facilitate as students respond to one another. My role during the discussion is primarily that of moderator. I challenge inaccuracies with questions. I push students to reason their way to a more sound understanding. Periodically I interject a new subtopic to move the discussion in a different direction. Often, I need to ask a student to temporarily defer an idea until a current one has been successfully discussed and resolved. The more students talking, the better. However, I often need to weigh in on a debated point when the group hasn’t correctly explained it. The students still need the teacher as final arbiter and information resource. The key is to subjugate this role to allow as much as possible for student-constructed knowledge.

Typical Student Responses Table 1 provides some topics typically raised in student essays and in the discussion. The right side of the table includes appropriate application of selection theory to the topic. Ideally, these explanations will be generated by the students with varying degrees of instructor guidance. I have found students need continual reminders of the connection between evolution and genes. They forget that beneficial traits will not emerge or increase in frequency unless the same happens to genes that influence those traits. For instance, many students claim that humans “need to be” increasingly more intelligent to survive in a technology-rich world, and that this promotes the evolution of increased intelligence. If such an issue is not correctly resolved by other students, I will respond with a question. I might ask, “Does a Silicon Valley software designer leave more of his or her genes to the next generation than someone who doesn’t even use a computer?” There is, of course, no evidence that technological facility or even intelligence

Table 1. STUDENT-GENERATED TOPIC

APPROPRIATE EXPLANATIONS

Pinky/pinky toe getting smaller or disappearing. “Wisdom teeth” will disappear.

Variant forms do not provide survival advantage or disadvantage, so allele frequencies should not change (e.g., large and small pinky toes will be passed on with equal frequency).

Appendix getting smaller or disappearing.

It is possible that allele(s) for smaller/less likely-to-become-infected appendixes could be favored in areas without access to quality health care. But this is probably not an issue where infected appendixes are easily treated.

Aren’t humans taller than in the 1700s?

If true, this is based on differences in nutrition/health care, not natural selection.

Lethal diseases.

Alleles for lethal disorders or predispositions for diseases are selected against, especially if they kill the individual before reproductive age. Alleles that confer resistance are selected for. For example, individuals with 2 CCR5 alleles do not acquire AIDS even if infected with HIV (O’Brien & Dean, 1997).These alleles should increase in frequency where AIDS mortality is high (e.g., Africa).

People evolving to be smarter in response to new technologies.

Ability to use technology is not gene-based or normally related to survival/reproduction. “Intelligence” is difficult to define.

Modern medicine.

People with potentially lethal alleles may have life prolonged, thus maintaining the distribution of those alleles in populations. For example, the PKU allele probably exists in higher frequency in countries where the disorder is diagnosed and treated (Ridley, 2001).

Biotechnology: “Genius sperm banks” Embryo screening Gene therapy/“designer” babies.

These are forms of artificial selection – further removing humans from the influence of natural selection?

Skin color, race.

Skin color probably has little, if any, effect on survival today due to clothing, housing, behavior, nutrition, sunscreen, etc. Recent research suggests a benefit to light skin for vitamin D production in northern latitudes (Kirchweger, 2001). Existence of distinct human races is not supported by human genome analysis (Paabo, 2001).

Speciation/macroevolution

Not occurring. If anything, there is less gene pool isolation today due to advances in global travel (Hayden, 2002).

(a difficult to define and controversial concept) increases a person’s fitness in the modern environment. Students need to employ the selection theory litmus test: Does the trait affect an individual’s ability to survive into and through the reproductive years (relative to individuals lacking the trait), and does it affect an individual’s ability to produce fertile offspring? If so, then the frequency of occurrence of an allele that influences the trait will be altered, and the population will experience microevolution due to natural selection. Or, in simpler terms: The gene for that trait will be passed

on more often and it will be found in more individuals of the following generations. There are two important side issues raised by the software engineer example. One is the student proclivity to refer to the “need” for a species to evolve in response to an environmental pressure. Research has revealed this misconception to be prevalent even in medical students who have, presumably, a strong academic background in biology (Brumby, 1984). I usually interrupt when I hear students ask, “Do species need EVOLVING HUMANS 23

to adapt?” A short discussion then reminds students that populations often, in fact, don’t adapt to change and they are extirpated. Either they adapt, or they don’t, and “need” has nothing to do with it. Those that adapt do so because some individuals already possessed beneficial traits that were born of random genetic variations and gave them a survival advantage. A second issue relevant to these discussions centers on our status as a bizarre species. Unlike most other organisms, individuals of Homo sapiens do not usually strive to maximize their reproductive success. The software designer may, for many possible reasons, choose against passing genes to the next generation. Also, he or she may or may not choose to enhance inclusive fitness (a topic only explored in my AP level

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courses) by helping to rear the children of relatives. By choosing against reproducing, and thus against increasing our fitness, don’t humans somewhat distance our gene pool from the formative influence of the environment? Even in the case of lethal disorder alleles, humans have, in some cases, supplanted the environment as the selecting mechanism. Many individuals with nonlethal forms of alleles choose not to reproduce, while many with potentially harmful alleles are now, due to medical advances, able to survive and pass on those alleles. Or, are humans and their technologies just another element of the environment? Students become quite animated in discussing these ideas.

