Evolution Continues Today - Can We Control It?

Much of the immediate success of Darwin’s book was due to his careful comparison of his new idea of natural selection to the well-known breeding of animals. Darwin was especially interested in pigeons, and his observations of their many varieties inspired his own early thinking. Humans have relied on artificial selection ever since we first put seeds in the ground some ten thousand years ago. Today, our continuing efforts to develop crops and animals for food, work, and companions have expanded beyond breeding to include genetic engineering. Dismay about our effects on the environment is encouraging us to see ourselves more as a part of nature than above it; perhaps we will eventually abandon Darwin’s term “artificial selection” in favor of coevolution. Evolution by natural selection is not just an explanation of the history of life. The process of Darwin’s theory clearly continues, changing our world and ourselves - both despite and because of our best efforts to control it. And we have reached beyond Darwin’s wildest expectations; we now have direct observations of natural selection to add to the overwhelming evidence for evolution.

Artificial Selection - or Coevolution?

The range of variations induced in relatively short periods of time by animal breeders convinced Darwin that natural selection across geologic time could have produced the great diversity of present life. Domestication of animals has resulted in the remarkable variety of dogs (Figure 1) from wolves, as well as cattle, horses, llamas, camels, and a few evolutionary dead-ends, such as the donkey.

Figure 1: Selective breeding has led to dramatic differences among breeds in a relatively short time, yet dogs are still able to interbreed with wolves - the wild species from which they originated. Darwin used his observations of artificial selection, as he called it, to derive and promote his theory of evolution by natural selection.

However, artificial selection has resulted in the achievement that extends far beyond our immediate, intentional goals. Our initial cultivation of plants such as corn (Figure 2) played a role in the eventual development of human civilization.

Figure 2: Over time, selective breeding has modified teosinte’s few fruitcases (left) into modern corn’s rows of exposed kernels (right). Cultivation of crops such as corn and wheat gave early humans the freedom to develop civilizations.

Since Darwin’s time, selective breeding and hybridization – mixing of separate species - has become even more sophisticated. We have further hybridized high-yield hybrids with local varieties throughout the world, intentionally adapting them to local climates and pests. Unfortunately, our widespread destruction of habitat is eroding the species and genetic diversity which provides the raw material for such efforts. Moreover, against our intent, our hybrids sometimes interbreed with natural varieties in the wild, leading to what some call genetic pollution. An example is a tiger, thought to be pure Bengal but actually a Bengal-Siberian hybrid, released in India to demonstrate the survival abilities of captiveraised tigers. The tiger did survive – to pollute the genetically pure Bengal population in a national park with northern-adapted Siberian genes (Figure 3).

Figure 3: The natural genes which adapted the Indian Bengal tiger (Panthera tigris tigris, left) and the Russian Siberian tiger (Panthera tigris altaica, right) to their unique habitats were mixed or “polluted” when a captive hybrid was released into a national park in India. The “escape” of non-native genes into a wild population is genetic pollution.

The new field of biotechnology has dramatically changed our quest to improve upon natural selection. Ironically, one new development intentionally undermines the very foundation of Darwin’s theory. As the first mammal to be cloned, a sheep name Dolly showed breeders of animals from farms to racetracks that they could copy “ideal” individuals without the bothersome variation which accompanies sexual reproduction (Figure 4). Many people hope that future decisions about cloning will consider Darwin’s lessons about the value of variation in unpredictable, changing environments.

Figure 4: Dolly, the first cloned mammal, is preserved for public display after six years of public life. Cloning can copy animals we believe are superior, but it denies the importance of variation to survival of species – a point made clear in Darwin’s ideas about natural selection.

Another contribution of biotechnology is genetic engineering, the transfer of a gene from one organism to another. First, we inserted the human gene for insulin into bacteria, which – as bacteria use the same universal Genetic Code as we use – read the DNA and produced the human protein for use by diabetics. Many more cost-saving and designer medical advances have followed, including

• production of clotting factors for hemophiliacs
• vaccines for devastating diseases such as hepatitis B
• a breast cancer “designer drug,” herceptin
• the potential for cheap, effective vaccines in fruits such as bananas

We have extended genetic engineering to agriculture, improving range, nutrition, resistance to disease, and other aspects of life. Transgenic animals - which possess genes from another species - now produce vaccines and hormones, serve in scientific research, and entertain us as pets (Figure 5). However, as for traditional agriculture, fears surround potential cross-pollination and interbreeding with wild populations. Modified genes have been found in plants up to 21 km (13 miles) away from their source. If such transfers spread resistance to herbicides or pesticides to wild populations, they will have defeated their intended purpose.

