Evidence that Evolution Is Still Happening
It is common to hear people say that evolution is something that happened in the past, not today. In On The Origin of Species, Darwin himself wrote, “I have to admit that natural selection will always act extremely slowly.” He could never have imagined that evolution could be witnessed in real-time. Let us look at some examples of natural events and experiments that allow us to see evolution in action.
The Evolution of Laboratory Bacteria
Countless researchers are currently carrying out laboratory experiments to study how evolution works. They rely on artificial selection, increasing or decreasing the frequency of certain traits in a population. They decide in advance which specimens will reproduce and which will not.
Another method is to change the environmental conditions in a laboratory and simply let natural selection run its course. Bacteria reproduce asexually every twenty minutes, making them perfect candidates for this type of experiment.
The American researcher Richard Lenski is best known for his study on the bacterium Escherichia coli, which he started in 1988. The experiment continues to this day. Lenski began with identical strains of bacteria and changed the environmental conditions for each of them. He varied the temperature, food type, glucose levels, etc. The objective was to test the ability of bacteria to adapt to different environments. After 18 years of research, or 40,000 generations of E.coli, Lenski and his colleagues have identified the beneficial mutations that have promoted the survival of the bacteria.
Lenski’s bacteria are nothing like his original colonies. They are twice the size of their ancestors and reproduce 70% faster. They have evolved ways to manage their food intake. They slow down their growth rates if they are offered any sugar other than glucose. They also undergo mutations more often than their ancestors. Lenski has observed evolution in action in his own laboratory.
Richard Lenski in his labortory in 2016
Bighorn Sheep
Artificial selection has influenced many species vulnerable to both hunting and fishing. In the wild, the fittest individuals survive. Humans prefer the most beautiful, largest, and strongest individuals. We hunt and fish the best specimens of a species. Therefore, those that survive and reproduce are often the smallest or least conspicuous in a population.
The bighorn sheep population of North America, Ovis canadensis, has experienced a 20% decrease in their average horn size in the last 30 years because hunters tend to go after the sheep with the most spectacular horns.
Bighorn Sheep (Ovis canadensis).
North American Sockeye Salmon
When the glaciers melted after the last ice age 10,000 years ago, many bodies of water formed in the northern United States and Canada, including Lakes Superior, Michigan, Huron, Erie, and Ontario. Some of the sockeye salmon that inhabited the rivers took refuge in those lakes. The salmon evolved into a different species, adapting to the new environment. These salmon do not interbreed with river salmon and have acquired differences in their bodies. How fast did this happen?
A recent event in Washington State allowed researchers to answer this question. In 1937, a population of Baker Lake salmon was introduced into Lake Washington, which empties into the Cedar River. In 40 years, two different species emerged from the originally introduced species. One species spawns in the Cedar River and resembles river salmon. The other breeds on lake beaches and resembles lake salmon. Furthermore, the two species do not interbreed.
Salmon
Annual Influenza Vaccines
Influenza is a respiratory disease that can have serious complications. Sometimes, a patient’s life can be in danger, and hospitalization is required. The best way to prevent contagion is to get vaccinated once a year. We ask ourselves: Why is it necessary to be vaccinated every year? Wouldn’t it be easier to receive a single dosage of the vaccine, as is the case with the polio vaccine? You need to get vaccinated yearly because the influenza virus constantly evolves. The virus can change from season to season. These mutations make the previous flu vaccines ineffective.
Human Immuno-Deficiency Virus (HIV)
HIV is a very personal disease, both in its mode of transmission and in the stigma it carries. The intensity of its rampant destruction is different in every patient. It is personal because no two infections are alike. They may bear the same name and have the same consequences and prognosis, but each HIV infection evolves in a unique way. It is a trajectory rather than a disease.
HIV infection progresses so quickly that within two months, the virus can already hide from the immune system it is attacking. If two individuals are infected with identical viruses on the same day, the viruses immediately begin to follow two different evolutionary paths. Each virus will evolve to survive in its particular host.
It is no exaggeration to say that HIV is the fastest-evolving entity known. Two years after infection, the original virus is no longer a single virus but a mixture of viruses with more genetic diversity than the genomes of humans and chimpanzees.
It might still be called HIV, but its remarkable evolution is analogous to a single foot soldier becoming an invasive army of tanks, infantry, and missiles.
Evolution also acts on the individuals infected with the virus. Natural selection occurs as long as people reproduce before dying, leaving more offspring than others.