Convergence with Social Issues Usually students will steer the discussion toward a variety of important societal issues. If they don’t, I will initiate these meaningful contributions to the discourse. The most fundamental issue is raised when a student asks the group, “Which people are we talking about?” Differential survival and reproductive success obviously is influenced by socioeconomic factors such as access to nutrition and health care. Thus, are humans in underdeveloped countries or impoverished American regions under a greater influence of natural selection? It is critical here to dispel any race-based associations. High school students in some populations may need to be reminded that poverty comes in all colors. If race does become a topic, it would be useful to discuss Human Genome Project conclusions that there is not enough genetic variation between groups with different skin colors to even support the concept of race in humans (Paabo, 2001). The impact of biotechnology on human evolution yields some of the most intriguing discussion. For instance, with “genius sperm banks,” are we replacing natural selection with artificial selection? But haven’t humans always (in most societies) selected mates based on their traits? The issue of artificial selection is taken a step further with testing for disorder alleles, medical abortions, and embryo screening. And what about gene therapy and the future possibility of “designer” babies? Realizing that such technologies would be expensive, students often raise the issue of access. In a recent discussion, a student asked if genetic modification might create a new gene pool isolating mechanism separating rich from poor. Could such a process lead to evolutionary divergence between isolated populations? Such topics move the discussion into the realm of science fiction, but they also reinforce that stunning advances in genetic engineering need to be accompanied by thoughtful societal debate.

Conclusion Some high school teachers avoid open-ended discussions out of fear of the unknown. An unscripted lesson such as this one often generates questions outside of the teacher’s expertise. However, successful teachers accept that they will not be able to answer every student question. In fact, answers to many student questions are not completely known to anyone. If a student makes a claim in this activity, and the teacher is unsure of its validity, there is a simple way to respond. I respond to such claims with, “If that were true then … ?” questions. If a student says that humans have become taller than in the 1700s, I would say, “If that is true, is there a possible explanation other than natural selection for it?” Or, “If that is true, can you explain a survival benefit that would have favored certain height influencing alleles over others?” In this way the emphasis is shifted from the veracity of the claim to a valid application of selection theory to the issue. There are many valuable resources for the teacher intending to use this lesson. In The Cooperative Gene, Mark Ridley (2001) devotes the latter two chapters to the current and future evolutionary status of Homo sapiens. He argues that natural selection may be “relaxed” in today’s wealthy societies because of modern medical technologies. Jared Diamond (1992) applies Darwinian ideas to humans and human cultures in a number of books. The one that is most focused on the topics of this lesson is The Third Chimpanzee (1992). In Why We Get Sick: The New Science of Darwinian Medicine, Nesse and Williams (1995) provide a natural selection-based view of human illness and medical treatments. For general information on evolution, the PBS Evolution Web site is a rich resource. Shellberg (2001) offers an excellent lesson idea for teaching students how to accurately explain the evolutionary origin of traits. The topic of human evolution is inherently interesting to students. The discussion is as vibrant with 9th grade Introductory Biology groups as it is with 12th grade Advanced Placement classes. By the end of the lesson, students have debunked some of

their own misconceptions about evolution and they are better able to use Darwin’s theory to explain the natural world.

Acknowledgments I would like to thank Dr. Jerry Phillips for reviewing an early draft of this manuscript. My wife, Phyllis, made many editorial improvements in the article and helped in innumerable ways.

References Brumby, M. (1984). Misconceptions about the concept of natural selection by medical biology students. Science Education, 68(4), 493-503. Diamond, J. (1992). The Third Chimpanzee: The Future and Evolution of the Human Animal. New York:Harper Collins. Greenberg, J. (2001). BSCS Biology: A Molecular Approach, 8th Edition. Colorado Springs, CO: Everyday Learning Co. Hayden, T. (2002). A theory evolves: How evolution really works and why it matters more than ever. U.S. News and World Report 133(4), 42-50. Kirchweger, G. (2001). The biology of skin color: Black and white. Discover, 22 (2). Available online at www.discover.com/recent_ issue/index.html. Nesse, R. & Williams, G. (1995). Why We Get Sick: The New Science of Darwinian Medicine. New York: Times Books. O’Brien, S. & Dean, M. (1997). In search of AIDS–resistance genes. Scientific American, 277(3), 44-51. Paabo, S. (2001). The human genome and our view of ourselves. Science, 291(5507), 1219-1220. PBS Evolution Web site: www.pbs. org/wgbh/evolution/index.html. Ridley, M. (2001). The Cooperative Gene. New York: The Free Press. Shellberg, T. (2001). Teaching how to answer “why” questions about biology. The American Biology Teacher, 63(1), 16-19. Small, M. (1999). Ask the experts: biology. Scientific American Online. Available online at: www.sciam. com/askexpert_question.cfm?page =3&topicID=3.

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