20 On-going evolution in man: Are we still evolving?

Dr. Monika Saini and Prof. A.K. Kapoor

epgp books

 

Contents:

  • Introduction
  • Difficulties in measurement of evolution
  • Role of demographic transition in influencing evolutionary process in modern human population Signs of on-going evolution
  • Other examples of agriculture and domestication mediated human evolution Cultural and technological adaptations of modern life: Barriers to Natural selection
  • Summary

 

Learning Objectives:

  • To comprehend the difficulties of measurement of evolution
  • To understand the factors impacting evolutionary process in modern population To know about recent examples of on-going evolution
  • To understand the barriers of natural selection

 

Introduction

 

Evolution is often perceived as a slow process which requires thousands or millions of years. During the last few decades a common consensus has been generated by world’s preeminent biologists that human evolution is over. It is often claimed that evolutionary process in modern humans have ceased because cultural and technological advancements have annihilated natural selection. Since modern Homo sapiens emerged 50,000 years ago, “natural selection has almost become irrelevant” to us, the influential Harvard paleontologist Stephen Jay Gould proclaimed. “There have been no biological changes. Everything we’ve called culture and civilization we’ve built with the same body and brain”. In contrast, recent studies show that selection can be strong in contemporary populations. Growing genomic revolution supports that microevolution, defined as a genetic change from one generation to the next in response to natural selection, can lead to changes in the phenotypes (observable characters) of organisms over just a few years or decade. This likely applies to humans as well because (a) natural selection operates on several morphological, physiological, and life-history traits in modern societies through differential reproduction or survival (Pettay et al., 2007 & Stearns et al., 2010), and (b) a number of these genetic traits present variations, attesting the potential for a microevolutionary response to selection (Milot, 2011 & McAuliffe, 2009).

 

According to Henry Harpending (Anthropologist, University of Utah) “It is likely that human races are evolving away from each other and we are getting less alike, not merging into a single mixed humanity.” Harpending theorizes that the attitudes and customs that distinguish today’s humans from those of the past may be more than just cultural, as historians have widely assumed. He says that “We aren’t the same as people even a thousand or two thousand years ago”. “Almost every trait you look at is under strong genetic influence” (McAuliffe, 2009).

 

 

Difficulties in measurement of evolution:

 

The measurement of evolution in a population is beset with many technical problems. The main difficulties which are involved in the measurement of evolution are following:

 

(a)  First evolution requires information on phenotype, pedigree links, and fitness over a sufficient number of generations which is rarely available. Humans are slow-reproducing species with generation times of about 20 years or more. It is therefore difficult to observe intergenerational evolutionary change. Only two reproductive generations have passed since the discovery of the structure of DNA (Pemberton, 2008 & Stock, 2008).

 

(b) Second, robustly demonstrating a response to selection is challenging. Typically, phenotypic trends observed in populations are compared with evolutionary predictions based on selection and heritability estimates, for example, using the breeder’s equation (Falconer, 1975 & Lynch and Walsh, 1998).

 

(c)   Third, selection measured at the phenotypic level does not necessarily imply a causal relationship between the trait and fitness (Wade and Kalisz, 1990 & Rausher, 1992) and, as a consequence, such predictions will often be inappropriate in the case of natural populations (Morrissey et al., 2010). This also implies that phenotypic changes, even those occurring in the predicted direction, may not provide robust evidence of evolution, as they may not be indicative of underlying genetic trends (Gienapp et al., 2008, Merilä et al., 2001 & Wilson et al., 2007).

 

These problems are likely exacerbated in long-lived species such as humans, where within-individual plastic responses to environmental variation, or viability selection, can drive phenotypic changes over the timescale of a study in the same direction as that predicted for genetic responses to selection (Gienapp et al., 2008 & Milot, 2011).

 

 

Role of demographic transition in influencing evolutionary process in modern human population:

 

The demographic transition is one major step in the history of modern human populations that impacts evolutionary processes. It typically begins with a sharp decrease in mortality arising from technological and societal improvements in living conditions, such as hygiene, disease prevention and health care, followed by a decrease in fertility. Typically this causes population size to grow then to stabilize or even decline towards the end of the transition. Figure 1 shows how contemporary evolution interacts with the demographic transition.