In places like Africa, many people die before having children. AIDS kills children and adults of reproductive age. A mutated gene has appeared among the inhabitants of Africa, an allele called CCR5-Delta32, that offers its carrier protection against HIV infection. If mortality from AIDS continues, the frequency of this allele will increase, and the population will gain resistance to the virus over time.
Human Immuno-Deficiency Virus
Antimicrobial Resistance
It is commonly thought that microbial resistance happens when individuals no longer respond to their prescribed medication. This is false. Resistance is due to the evolution of microbes, and it is the best example of natural selection in our daily lives.
Why do doctors insist that patients finish their entire prescription of antibiotics?
It is not because they are in collusion with pharmaceutical companies. Like any population of living things, the microbes targeted by the medication have variation.
The individual microbes which survive the first few doses of medication have characteristics that make them resistant. Some people stop taking their medication when they feel better, but not all the germs have been killed. Only a few germs resistant to the antibiotic survive. These germs are now free to reproduce and produce entire populations of drug-resistant microbes. The same medication may not work the next time these people get sick because the microbes have evolved resistance.
Penicillin was introduced in the early 1940s to cure infections caused by Staphylococcus aureus. Seventy years later, the bacteria Staphylococcus aureus has evolved and has acquired mutations making 95% of current strains resistant to penicillin.
The problem is further complicated by the way bacteria exchange genes. Darwin built his theory based on the idea that organisms inherit traits from their parents.
If a bird has a beak that allows it to crack seeds more easily than the rest of the population, it will have an evolutionary advantage and pass that trait on to its offspring. Their offspring will, in turn, pass it on to the next generations. Darwin never imagined exceptions to this rule. Bacteria are capable of exchanging genes horizontally, or laterally, with other bacteria. This is how bacteria acquire resistance to antibiotics. The misuse of prescription medication has become a medical challenge for the pharmaceutical industry.
Bacterias
The Reappearance of Tuberculosis
Because tuberculosis could be easily treated with antibiotics, scientists believed it could be eradicated. Unfortunately, it is becoming increasingly common for doctors to treat their patients with all kinds of medications without success. Unfortunately, the bacteria responsible for tuberculosis has evolved to become resistant to all antibiotics, making tuberculosis a deadly disease once again. About two million people die each year from tuberculosis. If new antibiotics are not developed by pharmaceutical laboratories soon, deadly bacterial diseases may once again make humanity suffer the deadly epidemics our ancestors endured for millennia.
Tuberculosis bacteria resistant to antibiotics
The Finches of the Galapagos Islands in Ecuador
The finches of the Galapagos Islands are the famous inspiration for Darwin’s theory of evolution by natural selection. He traveled throughout the southern hemisphere from 1831 to 1836 and visited the Galapagos for five weeks. The different species of tortoises and finches caught his attention. He noted how the populations varied from one island to another. Darwin sent specimens back to England, which were studied by the best naturalists of the time. They discovered that the Galapagos finches identified by Darwin actually belonged to different species.
Perhaps the best-documented example of current evolution is the study of the Galapagos Islands finches carried out by the married couple Peter and Rosemary Grant. Since 1973, they have visited Daphne Major Island every year for six months at a time, dedicating themselves to identifying each medium ground finch on the island. They take blood samples of each bird and measure their characteristics, such as the size of the beak, head, wings, body, etc. The Grants know the entire family history of the medium ground finches on Daphne Major. For more than 40 years, they have documented changes in the beak size of the finches. For example, small and easily opened seeds are unavailable in the dry season, and the finches that feed on these smaller seeds can find only hard, large ones. In this scenario, nature selects for finches with larger, thicker beaks, and in a very short period, the average beak size increases by up to 10%, along with an increase in body size.
When the dry season ends, the smaller, softer seeds become available; the finches evolve again to have smaller beaks within just a few years.
One Galapagos finch. Photograph by Armando Jinich
Chinch Bugs
A red-backed bug called a chinch bug, Jadera haematoloma, can be found in the southern parts of the United States. It lives on two types of plants. Its beak-shaped mouthparts allow them to penetrate the seeds’ outer coat and suck up the contents inside. In the last fifty years, this insect has adapted to three new plant species introduced to the area. The introduced plants produce seeds that are different from those of the native plants: two larger ones and a smaller one.
