Science, December 4, 2009
In the final words of the final sentence of On the Origin of Species, Charles Darwin gave a nod to the future. “There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”
Darwin recognized that as long as the ingredients for the evolutionary process still exist, life has the potential to change. He didn’t believe it was possible to forecast evolution’s course, but he did expect humans would have a big effect.
In his day, they had already demonstrated their power with the triumphs of domestication, such as breeding dogs from wolves. Darwin recognized that we humans can also wipe out entire species. He knew the dodo’s fate, and in 1874 he signed a petition to save the last surviving Aldabra giant tortoises on the Seychelles Islands in the Indian Ocean.
Darwin also expected that our own species would change. As Western powers colonized other parts of the world, he predicted that some populations would become extinct. But Darwin also felt a cautious optimism. “Looking to future generations,” he wrote in The Descent of Man, “there is no cause to fear that the social instincts will grow weaker, and we may expect that virtuous habits will grow stronger.” And unlike other species, humans could bring about this change consciously, through cultural evolution.
As the world celebrates the 150th anniversary of the publication of On the Origin of Species this year, scientists continue to think deeply about what comes next. But the complexity of evolution still makes forecasting hard. “As Yogi Berra once said, ‘Prediction is very difficult. Especially about the future,’” says Stephen Stearns, an evolutionary biologist at Yale University.
Yet evolutionary biologists also feel a new sense of urgency about understanding what lies ahead. Since Darwin’s day, humans have gained an unprecedented influence over our own evolution. At the same time, our actions, be it causing climate change, modifying the genomes of other organisms, or introducing invasive species, are creating new sources of natural selection on the flora and fauna around us. “The decisions we and our children make are going to have much more influence over the shape of evolution in the foreseeable future than physical events,” says Andrew Knoll, a paleontologist at Harvard University.
Shaping our genome
If there’s one thing that’s certain, it’s that humans, like other living things, will continue to evolve. “Evolution is unstoppable,” says Lawrence Moran of the University of Toronto in Canada. But that doesn’t mean that humans are marching on a path toward becoming giant-brained, telepathic creatures out of Star Trek. All it means is that the human genome will continue to change from generation to generation.
A background mutation rate guarantees this process. Each baby’s DNA carries about 130 new mutations. Most of them have no effect on our well-being. People can pass these neutral mutations down to their offspring without harm, and over time, a small fraction of them will end up spreading across entire populations, or even the entire species, thanks to random luck.
Natural selection can cause mutations that help individuals survive and reproduce to spread much faster than neutral ones. Exactly which mutations natural selection will favor, however, depends on the environment in which we live. And over the past 10,000 years, we humans have dramatically changed that environment. We have fostered new diseases to which humans have adapted, for example. But in other cases, civilization has shielded us from the environment, weakening the power of natural selection.
One of the best known examples of human-driven evolution is malaria. Early farmers cleared forests and created fields where malaria-carrying mosquitoes could lay eggs in pools of water. As malaria spread, natural selection favored those humans with defenses against the disease. One such defense comes from a variant of a hemoglobin gene that makes it hard for parasites to reproduce in blood cells. One copy of the gene reduces your chance of contracting malaria. Two copies cause sickle cell anemia.
On the other hand, civilization has also blunted some of natural selection’s power over humans, particularly in the 150 years since Darwin published On the Origin of Species. Back then, for example, some children had the misfortune to be born with defective copies of a gene for an enzyme that breaks down amino acids in the food they ate. This disorder, known as phenylketonuria, generally led to severe brain damage. Few people with severe phenylketonuria were able to pass on their genes. But today, now that scientists know what causes the disease, people with phenylketonuria can enjoy fairly normal lives simply by being careful about the foods that they eat, and they pass their genes on to their children. Other medical advances, from eyeglasses to antibiotics, may also allow some potentially detrimental genes to become more common than in the past.
Yet medical advances and other changes to human life have not stopped natural selection, nor will they in the future. HIV, for example, first evolved into a human pathogen in the early 1900s and today takes a devastating toll in many parts of the world. Genes that provide some resistance to the virus may be favored by natural selection in places where HIV is particularly common.
