If you have seen science fiction movies like Jurassic Park, you may be able to imagine what Earth was like during the time of the dinosaurs—before human societies existed. In the 1800s, however, most people couldn’t imagine a time before the Biblical descriptions of life on Earth. Shell fossils on mountain tops were viewed as evidence of the Great Flood, and dinosaur bones were believed to be the remains of dragons.
In this chapter, we will learn how two self-taught scientists expanded our knowledge of the natural processes of evolution. Their research provided the groundwork for understanding why biodiversity is so important to Earth’s living systems.
I. Darwin’s Voyage of Discovery
Charles Darwin grew up exploring nature. He was a keen observer of all forms of life, and enjoyed collecting and identifying insects, especially beetles. People who knew him well described him as a person of boundless curiosity. His father wanted him to become a minister, so Charles entered Cambridge University to study religion.
In 1831, at the age of twenty-two and fresh out of the University, Darwin signed on as a ship’s naturalist aboard the H.M.S. Beagle. Darwin’s education and beliefs stemmed from the view that living things were fixed and unchangeable products of God’s Creation. Captain Robert FitzRoy, a naturalist and clergyman himself, had enlisted Darwin to help collect evidence supporting that view of life.
Among the books Darwin packed for his trip was the recently published Principles of Geology by Charles Lyell. In Lyell’s book was evidence that Earth was much older than anyone had thought possible, and that certain plants and animals had become extinct, while new species came into being. Even at the outset of his journey, Darwin was very interested in the mysteries locked in the stone layers revealed along cliffs and riverbanks.
Darwin’s voyage as ship’s naturalist presented him with an exciting but difficult task. Most of the plants and animals caught in the ships nets and captured on overland journeys were new to science. Everything had to be described and catalogued, then preserved.
Map of the voyage of the H.M.S. Beagle
The work had to progress despite bad weather and months of seasickness, because there was an endless variety of exciting new creatures.
The H.M.S. Beagle sailed around the world, visiting many environments.
Darwin wondered about the geological history of the landscapes and the relationships between similar species. He later wrote “When I was on board the Beagle I believed in the permanence of species, but as far as I can remember, vague doubts occasionally flitted across my mind…”
In 1835, while exploring the Galapagos Islands, Darwin noticed that there were several different species of land tortoises throughout the islands, each specialized for feeding on a certain kind of vegetation. One species of tortoise had a notched shell that enabled its long neck to reach up easily to browse on bushes and trees. Another species of tortoise that lacked the notched shell fed on low-growing plants and fruits.
Darwin’s careful observations of how similar species had slightly different ways of living provided the clues for his later hypothesis about the origin of species.
Galapagos tortoise in a farmer’s field. Photo by Christy Giuliano
What kinds of food do you think these finches eat?
Which finch do you think was most like the South American ancestor that fed on insects?
Finches adapted from Darwin’s “Journal of Researches.” Returning to England, Darwin had scientists examine the finch specimens from the Galapagos Islands. They found that the birds were very closely related. Darwin concluded that the birds were the descendents of a common ancestor from the coast of South America.
II. Searching the Globe for Evidence of Evolution
Alfred Wallace was another self-taught naturalist who pursued his curiosity about the origins of life. Unlike Darwin, Wallace was born into a working class family, and as a child worked for his brother as an apprentice surveyor. Throughout his life Wallace struggled with financial difficulties. He managed to fund his explorations with money borrowed from wealthy people and museums interested in buying specimens from exotic parts of the world. Wallace was an independent thinker who often had ideas that were considered “far out” by the scientists of his day. He read widely about the idea of evolution, and may have drawn some of his plans to explore new worlds from Darwin’s book, Voyage of the Beagle, which he read twice.
In 1848, at the age of 25, Wallace sailed to the Amazon in hopes of finding explanations for how the amazing variety of plants and animals arose to populate all of Earth’s environments. Exploring and mapping the Amazon River and the headwaters of the Rio Negro, he collected and identified hundreds of species of insects, birds, and plants that were new to science. Weakened by malaria, he headed back to England in 1852, but the ship sank off the coast of South America and with it most of his specimens and documents.
Saved by a passing ship, Wallace returned to London penniless, but determined to pursue his dream of solving the puzzle of evolution. He must have been very convincing, for his sponsors funded a new expedition to the Malay Archipelago in what is now Indonesia. In eight years he explored dozens of islands, traveling more than 14,000 miles and collecting more than 125,000 species of insects and birds.
Wallace discovered that some islands such as Borneo, Bali and Java had plants and animals closely related to species from Malaysia to the north. Nearby islands such as Lombok and Sulawesi had very different kinds of plants and animals which were related to species from Australia and New Guinea. He had discovered what came to be known as “Wallace’s Line,” a boundary between the Oriental and Australian regions.
We now know that the Australian land mass moved northward meeting Asia and resulting in dramatic differences in the species that inhabit neighboring islands.