The prediction that the beak size of the chinch bug would evolve much like the beaks of the Galápagos finches has been confirmed. Bugs that feed on large seeds have evolved longer beaks, and those that feed on small seeds have evolved shorter beaks. After just a couple of decades, or 100 generations, the average beak size now varies by 25%. This may not seem like much, but if we put it in perspective, after 1,000 generations, they could have a beak more than double the original size.
After 10,000 generations, the beak would become a spike, 20 times longer than the original. This hypothetical example shows the cumulative power of small changes in a population over time.
Sketch of a Chinch bug, Jadera haematoloma
The Black Death of the Fourteenth Century
The most devastating pandemic in recorded history is the so-called black death, or bubonic plague, of the mid-14th century. It was caused by a bacterium, Yersinia pestis, and transmitted to humans by the fleas of infected rodents. Between 1347 and 1400, the plague was introduced to Europe on several occasions by ships arriving from Asia carrying goods and rodent passengers. Current genetic sequencing studies have revealed that the disease killed almost half of the inhabitants of the Western world, twenty-five million people, instead of only a third as previously believed.
Why hasn’t the plague made a deadly comeback? Because those who survived were fortunate enough to have the genes that made them resistant to the disease. These genes were inherited by their descendants, us! This is an excellent example of natural selection in action in relatively recent times.
Sketch of the plague
Herbicide Resistance
At the beginning of the 20th century, the use of chemicals to destroy certain unwanted species of plants, fungi, and animals became popular. There are insecticides for killing insects, fungicides for killing fungi, pesticides to eliminate plant diseases, and herbicides to eliminate unwanted plants. The latter is the most common and is used to control weed growth on crops and along roadsides.
Herbicides work in many ways, from inhibiting photosynthesis to blocking the production of specific proteins. The herbicides cannot kill all the target weeds.
Since genetic variability always exists, some weeds do not die when the herbicide is first used. These weeds reproduce, and a resistant variety emerges in a relatively short time, making the same product ineffective on the next occasion.
The California Salamander
In California, the mountainsides surrounding the Central Valley make an inverted U-shape. Salamanders of the Ensatina species populate this territory. They do not cut across the valley; they live on the mountainsides to the east, north, or west of the valley.
Southeastern salamanders and southwestern salamanders are located at the ends of the U-shaped territory. They are physically isolated from one another since their territories are not continuous. They are two different species that cannot interbreed with one another. However, all along the U-shaped territory, neighboring populations of the species vary slightly but can still interbreed. These salamanders are what biologists call a ring species. They can interbreed with the adjacent populations but not with distant ones. The further away they are from each other, the more genetic differences exist between populations. The differences are so significant that salamanders can no longer interbreed at the two ends of the inverted U. This is an excellent example of parapatric speciation.
Sketch of an Ensatina salamander
Sea Gulls of the Northern Hemisphere
In Great Britain, there are two species of gulls, the herring gull, Larus argentatus, and the black-backed gull, Larus fuscus. Something interesting happens if you follow the herring gull populations west toward North America. The herring gull in North America begins to look less like the English herring gull to its east and more like those in Siberia to its west. In Eastern Europe, the gulls look less and less like the American herring gulls and more like the black-backed gulls to their west.
Neighboring populations of subspecies can interbreed and have similar physical characteristics. However, the gulls at the ends of the ring, the herring gull and the black-backed gull, are two different species incapable of interbreeding. The ring starts in England, loops around the Arctic, through the Northern Hemisphere, and returns to England. This is another good modern-day example of parapatric speciation.
Herring gull, Larus argentatus.
Black-backed gull, Larus fuscus
Lizards on the Islands of the Adriatic Sea
Just off the coast of Croatia in the Adriatic Sea is a small island called Pod Mrcaru. It covers an area of 2.8 hectares (7 acres) and is covered in bushes. There are no human settlements on the island; its only inhabitants are lizards, and their origins are fascinating.
There were originally no lizards on Pod Mrcaru. However, a neighboring island, Pod Kopiste, had a population of lizards that fed primarily on insects. Pod Kopiste has a rocky terrain and no vegetation. In 1971, Israeli researcher Eviatar Nevo decided to export five adult pairs of Pod Kopiste lizards to Pod Mrcaru to study how they would survive on an island with entirely different conditions. Unfortunately, Nevo did not live to see the results of his study. The war in Yugoslavia began in 1991, preventing his team from visiting the island for years.
