HOW EVOLUTION SHAPES OUR LIVES
Richard E. Lenski
Many people think of evolution as a fascinating topic, but one with little relevance to our lives in the modern world. After all, most people first encounter the idea of evolution in museums, where they see the fossilized remnants of organisms that lived long ago. Later exposure to evolution may come in courses that present the basic theory along with evidence from the tree of life and the genetic code shared by all life on earth. For those enamored of wildlife, evolution might also be discussed in programs about exotic organisms in far-away lands, often showing nature “red in tooth and claw.” So it is easy to overlook the fact that evolution is important for understanding who we are, how we live, and the challenges we face.
The comic strip shown here comes from Garry Trudeau’s Doonesbury
series, and it reminds us that evolution is highly relevant to our lives and to society. In fact, it touches on several themes in this volume. The conversation between the doctor and patient reminds us that despite our efforts to control nature, we remain targets for organisms that have evolved, and continue to evolve, to exploit our bodies for their own propagation. At the same time, the cartoon emphasizes that humans have acquired another mode of response—the use of technology—that allows us to combat diseases far more quickly (and with less suffering)
than if we had to rely on a genetically determined evolutionary response. More subtly, the technology, institutions, and language (including humor) that make human societies what they are today all reflect a process of cultural evolution that emerged from, and now often overwhelms, its natural counterpart by virtue of the speed and flexibility of cultural systems. Finally, Trudeau jabs us with the needle of the conflict between evolutionary science and religion that dominates many discussions of evolution in the public sphere, despite the overwhelming and continually growing body of evidence for evolution.
Figure 1-1. Cartoon strip reminding us that evolution is highly relevant to our lives. (DOONESBURY Copyright 2005 G. B. Trudeau. Reprinted with permission of Universal Uclick. All rights reserved.)
1. BIOLOGICAL FOUNDATIONS
To set the stage, we begin this volume with a question: What is evolution? From subtle shifts in the genetic makeup of a single population to the entire tree of life, evolution is the process by which life changes from one generation to the next and from one geological epoch to another.
The study of evolution encompasses both the historical pattern of evolution—who gave rise to whom, and when, in the tree of life—and the ecological and genetic mechanisms that underlie the evolutionary process (see chapter 2
When Charles Darwin published The Descent of Man
in 1871, he used the comparative method to make sense of the evolution of our species. That is, he looked to the similarities and differences in the appearance and behavior of humans and our living relatives to understand how we came into being. Some Neanderthal bones had been discovered, but otherwise there was no fossil record of hominids—the taxonomic family that includes humans and the great apes—in his time. It would be several decades before much older fossils were discovered that began to fill in some of the so-called “missing links,” and the insights from DNA lay more than a century ahead. Today we have a bounty of fossil hominids, and DNA from both living organisms and fossils is providing new insights into what makes us human, where we came from, and even who mated with whom (see chapter 3
One of the important attributes of humans is that we live in social groups that require a substantial level of cooperation—in hunting game, rearing a family, and dividing the tasks of labor seen in modern societies. But cooperation is not unique to humans, or even to our close relatives (see chapter 4
). Social insects, for example, exhibit remarkable cooperation and division of labor. Within ant colonies and beehives, most individuals forgo reproduction while supporting reproduction by their queen, who is usually the sister of the nonreproductive workers. In other cases, unrelated individuals cooperate, as in mutualisms involving different species, such as the fungi and algae that together make lichens. Understanding the evolutionary forces that promote these different forms of cooperation sheds light on our own behaviors as humans, as well as on some of our commonalities with other organisms whose behaviors have been shaped by these same forces.
One approach to understanding the evolution of human behavior is to ask whether our actions are, in some sense, optimal. Do humans consume food in a manner that optimizes nutritional status? Are children born at intervals that maximize reproductive success? The recognition that there might be trade-offs—for example, between the number of
children and the probability they will survive to adulthood—allows the possibility, at least, of defining optimal strategies in mathematical terms (see chapter 5
). However, the conditions in which our species exists have changed greatly as a result of our ancestors’ migrations across the planet, as well as technological innovations that affect food availability, life expectancy, and so on. As a consequence, there might be critical mismatches between the behaviors that were evolutionarily advantageous in the past—which continue to influence how we respond—and those that would be most beneficial at present (see chapter 6
). By understanding how evolution helped shape our psychology, individuals and societies might make better decisions about how to respond to the challenges and choices we face today.