Figure 1 Schematic view of the dynamics between a demographic transition and evolutionary forces

(Milot and Pelletier, 2013)

 

The demographic transition is a change in vital rates, i.e. in age-specific fertility and mortality (bottom graph), driven by environmental changes in modernizing societies, often with a transient increase in population growth rate due to the typical delay in fertility decline relative to mortality. This has two effects on natural selection. First, demographic changes per se alter the variance in relative fitness (the opportunity for selection). Second, the latter, as well as environmental changes per se, may modify the covariance between phenotype and fitness (the strength of selection) (Milot and Pelletier, 2013).

 

Signs of on-going evolution:

 

During last few decades a team of researchers found an abundance of recent adaptive mutations etched in the human genome; even more shocking, these mutations seem to be piling up faster and ever faster, like an avalanche. Over the past 10,000 years, their data show, human evolution has occurred a hundred times more quickly than in any other period in our species’ history. Geneticists have identified more than two dozen genes that appear to have come under selective pressures since the rise of Homo, and several of them may still be subject to such pressures today. Some of these favoured alleles apparently arose at highly critical periods in human evolution. Such is the case of FOXP2, the so-called speech gene, which is implicated in the ability to talk, shows signs of strong selection, and arose no more than 200,000 years ago, coinciding closely with the first appearance of Homo sapiens. Other genes under selection are linked to cognition and behaviour, and still others are involved in defense against diseases such as hypertension, malaria, and AIDS. The new genetic adaptations, some 2,000 in total, are not limited to the well-recognized differences among ethnic groups in superficial traits such as skin and eye color. The mutations relate to the brain, the digestive system, life span, immunity to pathogens, sperm production, and bones—in short, virtually every aspect of our functioning (Balter, 2005 & McAuliffe, 2009). Following examples clearly show the signs of on-going evolution:

(a) There is growing evidence that epidemics are exerting selective pressure on our species. New methods for studying genetic variability—which can be used to study long-lived species with long generation times—have demonstrated directional natural selection on human genes by looking for signatures of selection in the genes of present populations. These include the glucose-6-phosphate dehydrogenase (G6PD) gene, which confers resistance to malaria, and the chemokine receptor 5 (CCR5) among Europeans, which confers resistance to the human immunodeficiency virus (HIV). The latter is likely to have evolved within the past 2,000 years, in response to an infectious agent that uses the CCR5 receptor to infect host cells. Numerous other studies have also provided evidence for recent natural selection on the human genome through comparisons of large sections of DNA (Stock, 2008).

 

(b) Another most incendiary aspect of the fast-evolution research is evidence that the brain may be evolving just as quickly as the rest of the body. Stronger evidence that natural selection has continued to shape the brain in recent epochs comes from studies of DRD4, a mutation in a neurotransmitter receptor that is linked to attention-deficit/hyperactivity disorder (ADHD). Children diagnosed with ADHD are twice as likely to carry the variant gene as those without the diagnosis. DRD4 makes a receptor in the brain less effective in bonding to dopamine, is often helpful in treating the problem. Children with the mutation tend to be more restless than other youngsters and to score higher on tests of novelty-seeking and risk-taking, all traits that might have pushed those with the variant to explore new frontiers. Sequencing studies suggest that the DRD4 mutation arose 50,000 years ago, just as humans were spreading out of Africa. Its prevalence tends to increase the farther a population is from Africa, leading some investigators to dub it “the migratory gene.” At least one allele (or copy of the gene) is carried by 80 percent of some South American populations. In contrast, the allele is present in 40 percent of indigenous populations living farther north in the Americas and in just 20 percent of Europeans and Africans (McAuliffe, 2009).

 

(c)   Our exodus out of Africa, paved the way for one of the most obvious markers of race, skin hue. People with dark skin have trouble manufacturing vitamin D from ultraviolet radiation in northern latitudes, which makes them more susceptible to serious bone deformities. Consequently, Europeans and Asians over the last 20,000 years evolved lighter skin through two dozen different mutations that decrease production of the skin pigment melanin (McAuliffe, 2009).