III. The Theory of Natural Selection
Wallace wrote to Darwin, sharing his ideas of evolution and development of new species. Darwin had been working on his own case for the origin of species for twenty years but had been reluctant to publish his findings. He knew his ideas would cause great controversy, because they did not fit with the biblical explanation of creation. He had been patiently amassing a body of evidence that he hoped would prove his claims beyond a doubt.
Wallace and Darwin did not fight over who had prior rights to the ideas. Instead they agreed to publish their findings together in a joint article in 1858.
The key ideas of their theory include:
- Every population includes individuals with a variety of physical and behavioral characteristics, or “traits” that are inherited.
- Every population produces more offspring than can survive.
- Competition for scarce resources and the natural hardships of living favor the survival of individuals whose traits make them well-adapted to their environment.
- The better-adapted offspring survive in greater numbers to pass on their characteristics. Over many generations, the population is dominated by individuals that have inherited the most favorable traits for that environment.
- Organisms less suited to the environment have a greater risk of dying young from predation, starvation, and other mortality factors or of failing to produce offspring.
A year later, when Darwin published his large book, The Origin of Species by Means of Natural Selection, he wrote “This preservation of favorable variations and the rejection of injurious variations, I call Natural Selection.” Darwin and Wallace theorized that over long periods of time, this process resulted in the evolution of the millions of species of organisms alive today. The theory that organisms evolved from a common ancestor through the process known as “natural selection” became a major unifying concept of modern science. It explained the observations of scientists studying geology, paleontology, biochemistry, and biology.
The concept of a population is a key to understanding the theory. Darwin and Wallace realized that individuals do not evolve into something different. Instead beneficial traits are passed to the next generation causing an overall increase in the proportion of the successful trait within the population. After generations, a greater proportion of the population shows the helpful trait.
Excerpt From: Charles Darwin, “On the Origin of Species.”
“I have called this principle, by which each slight variation, if useful, is preserved, by the term natural selection, in order to mark its relation to man’s power of selection. But the expression often used by Mr. Herbert Spencer, of the Survival of the Fittest, is more accurate, and is sometimes equally convenient. We have seen that man by selection can certainly produce great results, and can adapt organic beings to his own uses, through the accumulation of slight but useful variations, given to him by the hand of Nature. But Natural Selection, we shall hereafter see, is a power incessantly ready for action, and is as immeasurably superior to man’s feeble efforts, as the works of Nature are to those of Art.
“A struggle for existence inevitably follows from the high rate at which all organic beings tend to increase. Every being, which during its natural lifetime produces several eggs or seeds, must suffer destruction during some period of its life, and during some season or occasional year, otherwise, on the principle of geometrical increase, its numbers would quickly become so inordinately great that no country could support the product. Hence, as more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life. It is the doctrine of Malthus applied with manifold force to the whole animal and vegetable kingdoms; for in this case there can be no artificial increase of food, and no prudential restraint from marriage. Although some species may be now increasing, more or less rapidly, in numbers, all cannot do so, for the world would not hold them.
“There is no exception to the rule that every organic being naturally increases at so high a rate, that, if not destroyed, the earth would soon be covered by the progeny of a single pair. Even slow-breeding man has doubled in twenty-five years, and at this rate, in less than a thousand years, there would literally not be standing room for his progeny. Linnaeus has calculated that if an annual plant produced only two seeds—and there is no plant so unproductive as this—and their seedlings next year produced two, and so on, then in twenty years there would be a million plants. The elephant is reckoned the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase; it will be safest to assume that it begins breeding when thirty years old, and goes on breeding till ninety years old, bringing forth six young in the interval, and surviving till one hundred years old; if this be so, after a period of from 740 to 750 years there would be nearly nineteen million elephants alive descended from the first pair.”
IV. The Case of the Pepper Moth
One of the first cases of natural selection observed in the wild involved the pepper moth in England. This white speckled little moth hid among the lichens growing on the trunks of trees. As industrial pollution killed the light-colored lichens in the surrounding countryside, the light-colored moths, which were no longer camouflaged on the bark, became easy prey for birds. As the lichens disappeared, so too did the white moths.
There were some darker varieties of the pepper moth that survived in greater numbers because they could easily hide on the dark bark of the trees. These dark moths soon became dominant in the population. However, some white moths survived in small numbers.
Explain how the changing moth and lichen populations provide an example of natural selection at work.
What do you think has happened to the moth population in those regions where pollution has decreased?
What further research activities do you propose to strengthen evidence for natural selection in pepper moth populations?
V. Continuing to Search for Clues
Darwin continued throughout his life to gather evidence of evolution. He was very interested in anatomy and studied the bodies and bones of thousands of organisms. He noticed that the arm bones of very different animals such as birds, whales, and alligators have a similar structural pattern. He proposed that these similarities were because the organisms shared a common ancestor.
Wallace was also a careful observer and recorder of nature. He collected and catalogued 125,660 species, mostly birds and insects. As he traveled from island to island throughout Southeast Asia, he noted that each species had very similar relatives living nearby.