A group of researchers returned to Pod Mrcaru in 2004 when the war ended. They were surprised to find that the lizards on the island were very different from those of Pod Kopiste. In just 33 years, the Pod Mrcaru lizard population had adapted to eating leaves instead of insects. They developed larger heads and a more substantial bite. Their legs were shorter, and they moved more slowly. DNA analysis confirmed that the lizards from Pod Mrcaru were indeed the descendants of the five pairs taken from Pod Kopiste. The most astounding fact was discovered when they dissected some of the lizards. Their digestive system had radically changed. The lizards had developed a new stomach cavity colonized by bacteria, which allowed them to digest the cellulose in the leaves. Their behavior had also changed. Pod Mrcaru’s abundant vegetation meant there was no need to compete for food or protect territories, nor did the lizards have to worry about predators. As a result, they became less aggressive.
By contrast, the Pod Kopiste lizards are very aggressive, competing for a limited food supply while fiercely defending their territories.
Natural selection brought about amazing adaptations in just 33 years and around 20 generations. The example of the Pod Mrcaru lizards is fascinating because it shows how complex traits and changes in behavior can emerge in a short time.
One Adriatic lizard
The Pigeons of California
Alberto Palleroni, a researcher at Harvard University, dedicated seven years of his life to studying the predation of wild pigeons by the peregrine falcon, Falco peregrinus. He observed that pigeons could have one of six different patterns of plumage. Pigeons with the white feather pattern on their backs make up 20% of the population but suffer only 2% of attacks. The researchers disguised the pigeons born with different patterns by carefully cutting, stripping, and gluing white feathers on their rumps. The pigeons with white feathers fared much better than those that received blue or gray feathers. The white pattern prevented them from being attacked during flight. We can conclude that the pigeons are evolving under natural selection; their modified plumage makes them better suited to escape predators.
Peregrine falcon, Falco peregrinus, on its nest in Alaska.
The Peppered Moths of Great Britain
The peppered moths of Great Britain, Biston betularia, are a classic example of evolution in action. The typical form of the peppered moth has light-colored wings speckled with small dark spots. The pale moths were well camouflaged on the white bark of birch trees common to the area, enabling them to avoid the hungry birds who share their habitat. The air pollution created by the Industrial Revolution made the trees darken in color as soot accumulated in the areas surrounding the factories. The moth population changed in response to the darker birch trees. Moths with darker or dusky wings became more common. The dusky wings made it difficult for birds to spot the moths on the soot-covered tree trunks.
As air quality improved in the 1960s, the soot disappeared, and the coloration of the trunks became lighter again. The dusky moths, which once represented 90% of the population, became easy prey for their predators and their numbers dropped to 10%. The population evolved in response to changes in the environment, twice.
Peppered moths, Biston betulari, of Great Britain, light and dark morphs.
The Eastern Diamondback Rattlesnake
Humans are often the most powerful agents of artificial selection for wild animals. The Eastern Diamond Rattlesnake, Crotalus adamanteus, studied by biologists in Florida, is an interesting example. These snakes tend to be fearful by nature and rattle their tails to warn predators to stay away. This signal makes them easy prey for humans. In many regions of Florida, biologists found populations of predominantly silent rattlesnakes. The snakes that made noise gave away their presence and were killed by humans, dying before reproducing and leaving behind no offspring in the population.
The Eastern Diamond Rattlesnake, Crotalus adamanteus.
The Cichlids of Lake Victoria
One of the best examples of explosive speciation today is the cichlids of Lake Victoria in East Africa. There are approximately 500 different species of cichlids in the lake. In 1995, a group of geologists decided to research the origin of the fish by studying the composition of the lake’s muddy bottom. The lake is fed by rivers that carry pollen, dust, and dirt, which settle and accumulate on the bottom over time. The geologists drilled into the subsoil and obtained a vertical sample of the sediments that collected over millions of years. They were surprised to obtain a core sample only nine meters deep, equating to about 14,500 years of deposition. At greater depths, evidence of buried grasslands appeared. The lake is only 14,500 years old.
DNA analysis of the fish shows that their genes are very similar to each other and that they all descended from a single species, which must have inhabited a river before the formation of the lake. During the time it took for humans to become civilized, a single fish species gave rise to 500 different species. When the fish entered the lake 14,000 years ago, they were liberated from the limitations of river life, evolving adaptations to the variety of habitats offered by Lake Victoria, the largest tropical lake in the world. Indeed, sexual selection played an important role since there are cichlids of all colors and behaviors. Unfortunately, many cichlid varieties are being wiped out because of the introduction of a top predator, the Nile perch.