 

(d)  Similarly, the gene for blue eyes codes for paler skin coloring in many vertebrates and hence might have piggybacked along with lighter skin. Clearly something made blue eyes evolutionarily advantageous in some environments and it is well known fact that 10,000 years ago no one had blue eyes on earth.

Figure 2: Recent examples of selection in Humans

Source: Centers for Disease Control and Prevention, United States (CDC)

 

 

(e)   It is also claimed that human sperms may also be evolving at high speed, driven by the race to get to the egg before another man’s sperm. In the modernized cities there is a fiercer competition among sexual partners. Because sperm can fertilize an egg up to 24 hours after being ejaculated in the vagina, a woman who copulates with two or more partners in close succession is setting up the very conditions that pit one man’s sperm against another’s. According to John D. Hawks (Evolutionist, University of Wisconsin—Madison), “sperm today is very different from sperm even 5,000 years ago.” Newly selected mutations in genes controlling sperm production show up in every ethnic group. The selection for such kind of “super sperm,” provides further corroboration that our species is not particularly monogamous—a view widely shared by other anthropologists (McAuliffe, 2009).

 

Other examples of agriculture and domestication mediated human evolution:

 

There are some examples of human evolution that occurred subsequent to the invention of agriculture, and that involve the co-evolution of cultural and genetic systems with changes in subsistence strategies.

 

(a)   The example that is most often cited is the natural selection of heterozygous carriers of the sickle-cell gene to maintain sickle-cell anaemia in populations that are exposed to malaria. The disease of Malaria is about 35,000 years old, with the most lethal form of it just 5,000 years old. Yet in sub-Saharan Africa and other regions where it is endemic, “people have already developed 25 new genes that protect against malaria, including the Duffy blood type, an entirely new blood group. This natural selection is particularly visible in regions of central Africa where tropical forests have been cleared for agriculture, which, in turn, has caused the proliferation of mosquitoes that transfer the malaria-causing Plasmodium parasite (Stock, 2008 & McAuliffe, 2009).

 

 

(b) Another example of more recent evolution within the human genome is provided by evidence for strong natural selection on the gene that controls lactase production. Once people began keeping cattle herds, it became an advantage to derive nutrient calories from milk throughout life rather than only as an infant or toddler suckling at its mother’s breast. A mutation that arose about 8,000 years ago in northern Europe allowed adults to digest lactose (the main sugar in milk), and it propagated rapidly, allowing the rise of the modern dairy industry. Today the gene for lactose digestion is present in 80 percent of Europeans but in just 20 percent of Asians and Africans (Bersaglieri et al., 2004 & McAuliffe, 2009).

 

 

(c) As agriculture became established and started creating a reliable food supply, more men and women would have begun living into their forties and beyond—jump-starting the selection pressure for increased life span. Robert Moyzi (Biochemist, University of California) is currently performing a genetic analysis of men and women in their nineties who are of European ancestry. He has traced many early onset forms of cancer, heart disease, and Alzheimer’s to older human gene variants. According to him people with more modern variants tend to have greater resistance to these chronic illnesses of old age and should be overrepresented in the age 90-plus population (McAuliffe, 2009).

 

These examples clearly demonstrate that natural selection has recently acted upon humans after the origin of agriculture and the domestication of animals, and independently among different populations.

 

 

Cultural and technological adaptations of modern life: Barriers to Natural selection:

 

According to many scholars humans are no longer subjected to the random mechanisms of variation and selection and, as a species, they are depended on culture and technology for survival. In this respect, humans have been regarded as a species so dependent on culture and technology that cultural adaptation has replaced biological adaptation. During the past 12,000 years, humans have increasingly used culture and technology—built upon agriculture and animal domestication—to control and modify the natural environment. Therefore, culture has an important role in understanding whether evolution is still influencing the biology of our species.

 

Figure 3: Model of the relationship between the environment and human adaptation (Stock, 2008)

 

 

Culture and technology were clearly crucial to the successful colonization of the world by our species. They allowed us to occupy most regions of the planet through the use of fire, housing, watercraft, versatile tools and cognition, which enormously improved our ability to hunt and forage for food in markedly different environments and, in the process, to occupy more environmental niches than most other species.