Figure 5: Genetic engineering has influenced our practices of medicine, research, agriculture, and animal husbandry – and recently the pet world. Zebra fish (natural species lower right) have received genes from jellyfish (green and yellow) or a coral (red) so that they glow. Originally “designed” for research, they are now bred for aquarists. Did we choose them, or did they choose us?

In his book, The Botany of Desire, Michael Pollan questions our feelings of superiority over our domesticated plants and animals. Discussing our domestication of the apple for its sugar, the tulip for its beauty, marijuana for its psychogenic effects, and the potato for its food value, Pollan takes the plants’ view of the evolving relationships. Could it not be that, as we have selected and modified these plants, they have also selected us for our powers to ensure their survival and reproduction – and changed us in the process? Are domestication of animals, cultivation of plants, and selective breeding actually forms of coevolution? Pollan’s delightful yet sobering treatise may reflect a growing realization that we humans are as much a part of nature as any other species. Yes, we can influence evolution in a number of ways. However, we remain subject to natural selection, and every choice we make has effects on evolution – including our own. As we have already seen, our choices often have unintended effects.

Evolution of Resistance

In almost unprecedented actions during May 2007, United States government agencies put a US citizen on a no-fly list, urged border agents to detain him, failed to detect his re-entry into the US, and eventually ordered him into involuntary isolation, urging individuals who had flown with him on several international flights to be tested for XDR-TB. Why were such drastic measures needed? What is XDR-TB, and how did it originate? The answers show evolution in action today - in a way that all of us need to understand for our own well-being.

Tuberculosis (TB) has infected and killed humans since at least 4000 BCE. Today, over one- third of the world’s population has been exposed to the bacterium which causes tuberculosis (Figure 6), but 90% of those carry the microorganism without symptoms. In the past, the 10% who did develop the characteristic lung infection had a 50% chance of dying. The advent of antibiotics in the mid-20th century dramatically improved survival, although the slow-growing bacteria required treatments of 6-12 months rather than days. Just 40 years later, in the 1990s, a new strain appeared with a mortality rate comparable to lung cancer – up to 80%. MDR-TB, or multi-drug resistant TB, is not treatable by two of the most effective anti-TB antibiotics. Then, about the year 2000, a second, more menacing strain emerged. XDR-TB, or extensively drug-resistant TB, is not treatable by either the two major drugs or the less-effective “second line” drugs now used to treat MDR-TB. Late in 2006, an epidemic of XDR-TB developed in South Africa. Currently there are no available drugs that can effectively treat this strain of TB.

Figure 6: An electron microphotograph reveals the rod-shaped cells of the bacterium which causes tuberculosis (TB). We cannot, however, distinguish the antibiotic-resistant varieties by appearance; only chemical analysis can discover which patients are infected with XDR-TB. Natural selection, however, can distinguish the resistant varieties from those which are sensitive to antibiotics. Or would that be considered artificial selection, because we are (albeit inadvertently) choosing which bacteria survive?

Clearly these strains of TB are new, and changing rapidly. The evolution of resistance is a growing problem for many disease-causing bacteria and also for parasites, viruses, fungi, and cancer cells. The “miracle” of drug treatment which appeared to protect humans from disease may be short-lived. How does resistance happen? How can we prevent it?

First, recognize that resistance describes the bacterium (or other microorganism) – not the human. Bacteria multiply much more rapidly than humans, and therefore can evolve much more rapidly. Consider a population of bacteria infecting an individual with tuberculosis. Like all populations, individuals within that population show variation. Mutations add more variation. By chance, mutation may change the chemistry of one or a few bacteria so that they are not affected by a particular antibiotic. If the infected human begins to take antibiotics, they change the environment for the bacteria, killing most of them. However, the few bacteria which by chance have genes for resistance will survive this change in environment - and reproduce offspring which also carry the genes. More and more of the bacterial population will be resistant to antibiotics, because the antibiotics select for resistance. The bacteria are merely evolving in response to changes in their habitats! If the resistant bacteria are transmitted to another human “habitat,” their population continues to expand, and if the new “habitat” takes different drugs, natural selection may result in multi-drug resistance (Figure7).

Figure 7: The development of resistance to antibiotics is a classic example of natural selection. Before selection, a number of heritable variations in level of resistance exist within the population. After selection by antibiotics, only those bacteria resistant to antibiotics survive. Only these resistant bacteria reproduce, so that the final population contains a greater proportion of resistant bacteria.

How widespread is the problem? Staphylococcus aureus bacteria first showed resistance to penicillin just four years after the drug was put into use; today, some strains have shown resistance to nearly all antibiotics. These are now known as one of several “superbugs.” The Human Immunodeficiency Virus (HIV) has become resistant to several antiviral drugs, and cancer cells within an individual often evolve resistance to chemotherapy drugs. Pesticide resistance is evolving in a similar manner; U.S. crop losses to insect pests have increased from 7% in the 1940s to more than 13% in the 1980s, despite the use of more types of pesticides in the 1980s.