Cichlid fish of Lake Victoria, Pundamilia nyererei.
Australian Rabbits
Another current example of evolution is the Australian rabbit, Oryctolagus cuniculus. Rabbits are not native to Australia. Small kangaroos called wallabies occupied the ecological niche that they fill today. In 1859, an Englishman named Thomas Austin, who liked to hunt rabbits, decided to import 24 rabbits from England. He released them on his property in the state of Victoria. The rabbits reproduced with great speed. By 1866, rabbits had spread over all of Victoria and New South Wales, and by 1907, there were millions of rabbits all over Australia.
The government decided to release mosquitoes with a virus called myxomatosis to control the plague. The goal was to kill the invasive rabbits without affecting any other mammal. The campaign was a resounding success, killing 99.9% of the rabbits. However, 0.1% of the rabbits that survived had resistance to the virus, a mutation that European rabbits do not possess. This small population of Australian rabbits reproduced literally, like rabbits. It is estimated that approximately 200 million feral rabbits inhabit Australia today. In just over one hundred years, the rabbits of Australia evolved a genome very different from any other rabbit species in the world.
European rabbit, Oryctolagus cuniculus.
Guppies
Guppies, Poecilia reticulata, are a favorite in home aquariums. As a result of sexual selection, they have developed an enormous variety of colors. Females prefer to mate with the most colorful males. However, male guppies face a big dilemma in the presence of predators. The more colorful they are, the more they mate, but the more quickly their enemy locates and devours them. Without color, they survive, nobody eats them, but they do not mate either.
John Endler, an American researcher, designed a series of experiments that allowed him to witness the evolution of fish in front of his very eyes. He started with a population of wild guppies in which the males had a certain average number of spots. He then placed a population of guppies in three different ponds: one with no predators, one with very aggressive predators, and one with less aggressive predators. After a few months, the males in the population with no predators and those with less aggressive predators had a higher-than-average number of spots. In contrast, the males of the population that lived with the aggressive predators had a lower-than-average number of spots. Both natural and sexual selection played a role in the composition of each of the populations in Endler’s ponds.
Sketch of a guppy, Poecilia reticulata.
The Tibetan People
Tibet, the roof of the world, is located on the northern side of the Himalayan mountains, at an average elevation of 4,900 meters (16,000 ft) above sea level. In less than 3,000 years, the region’s human population has developed an adaptation that allows them to survive with 40% less oxygen than the rest of humanity.
It has long been suspected that people indigenous to Tibet simply don’t require as much oxygen as other people. Their muscles are more efficient. Recent studies have found that Tibetans carry approximately thirty unique mutations in their genes, making them distinct from the rest of the Chinese and Southeast Asian populations. In particular, they carry a gene called EPAS1, which plays a critical role in the body’s ability to adapt to changes in oxygen levels, an important adaptation for people living in high-altitude areas.
One Tibetan family
The Anole Lizards of the Bahamas
Anole lizards, misnamed chameleons, were introduced to the Bahamas in recent times. They come from nearby islands with plentiful vegetation and easy access to food. Therefore, they were adapted to life on tree branches and can move quickly on small surfaces. In the Bahamas, the lizards underwent a series of adaptations in less than 25 years. The topography and vegetation are very different from their native islands, with few trees and many rocks. The proportions of the anole’s hind limbs changed; they have longer legs than their closest relatives, allowing them to move over broad surfaces looking for food.
Silver foxes in Russia
In 1959, a Russian researcher named Dmitri K. Belyaev began a program to domesticate silver foxes, Vulpes vulpes, using the foxes he obtained from Soviet furriers. Systematically, he dedicated himself to choosing and reproducing the most docile specimens. The success of the experiment was surprising. By mating the most docile foxes, they produced foxes that behaved like border collies in just 20 years. They approached humans without fear, wagging their tails nonstop. What was most interesting about the experiment was not only the speed with which the docile foxes underwent artificially selection but also the unexpected consequences.
In addition to behaving like border collies, the most docile foxes looked like border collies. They grew black and white fur with white snouts. Instead of pointy ears, they developed floppy ears. Their heat cycle changed; instead of going into heat once a year, they began to go into heat more often, like dogs. And as aggressiveness decreased, serotonin levels increased. Serotonin is a neurotransmitter whose concentration is reduced by stress. Foxes became pseudo-dogs in twenty years. We say pseudo-dogs because dogs descended from wolves, not foxes.
Sketch of a Siberian silver fox, Vulpes vulpes.