 

Since the origins of agriculture, the rate of technological progress has increased exponentially. Agriculture originated independently within the past 12,000 years in various parts of the world, and the surplus of food resulting from agriculture has allowed people to specialize in different tasks, and has provided greater scope for innovation and cultural transmission. The technological achievements of our contemporary and industrialized society still rest on our agricultural production system, and the effective distribution of food resources. In turn, these technologies allow us to modify our environment so effectively that many have argued that we have removed our species from nature. Gene frequencies might still change over time through random factors such as genetic drift, but if our culture effectively removes us from environmental stress, then natural selection will no longer occur. However, it is important to remember that our ability to adapt to environmental stress is contingent on the availability and distribution of resources and energy.

 

Regarding another tier of environmental buffering, there is evidence that humans have physiological characteristics that allow them to adapt efficiently to different or changing environments (Wells and Stock, 2007). The ability to cook food provides humans with a greater dietary flexibility than chimpanzees, gorillas or orang-utans. This dietary flexibility and the extensive use of meat have allowed humans to converge on a common adaptive niche, and to survive in a greater range of environments. Humans also show greater flexibility in growth and have larger stores of body fat than many other species, both of which increase our ability to survive short-term environmental fluctuations. Humans have greater variation in fertility and birth spacing, which allows populations to bounce back quickly after periods of high mortality, and there is increasing evidence that environmental conditions can alter the regulation of specific genes. All of these physiological features allow us to respond to environmental stress without the need for genetic adaptation by natural selection. Considering the strong evidence that our species has a greater range of both technological and physiological mechanisms for buffering the effects of environmental stress, one could argue that genetic evolution is no longer influencing our species. However, it is clear that most of our non-genetic methods for mediating environmental stress depend on our access to the resources provided by agriculture. As a result, these means of environmental buffering might not be sufficient in all circumstances (Stock, 2008).

 

Summary:

 

To many researchers, the limited but growing evidence that natural selection is currently acting on the human genome means that humans are still evolving, even if in subtle ways. Whether or not these patterns will make a significant difference in the way humans look or live is another question.

 

According to Ian Tattersall (Anthropologist of the American Museum of Natural History in New York City) “There will be minor fluctuations over time and space in the makeup of local human gene pools as humans respond to local conditions but they won’t be directional. I find it hard to foresee that under current conditions a qualitatively new kind of human is ever likely to emerge. But if conditions change, all bets are off.” Evolutionary predictions are tied to speculation about just what kind of environment humans may face. Some researchers suggest that changing climate conditions may diminish the benefits of culture and medicine, creating a new era of natural selection. “There has been a relaxation in selective pressures in industrialized societies,” says evolutionary geneticist Peter Keightley of the University of Edinburgh, U.K. “But our ability to sustain that relaxation is probably temporary. We are using up our energy resources, our population is growing, and the climate is changing. All this is bound to lead to greater difficulties and renewed selective pressures.” Despite such concerns, however, most scientists remain leery of long-term forecasts, in part because of the way evolution works. “Evolution is not directed towards a goal,” says Tyler-Smith. “It always takes the short-term view, operating just on what allows us to survive and reproduce better in this generation.” For now, predicting humanity’s evolutionary future may be little more than crystal ball gazing—better suited to science fiction than scientific research. The accuracy of forecasts, for instance those pertaining to demography or epidemiology, and on which public policy may rely, could well depend on our knowledge of contemporary evolution (Milot, 2011). The evolutionary potential of modern humans has major implications. First, it signifies that we should consider the role of evolutionary processes that might underlie any observed trends in phenotypes. Second, it may produce eco-evolutionary feedbacks modifying the dynamics of modern populations (Carroll et al., 2007 & Pelletier et al., 2009). Not surprisingly, the new findings have raised hackles. Some scientists are alarmed by claims of ethnic differences in temperament and intelligence, fearing that they will inflame racial sensitivities. Other researchers point to limitations in the data. Yet even sceptics now admit that some human traits, at least, are evolving rapidly, challenging yesterday’s hallowed beliefs (McAuliffe, 2009 & Balter, 2005).

you can view video on On-going evolution in man: Are we still evolving?

