Paleofantasy: What Evolution Really Tells Us about Sex, Diet, and How We Live (31 page)

Gene flow is simply the movement of individuals and their genes from place to place, an activity that can itself alter gene frequencies and hence drive evolution. Instead of a rock falling on part of our hapless hamster troop, some additional whisker-wiggling individuals could have wandered into the population from another place. To use an example slightly more relevant to the matter at hand, Vikings may have spread the
CCR5
-D gene as they moved south through Europe—a factor that could have increased the frequency of that variant apart from any influence of selection. Although Jones suggested that the increased gene flow in the modern world would slow or even halt evolution in the cosmopolitan world of today,
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Hawks notes that, as with larger populations meaning that more fathers contribute their mutations, more mutations will also be entering the more mobile populations, and once again such increased variability facilitates evolutionary change.
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The final way that evolution sans natural selection can occur is via those mutations, changes in genes that are the result of environmental or internal hiccups that are then passed on to offspring. Such genetic alterations, such as the one that causes Tay-Sachs or other genetic diseases, are usually harmful, simply because random changes to complex machinery are rarely an improvement, which means that their bearer does not get an opportunity to reproduce. A small minority, however, are not harmful, and they, too, reform the genetic landscape and cause a change in gene frequencies.

The suspicion that humans might not be evolving may stem from the misconception, discussed earlier, that evolution is progressing toward a goal. A related fallacy is that all of life has been aimed at the production of humans, the pinnacle of evolution. If humans are seen as an end point, then presumably there is no need for further modification, making continued human evolution a sort of gilding of the anthropological lily. Such a notion is scientifically indefensible, of course; nature has not singled out humans for special treatment, and human beings are not the most recently evolved species on the planet in any event. That last honor, if it can be viewed as such, would likely belong to a virus, a bacterium, or another microorganism, since their short generation times allow them to evolve, almost literally, in the blink of an eye. Humans are no more of an end point to evolution than they are its most recent product.

The bottom line is that evolution in modern-day humans is easily demonstrated according to the criteria of change in gene frequency in our population; numerous surveys of the human genome and comparisons to ancient DNA or the genes of our closest relatives, the great apes, illustrate that the frequencies of many genes have changed. In addition, it simply stands to reason that our genes, like those of other species, are buffeted about by random forces, so we should not expect humans to be special in that regard. But to many people, this answer, that we are evolving because our genes have changed, often through random forces, is unsatisfying. What they really want to know is whether natural selection, the remaining evolutionary mechanism, is still in force. Here, too, the answer is yes.

Plumper, but with lower blood pressure

Upon reflection, it should come as no surprise to hear that the evidence of recent selection is all around us. It may help to do as Mary Pavelka, a primatologist at the University of Calgary in Canada, suggested in a 2005 article: “The question ‘Are humans still evolving’ should be rephrased as ‘Do all people have the same number of children?’ ”
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The answer is obviously no, which means that differential reproduction is occurring. If people have genetic differences in their fertility, natural selection has an opportunity to act. The use of contraception complicates matters somewhat, but that opportunity does not go away.

I have already discussed one of the best-documented examples of recent selection in humans: the rapid increase in the ability to digest lactose. But another illustration of our continual evolution is much closer at hand, both in time and space. What’s more, uncovering the evidence relied on rather unglamorous techniques that have been around far longer than the flashy genomic technology of the twenty-first century.

The Framingham Heart Study is an ongoing survey of about 14,000 residents of Framingham, Massachusetts, that began in 1948. Sponsored by the National Heart, Lung, and Blood Institute and Boston University, the study is the longest-running multigenerational study of its type. It was originally intended to uncover the risk factors for cardiovascular disease by tracing the lives, and deaths, of multiple generations of Framingham residents. People enrolled in the study are given medical exams every two to four years, and information about their blood pressure, cholesterol levels, and other health markers, as well as basic statistics on height, weight, and the number of children they have, is carefully recorded. Over the last half century, researchers using these data have made a number of important discoveries, including genes associated with Alzheimer’s disease and links between sleep apnea and the likelihood of having a stroke.

The scientists who began the Framingham study were not particularly interested in evolution, but the information from the study turns out to be exactly what biologists who look for evidence of natural selection in any population, human or not, need. The first and second generations of women from the study are now postmenopausal, which means they have already had all their children. A third generation is in the midst of data collection and still reproducing. The information being generated by the Framingham residents is not terribly different from that collected by scientists examining evolutionary change in fruit flies or mice: researchers look for a correlation between the number of offspring an individual has had and her own characteristics, such as body size (the height and weight measures taken for the Framingham study) or health. If women with a certain trait reproduce more, and the trait itself is one that can be passed from parents to offspring, natural selection occurs.

Much as the Grants and their colleagues did with finches on the Galápagos Islands (see Chapter 3), a team of scientists led by Stephen Stearns of Yale University calculated which characteristics were associated with having more children.
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Using humans instead of birds has its advantages and disadvantages in this type of study. On the plus side, the same individuals could be unquestionably identified and measured over and over again—a difficult task when attempting to catch finches in a net. On the minus side, humans smoke, use medications, and are educated about their health—all components of human existence that are conspicuously absent in avian society and are potential sources of interference in drawing conclusions. What if, for example, smokers happened to have more children because they were wealthier, or highly educated people were more likely to be taller? Luckily, the latter complications could be dealt with in the statistical analysis of the data. Similar accounting methods were used to ensure that early deaths, before people could reproduce, did not bias the sample.