Even in affluent parts of the world like the United States, natural selection has not stopped. Subtle differences in people’s health influence how many children they have and thus can gradually change entire populations.
In a report published online 26 October in the Proceedings of the National Academy of Sciences, Stearns and his colleagues documented natural selection in 2238 U.S. women. The women were subjects in the Framingham Heart Study, which has tracked the health of thousands of people in Framingham, Massachusetts, since 1948. The scientists searched for traits that were correlated with having a higher number of children. Then they checked to see whether those traits tended to be passed down from mother to child—in other words, whether they were genetically based.
The scientists discovered that a handful of traits are indeed being favored by natural selection. Women with a genetic tendency for low cholesterol, for example, had more children on average than women with high cholesterol. A greater body weight was also linked with greater reproductive success, as was shorter height, lower blood pressure, an older age at menopause, and having one’s first child at an earlier age.
Stearns and his colleagues now know which traits are selected in the women of Framingham, but they have yet to determine exactly what advantage each trait confers—a situation that evolutionary biologists often face when documenting natural selection. Nevertheless, based on the strength of the natural selection they have measured, the scientists predict that after 10 generations, the women of Framingham will give birth, on average, a few months younger than today, have 3.6% lower cholesterol, and will be 1.3% shorter.
Of course, even this prediction is subject to change. Women with higher cholesterol may eventually be able to enjoy higher fertility rates thanks to cholesterol-lowering drugs, says Stearns, wiping out the differences in reproductive rates. “Selection is always operating,” says Stearns, “but the traits on which it operates shift with ecology and culture.”
Along with natural selection, it’s also conceivable that one day genetic engineering will change human DNA directly. In September, scientists at the Oregon National Primate Research Center reported that they could replace the DNA in the mitochondria of a monkey embryo with mitochondrial DNA from another monkey. In July, scientists at the Center for Regenerative Medicine in Barcelona, Spain, reported that they had repaired human stem cells carrying genes for an inherited blood disorder. Both studies hint that eventually scientists will be able to alter the genes of future generations.
But even if a child was born with engineered genes in our lifetime, that milestone wouldn’t mean much for the evolution of our species. Those engineered genes would be swamped by the billions of mutations that emerge naturally in the babies born every year. Yet although engineered genes aren’t likely to provide enough reproductive advantage to spread on their own, they may still become common. John Hawks, an anthropologist at the University of Wisconsin, Madison, speculates that if genetic engineering becomes cheap enough and provides an attractive trait—such as staying thin—economics could spread a gene even if natural selection can’t. “I think people would buy it,” says Hawks.
Human-powered evolution
Genetically engineered humans may still be science fiction, but genetically engineered animals, plants, and microbes are all here already. In 2008, farmers planted 125 million hectares of genetically modified crops. Many of these crops carry genes from other species. Corn, cotton, and other plants have been engineered to carry a gene produced by bacteria, for example, so that they can make an enzyme that can kill insects. With big countries such as China and India dramatically ramping up their use of genetically modified crops, this evolutionary trend will likely continue.
In the near future, scientists may start to engineer life in a more profound way, manufacturing new species from scratch. The idea would be to design a microbe on a computer, combining genes with different functions into genetic networks. Scientists could then synthesize the new genome from raw DNA and insert it into an empty microbial cell that would come to life. J. Craig Venter and his colleagues at the J. Craig Venter Institute in Rockville, Maryland, have taken a series of key steps toward that goal, such as performing a “genome transplant” on a microbe.
If Venter succeeds, his artificial would be a triumph of human ingenuity, but it would probably be a minor blip on the biosphere’s radar. Synthetic biologists want to make microbes to serve our own ends, such as making fuel and medicines. Burdened with genes for these functions, the microbes will likely be ill equipped to compete in the wild against species that have adapted for millions of years. For the foreseeable future, synthetic microbes will probably survive only in the refuge of a laboratory or a fermentation tank. “I will venture a prediction,” says Adam Wilkins, a biologist at the University of Cambridge in the United Kingdom. “This kind of biotech engineering might succeed in creating some rather weird and wonderful organisms. But the net effect on evolution will be nil—that is, outside the laboratory.”