What can we do about this particular instance of evolution which we have unwittingly encouraged? In general, we should reduce the use of antibiotics where possible and safe in order to lessen the selective pressure on bacteria. Here are some practices to keep in mind: 1. Don’t take antibiotics for viral infections such as colds and flu; they act only on bacteria.
2. When antibiotics are appropriate, take them exactly as prescribed, and complete the entire course.
3. Never take antibiotics which are left over from an earlier illness or prescribed for someone else.
4. Consider purchasing meats and other animal products from animals not treated with antibiotics.
5. Consider purchasing organic produce, which is not treated with pesticides.
6. Resist the use of pesticides in your own gardens.

We have unintentionally sped up the evolution of microorganisms, but at the same time, their development of resistance has given us a window into the process which underlies all changes in life, natural selection.

Evolution Continues, and We "Catch it in the Act"

Much more passively and with a clear understanding of our lack of control, humans have watched viruses rapidly evolve through mutation to cause frightening worldwide epidemics, or pandemics - from the 1918 “Spanish flu” through Severe Acute Respiratory Syndrome (SARS) and West Nile virus, to the widely anticipated “avian flu” caused by a highly pathogenic viral subtype of influenza A (Figure 8), known as H5N1, and the 2009 ”swine flu” caused by the H1N1 influenza virus. Figure 8 shows the increase in human infections and deaths from H5N1. Mutations have adapted it for life in birds and in humans, and for transmission from bird to bird and bird to human. If a future mutation adapts it for effective transmission from human to human, a serious epidemic could result. If, as some argue, influenza pandemics occur in cycles, we are overdue for a dramatic demonstration of evolution and natural selection.

Figure 8: Human infections and deaths from avian flu, caused by the H5N1 subtype of influenza A virus, are clearly increasing. Mutations have adapted the virus for life in birds and humans, and for transmission from birds to birds, and from birds to humans. Some scientists think the probability is high that the virus will also evolve the means for effective transmission between humans and cause a serious pandemic.

Peppered moths (Figure 9) are mostly white with black specks – a color pattern which hid them for centuries from predatory birds as they rest against lichen covered tree trunks. However, soot from the Industrial Revolution darkened the trees and destroyed their camouflage, selecting instead for the dark mutants which occasionally appeared. Gradually the population shifted to a dark color – an instance of natural selection that was directly observed by Englishmen of the time. Subsequent improvements in air pollution control have cleaned up the environment, and the English now note a new change: the trees have lightened, and moth populations are returning to their original coloration. These direct observations of natural selection would have delighted Darwin (except perhaps for the pollution) just a few years earlier.

Figure 9: The peppered moth population changed from mostly light (left) to mostly dark (right) as the lichen-covered trees in England’s forests absorbed soot from the Industrial Revolution. Now, as pollution is being cleaned up, the moth population is returning to its former proportion of light moths. These changes illustrate what famous idea?

Much more intentionally, biologists Peter and Rosemary Grant have devoted more than 30 years to a study of two species of Darwin’s finches on one of the Galapagos islands (Figure 10). Catching, weighing, and recording the seed species eaten by hundreds of these birds, they have witnessed changes in beak size which clearly correlate with changes in weather and availability of food. A severe drought and food shortage in 1977 led to a significant change. Birds whose small beaks could not crack the tough remaining seeds died, and the larger-beaked individuals who survived reproduced. The following year, offspring were larger bodied and larger-beaked, showing that natural selection led to evolution. A rainy winter in 1984-1985 reversed the trend; more soft seeds were produced, and the smaller beaked finches survived and reproduced in greater numbers than their large-beaked cousins.

Figure 10: A large cactus ground finch crushes a seed on the island of Espanola in the Galapagos archipelago. Peter and Rosemary Grant studied two closely related species of Darwin’s finches and recorded changes in beak size and body size which paralleled changes in weather. How fitting that they should demonstrate natural selection in action – something Darwin did not think possible – using one of the species he made famous!

Jonathan Winter eloquently describes the Grants’ work and discoveries in his Pulitzer Prizewinning The Beak of the Finch, A story of Evolution in our Time. He states:

“For all species, including our own, the true figure of life is a perching bird, a passerine, alert and nervous in every part, ready to dart off in an instant. Life is always poised for flight. From a distance it looks still, silhouetted against the bright sky or the dark ground; but up close it is flitting this way and that, as if displaying to the world at every moment its perpetual readiness to take off in any of a thousand directions.”