After a complex set of calculations, Stearns and his colleagues found evidence of natural selection on the women in the study (focusing on women is sensible because each has an unambiguous number of children, which is not necessarily true for men). The women of Framingham seemed to be getting shorter and slightly plumper, and both their cholesterol levels and blood pressure were decreasing. The age at which they had their first child was also going down, but the age at which menopause occurred was going up, leading to a longer reproductive period in the total life span. We can only speculate about the causes of these changes, other than to state the obvious: that women with the new set of characteristics tend to have more surviving children than do those with the older set. Certainly, lowered cholesterol and blood pressure are associated with better overall health, but the height and weight alterations are more difficult to explain.

The scientists were then able to use their results to predict the future. Or, put a bit more modestly, they could calculate what the women of Framingham would look like in ten generations, assuming the same associations between fertility and the various health measures. Average weight, for example, is expected to go up by 1.8 kilos (4 pounds), while total serum cholesterol should decrease by 2.35 milligrams per 100 milliliters, or about 3.6 percent. The other traits measured had similarly small changes, leading the authors to conclude, “Natural selection is acting to cause slow, gradual evolutionary change.”
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Of course, as the authors recognize, there is no guarantee that the environment will remain the same over those ten generations, and any environmental changes would throw off the predicted trajectory.

The authors of the study also identified a number of other sources of data that could be used to examine natural selection on human populations. In a review paper published in 2010, the same team noted that while the Framingham Heart Study was one of the earliest initiated, similar long-term surveys are being made of people in Denmark, Gambia, Great Britain, and Finland, as well as several other US states.
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The surveys have a variety of goals, ranging from collecting data on general health to understanding cardiovascular disease in African Americans, but all are a gold mine of information for evolutionary biologists, as long as they also include data on how many children the participants produce.

A few characteristics were consistent across several of the studies. People, both men and women, who began having children at an earlier age were favored by selection, meaning that their genes were more likely to appear in succeeding generations. In at least one preindustrial population (Finland during the seventeenth through nineteenth centuries) and two postindustrial populations (twentieth-century Australia and the United States), selection also favored women either having their last child at a later age or being older at menopause.

In keeping with these results, a 2011 paper by Canadian researchers found a decrease in the age at which women living on Île aux Coudres, a small island north of Quebec City along the St. Lawrence River, had their first children.
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The researchers, led by Emmanuel Milot from the University of Quebec in Montreal, reviewed church records for women married after 1799 and before 1940. The population was isolated from the rest of the province and hence provided a nicely demarcated sample. Over the 140 years surveyed, the age at first reproduction went from twenty-six to twenty-two. This change had a genetic component, probably because characteristics related to child-bearing ability, such as the growth rate or age at first menstruation, are themselves heritable. In addition, the average number of children per woman went from three to four during the same period.

All of these findings suggest that what Stearns and his colleagues call “the temporal window of reproductive opportunity”
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is growing larger, enabling people to have children throughout more of their lifetime. Of course, people may not take advantage of that open window, which means that culture, again, may end up ameliorating the effects of selection in the long term.

The results of the review by Milot and colleagues also put to rest the notion that modern medicine and public health improvements such as sewage treatment have halted evolution in humans. Recall Pavelka’s question: Are all people having the same number of children? Whether or not survival increases, differences in reproduction among individuals are what matter. Culture, in the form of contraception or assisted reproduction, may complicate the story, but ultimately culture just forms part of the environmental backdrop against which evolution occurs. Environments always vary in the intensity of selection: if a storm kills off half the population, as it did with Bumpus’s sparrows, selection is extreme, while if a shift in seed size means that some birds are able to lay one egg fewer than usual, selection is weaker. Culture may change the strength of natural selection, or the traits upon which its action is most apparent, but it will not completely eliminate it. This means that instead of dismissing evolution in humans as too subject to the vagaries of culture to consider, new research areas present themselves. For example, how does
in vitro
fertilization with donor sperm or eggs change the rate of evolution? What about the use of surrogates? These questions are part of a new frontier in biology.

Apropos of this point, although natural selection is acting in modern human populations, it’s important to recognize that it does not act with the same intensity, or even in the same direction, everywhere. Studies measuring selection on height offer an example: In some places, such as Gambia, women’s height was increasing; in others, such as Poland or some parts of the United States, it was decreasing. In yet others, selection was stabilizing, meaning that the average height was not projected to change much at all.
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This set of results is typical for a large, heterogeneous population of organisms. Some environments favor being tall, some favor being short, and the same environment can do both at different times. The review authors suggest that women in preindustrial populations are under selection to become taller, while those in more industrialized places, such as Massachusetts, have more children if they are shorter.
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We do not know the reason for this difference; perhaps taller women do better at the physically demanding tasks of a less technologically dependent society. Regardless, environmental variability drives the difference in selection pressure. More generally, this variation underscores the point that there is no single optimum state for humanity—being tall can be better than being short, or vice versa, for different individuals under different circumstances.

Evolving up where the air is clear—and thin

Elegant though Stearns’ study is, it did not examine genetic change directly; that was not its intent. None of the researchers examined DNA itself, instead relying on the more indirect, if time-honored, methods of studying the changes in the characteristics of a population to infer genetic change. We already know that height and blood pressure are partly inherited from our parents, so inferring that changes in averages of those traits is due to selection is not too far a leap. But other recent research has taken another step, revealing the exact nature of human evolution in action by examining the genes themselves.