2. EVOLUTION IN HEALTH AND DISEASE
Diseases are usually studied with a focus on proximate causes. For example, what organ is having problems, and how can it be repaired? Did an infectious agent cause the problem, and if so, how can it be eliminated? But one can also ask questions about the evolutionary forces that shape disease, although this is rarely done, and that failure may leave important stones unturned (see chapter 7
). For example, why might one group of people be more susceptible to a particular disease than another group? Why are some diseases more prevalent now than in the past, despite improved sanitation and increased access to food? Why do we senesce, losing our reproductive capacity as we age (see chapter 8
)? And why, in particular, do human females lose the capacity to reproduce even while they are still healthy and vigorous?
Turning our attention to infectious diseases: why do some pathogens and parasites make us very sick, or even kill us, when closely related microbes are harmless? For many years the conventional wisdom was that evolution would favor those parasites and pathogens that were harmless to their hosts. If parasites killed their hosts (so the thinking went), then they would drive their hosts and themselves to extinction. From this perspective, a highly virulent parasite was seen as a transient aberration, perhaps indicative of a pathogen that had recently jumped to a new host—one that would, over time, evolve to become less virulent if
it did not burn out first. But this view has been challenged by more rigorous analyses. Even lethal infections do not usually drive their hosts extinct, and the optimum virulence, from the parasite’s perspective, likely depends on the balance between within-host growth and between-host transmission (see chapter 9
The antibiotics that scientific researchers and pharmaceutical companies developed to treat bacterial infections were hailed as a triumph of technology over nature. Only a few decades ago, the most dangerous infections were largely conquered in developed countries. Schools of public health shifted their attention from infectious diseases to other threats, while the public looked forward to a cure for the common cold (along with personal jet packs). This benign outlook was shaken, however, in the 1980s by the AIDS epidemic and the discovery that it was caused by a virus. And it continues to be shaken by reports of emerging and reemerging diseases that threaten denizens of even the wealthiest nations, from the SARS virus and bird flu (the H5N1 influenza virus) to multidrug-resistant strains of dangerous bacteria including Mycobacterium tuberculosis
and Staphylococcus aureus
. The reemergence of these bacterial pathogens reflects the evolution of varieties resistant to some or all of the antibiotics that were previously used to treat them (see chapter 10
). Thousands of tons of antibiotics are used each year, causing intense selection for bacteria that can survive and grow in their presence. As a consequence, pharmaceutical companies must spend vast sums to develop new antimicrobial compounds that will allow us, we hope, to keep up with fast-evolving microbes. Meanwhile, emerging diseases that are new to humankind typically derive from pathogens that infect other animals.
The toolbox of molecular evolution and phylogenetic methods is now widely used to determine the source of zoonotic infections and to track a pathogen’s transmission through the host population based on mutations that arise as the pathogen continues to evolve during an outbreak. And if these challenges were not enough, some terrorists have deployed pathogens. Investigators must identify the precise source of the microbes deployed in an attack, using evolutionary approaches similar to those used to track natural outbreaks. The “Amerithrax” case, in which spores of Bacillus anthracis
(the bacterium that causes anthrax) were spread via the US Postal Service, demonstrated the power of new genome
sequencing methods to discover tiny genetic differences among samples that may identify relevant sources (see chapter 11
Some 10,000 years ago, humans began to harness the power of evolution by selectively breeding various plants and animals for food, clothing materials, and transportation. However, humans were not the first species to invent agriculture. That distinction belongs to ants, some of which began cultivating fungi for food millions of years ago (see chapter 12
). Some ants even tend other insects, such as aphids. Humans and other farming species change the environment of domesticated species by providing them with shelter, nutrients, and reproductive assistance. Selection in this protective environment reshapes the morphology, physiology, and behavior of the domesticated varieties. While we usually think of the farmer as controlling the domesticated species, their relationship is really a mutualism; the farmer, too, may evolve greater dependence on agriculture. For example, humans have evolved an unusual trait among mammals that enables many (but not all) adults to continue to produce lactase, an enzyme that allows milk sugar to be digested.