 

References

  • Balter, M. (2005). Are humans still evolving?. Science, 309(5732), 234-237.
  • Bersaglieri, T., Sabeti, P. C., Patterson, N., Vanderploeg, T., Schaffner, S. F., Drake, J. A., Rhodes, M. Reich, D. E. & Hirschhorn, J. N. (2004). Genetic signatures of strong recent positive selection at the lactase gene. The American Journal of Human Genetics, 74(6), 1111-1120.
  • Carroll, S. P., Hendry, A. P., Reznick, D. N., & Fox, C. W. (2007). Evolution on ecological time‐scales. Functional Ecology, 21(3), 387-393.
  • Falconer, D. S. (1975). Introduction to quantitative genetics. Pearson Education India.
  • Gienapp, P., Teplitsky, C., Alho, J. S., Mills, J. A., & Merilä, J. (2008). Climate change and evolution:
  • disentangling environmental and genetic responses. Molecular ecology, 17(1), 167-178.
  • Lynch, M., & Walsh, B. (1998). Genetics and analysis of quantitative traits (Vol. 1, pp. 535-557).
  • Sunderland, MA: Sinauer.
  • McAuliffe, K. (2009). Are we still evolving. Discover.
  • Merilä, J., Kruuk, L. E. B., & Sheldon, B. C. (2001). Cryptic evolution in a wild bird population. Nature, 412(6842), 76-79.
  • Milot, E., & Pelletier, F. (2013). Human evolution: new playgrounds for natural selection. Current Biology, 23(10), R446-R448.
  • Milot, E., Mayer, F. M., Nussey, D. H., Boisvert, M., Pelletier, F., & Réale, D. (2011). Evidence for evolution in response to natural selection in a contemporary human population. Proceedings of the National Academy of Sciences, 108(41), 17040-17045.
  • Morrissey, M. B., Kruuk, L. E. B., & Wilson, A. J. (2010). The danger of applying the breeder’s equation in observational studies of natural populations. Journal of evolutionary biology, 23(11), 2277-2288.
  • Newson, L., Postmes, T., Lea, S. G., & Webley, P. (2005). Why are modern families small? Toward an evolutionary and cultural explanation for the demographic transition. Personality and social psychology review, 9(4), 360-375.
  • Pelletier, F., & Coulson, T. (2012). A new metric to calculate the opportunity for selection on quantitative characters. Evolutionary Ecology Research, 14(6), 729-742.
  • Pelletier,     F.,    Garant,     D.,    &     Hendry,    A.     P.     (2009).     Introduction:     Eco-Evolutionary Dynamics. Philosophical Transactions: Biological Sciences, 1483-1489.
  • Pemberton, J. M. (2008).  Wild pedigrees: the way forward. Proceedings of the Royal Society of London B: Biological Sciences, 275(1635), 613-621.
  • Pettay, J. E., Helle, S., Jokela, J., & Lummaa, V. (2007). Natural selection on female life-history traits in relation to socio-economic class in pre-industrial human populations. Plos one, 2(7), e606.
  • Rausher, M. D. (1992). The measurement of selection on quantitative traits: biases due to environmental covariances between traits and fitness. Evolution, 46(3), 616-626.
  • Retrieved from: http://wps.prenhall.com/wps/media/objects/12330/12626747/myanthropologylibrary/PDF/BA_DSC_4 0_Mcaul_587.pdf
  • Stearns, S. C., Byars, S. G., Govindaraju, D. R., & Ewbank, D. (2010). Measuring selection in contemporary human populations. Nature Reviews Genetics, 11(9).
  • Stock, J. T. (2008). Are humans still evolving?. EMBO reports, 9(1S), S51-S54.
  • Wade, M. J., & Kalisz, S. (1990). The causes of natural selection. Evolution, 44(8), 1947-1955.
  • Wells, J. C., & Stock, J. T. (2007). The biology of the colonizing ape. American Journal of Physical Anthropology, 134(S45), 191-222.
  • Wilson, A. J., Pemberton, J. M., Pilkington, J. G., Clutton-Brock, T. H., Coltman, D. W., & Kruuk, L. E. B. (2007). Quantitative genetics of growth and cryptic evolution of body size in an island population. Evolutionary Ecology, 21(3), 337.