But humans, Wilkins is quick to point out, don’t need synthetic biology to have a big effect on the evolution of life. Chainsaws, fishing lines, and smokestacks do just fine. Many fisheries, for example, have established rules for keeping fish only above a certain size. As a result, natural selection has favored fish that become sexually mature at smaller sizes. On land, hunters have had a similar effect by going after big game. Bighorn sheep, for example, now grow horns 25% smaller than they did 30 years ago.
Humans have also triggered bursts of evolutionary change by introducing species to new habitats. In Australia, for example, cane toads brought in from South America in 1935 have became a continent-wide pest. They’re devouring some small native species, and their poisonous skin is killing off some of their predators. Scientists have discovered that the toads are evolving in their new home: Toads at the leading edge of the invasion are growing longer legs and moving faster than their ancestors, speeding up the invasion. The native species are responding as well. Australian snakes are evolving resistance to the cane toad poison.
Stephen Palumbi, a biologist at Stanford University in Palo Alto, California, expects that human-induced natural selection will become much stronger in the future. “In the last century, we were having a big impact, but it wasn’t everywhere,” says Palumbi. “But global climate change is an ‘everywhere’ impact, and that’s different.”
Plants and animals are already responding to the warming climate by shifting their ranges to find the most comfortable temperatures. But moving won’t be a solution for many species, which will face barriers such as deserts or cities. They will have to adapt to survive—a process scientists have already detected in some species, such as red squirrels in Canada, which have evolved to breed earlier in the spring.
Extra carbon dioxide is creating a second worldwide evolutionary pressure as it dissolves into the ocean. There it is turning into carbonic acid and lowering the pH. Continued acidification will make it more difficult for corals and other marine animals to build skeletons and shells from calcium carbonate. Organisms will need to adapt to survive in these new conditions.
“We know that things can evolve quickly, but can they evolve fast enough?” asks Palumbi. He and many other scientists suspect that for many species the answer is no. Unless we can ease up on the biosphere, they warn that the biggest feature of evolution in the near future will be extinctions.
Knoll points out some disturbing parallels between today’s crisis and a pulse of mass extinctions that occurred 252 million years ago, wiping out an estimated 96% of species in the oceans and 70% of species on land. A rapid increase in carbon dioxide in the atmosphere led, among other things, to ocean acidification. For animals that depended on calcium carbonate, “you had about a 90% chance of going extinct,” says Knoll. “Corals, sponges, brachiopods, they all kicked the can.”
Knoll doesn’t expect human-driven mass extinctions to be as bad as that ancient one. But they could still be unimaginably huge. “If we lose half the species on the planet, our grandchildren are not going to see them restored,” says Knoll. “It will take millions of years.”
A drop in biodiversity may bring with it a collapse of many ecosystems. Coupled with a rapid increase in global temperatures, ocean acidification, and other changes, we may be pushing the environment into a state we’ve never experienced as a civilization. Such a stress could put our species under intense natural selection as well.
Taking the long view
One way or another, life will survive this current crisis. But where is life headed in the very distant future? To find out, planetary scientist King-Fai Li of the California Institute of Technology in Pasadena and his colleagues built a model of Earth and the sun and watched it evolve for billions of years. In their simulation, the sun gets brighter, as it has since it first formed. The extra energy speeds up the rate at which carbon dioxide is drawn out of Earth’s atmosphere, cooling it off. But after about 2 billion years, this cooling mechanism breaks down, and Earth heats up, ending up like its lifeless neighbor, Venus.
But Li’s model does not include a clever species like our own, which can use its brain to influence the planet. Would it be possible to extend the life span of Earth’s biosphere? “I am not going to rule out any talented civilizations that will be able to do that,” says Li.
Copyright 2009 American Association for the Advancement of Science. Reprinted with permission.