Agriculture is the most familiar way in which humans use evolution for practical purposes, but it is not the only way. Over the past few decades, molecular biologists have developed systems that allow populations of molecules to evolve even outside the confines of living cells (see chapter 13
). For example, RNA molecules have been selected to perform new functions in vitro, such as binding to targets of interest. After a random library of sequences has been generated, sequences that have bound to the target are separated from those that have not. The former are then amplified (replicated) using biochemical methods that introduce new variants by mutation and recombination. This Darwinian process of replication, variation, and selection is repeated many times, allowing the opportunity for further improvement in binding to the target. Similar approaches allow the directed evolution of proteins, so that today RNA and protein molecules produced by directed evolution are used to treat certain diseases.
Perhaps even more remarkably, computer scientists and engineers are harnessing evolution to write code and solve complex problems (see chapter 14
). They do so by implementing the processes of biological evolution—replication, variation, and selection—inside a computer. This approach has been used in biology to test hypotheses that are difficult to study in natural systems, and in engineering to facilitate the discovery of solutions to complex problems. For example, the design for an antenna used on some NASA satellites was generated not by a team of engineers but in a population of evolving programs (variant codes) that were selected based on the predicted functional properties of the objects encoded in their virtual genomes. Of course, it was necessary to build the physical objects and test them to see whether they would perform as intended, which they did.
With the success of agriculture and other technologies, the human population has grown tremendously in size and pushed into geographic areas that would otherwise be inhospitable to our species. As a result, humans have altered—by habitat destruction, introduction of nonnative species, and pollution—many environments to which other organisms have adapted and on which they depend, leading to the extinction of some species and threatening many others. Evolutionary biology contributes to conservation efforts in several ways (see chapter 15
). For example, phylogenetic analyses are used to quantify branch lengths on the tree of life and determine, in effect, how much unique evolutionary history would be lost by the extinction of one species or another. Given limited resources for conservation efforts, this information can inform where those resources will have the biggest impact. Also, the mathematical framework of population genetics, which underpins evolutionary theory, is widely used in the management of endangered populations. In particular, captive-breeding programs and even the physical structure of wildlife reserves can be designed to maximize the preservation of genetic diversity and minimize the effects of inbreeding depression. However, the challenges are only growing. Through many of the activities we take for granted—the production of desirable products, our travel for work and pleasure, and the heating and air-conditioning that keep us comfortable—we are changing the earth’s climate (see chapter 16
). As we do so, we impose selection on many other organisms. Which species and lineages will survive, and which will go extinct because they cannot
cope with the changes? And how will the survivors cope? Might they simply migrate to new habitats and locations that match their previously evolved requirements? Or will they evolve new preferences and traits that allow them to tolerate their altered world?
4. EVOLUTION IN THE PUBLIC SPHERE
Evolutionary biology attracts substantial public attention for two reasons. First, many people find it fascinating to understand how humans and other species came into being. Indeed, that question has interested people since the dawn of history, with different cultures and religions providing diverse narratives about the origins of the world and its inhabitants. Second, evolutionary biology often attracts attention because its findings are inconsistent with those narratives. The resulting tension is complex, with many different positions held by scientists and nonscientists alike (see chapter 17
). Some people reject religions whose narratives are contradicted by established bodies of scientific evidence. Others emphasize the difference between evidence and faith, viewing them as separate domains of human understanding; these people may retain some religious beliefs and sensibilities while rejecting the literal interpretation of prescientific narratives. Yet other people have suggested that evolution may, in fact, illuminate theology by providing a deeper understanding of nature as it was created.
While some view evolution as an affront to their religion, evolutionary biologists often feel that their field of study is under attack, especially in the United States, where opposition to teaching evolution is a hotbutton issue that generates loud and emotional responses. However, the opposition is far from unified; instead, it is a coalition of creationists expressing views that are inconsistent not only with the scientific evidence but also with the beliefs of other coalition members (see chapter 18
). Efforts have been made to unify the creationist coalition and give it a veneer of credibility by hiding these differences and obscuring their religious basis under the gloss of “intelligent design.” However, several US court decisions have recognized the religious nature of the opposition to evolution and they have disallowed such nonscientific ideas to be presented as scientific alternatives to evolution in publi...