Photo by George Goertz.
Both scientists were greatly influenced by an article written by Thomas Malthus. In his “Essay on the Principle of Population”, Malthus noted that human populations tend to increase in size geometrically, yet their food supplies don’t increase nearly as rapidly. He proposed that human populations are kept in check by famine, plague, and war.
Both Darwin and Wallace realized that this implied a “struggle for existence” for humans. They proposed that this struggle to survive was probably true for other organisms. Darwin wrote: “It at once struck me that under these circumstances favorable variations would tend to be preserved and unfavorable ones to be destroyed. The result of this would be the formation of new species.”
VI. Comparing Ideas
How did Darwin’s and Wallace’s theory of natural selection differ from other ideas that came before? Early in the 1800’s, Jean-Baptiste de Lamarck had proposed an explanation for why animals exhibited adaptations to their environments.
In what ways do the “arms” of these vertebrates suggest a common ancestor?
Lamarck proposed that an animal’s activities would, over time, change its body, and that these changes could be passed on to the next generation. To contrast Lamarck’s ideas with those of Darwin and Wallace, let’s consider the evolution of the giraffe.
Lamarck explained that the long necks of giraffes resulted from the animals stretching to reach the leaves of tall trees. The stretching caused the necks to grow longer than the necks of giraffes that didn’t stretch as much. The giraffes with the most stretched necks had babies with the longest necks. Lamarck’s hypothesis quickly fell out of favor because it failed to be supported with scientific observations.
How would you explain the long necks of giraffes using the theory of natural selection?
Simulate Natural Selection with Beetles
To get a feel for the process of natural selection, experiment with a population of M&M candy or paper dot “beetles” to test how well each color is adapted to survive on a field of wrapping paper or fabric.
You will need a large piece of colorful patterned wrapping paper, and a large bag of M&M candy to represent beetles. Instead of candy, you may create a population of “dot beetles” by punching holes out of five different colors of paper.
- First, cover a table with a large piece of colorful patterned wrapping paper. This will simulate the environment.
- Next count out equal numbers of “beetles” of each color and scatter them across the paper environment. They will represent a population of “beetles” that show variation in the trait of color.
- Pretend you are a bird predator and collect half the “beetles” by picking them up and setting them aside. Don’t eat the “beetles” yet!
- Turn your eyes away each time you capture a “beetle” and try not to intentionally favor one color over another. Your actions represent a bird searching for the most visible food in its habitat.
- Sort the captured “beetles” into piles by color and count them.
- Which color was the easiest to spot against the colorful background?
- Which color was best camouflaged?
- If the surviving camouflaged “beetles” were to reproduce one offspring each, which color of beetle would be most common in the next generation?
- What do you predict will happen over several generations? How could you test your prediction?
- What might happen if you altered the environment by using another pattern for the environment or by dimming the lights?
- Scientists use simulations to test their ideas. What changes would you make in this activity to make it a better simulation of natural selection?
VII. Variation in Population
The Theory of Natural Selection explained how, as environments changed or species spread to new areas, traits best suited to the new conditions were favored. The concept of “variation in a population” was very important to this new theory. Darwin and Wallace had observed that each species of plant and animal had many visible characteristics that differed slightly from individual to individual. Evolution occurred when conditions changed so that some traits were favored over others.
Variation in Bison
Jerry McDonald, a graduate student at the University of California at Los Angeles, observed several traits in bison that varied from individual to individual. He became interested in the evolution of the bison and set out to learn more about the original variation in bison traits. He examined hundreds of fossil and modern skulls and looked at evidence from sixty museums in Canada, and Mexico, and the United States. In 1982, after four years on the trail of bison traits, McDonald published his doctoral thesis showing how new species of bison evolved in North America.
Jerry McDonald hypothesizes that the following characteristics favor grazing behavior and speed on the open plains: The skull of the modern bison has rotated downward so that it is easier for the animal to graze with its head down. The thicker wool on the front of the body makes the animals look larger and pads it against injury. The plains bison tended to travel in large herds over great distances; their bodies were better adapted to running from predators.
The ancestors of the modern bison were cattle-like animals with long massive horns, known in scientific terms as Bison latifrons.
How were these large animals protected against large predators such as saber-toothed cats and wild dogs?
Bison latifrons, the largest bison to ever live, was common
throughout North America 40,000 years ago
Following a series of advancing and retreating ice sheets, and the arrival of humans on the continent, a new species, Bison antiquus spread across the Arctic and southern woodlands into present day Mexico. It’s horns were smaller and more curved, and it had evolved a heavy woolly coat over the front half of its body. Shorter horns may be an adaptation resulting from herd behavior in which the species relies on social clusters for protection against predators.
Bison antiquus lived 11,000 years ago and was smaller, had smaller horns,
and was covered with a woolly coat on the front half of its body.
How else might shorter horns benefit the new species?
Its descendants, our modern Bison bison developed two distinct sub-species that were recognized by the early settlers and hunters. Bison bison athabasca, called the Woods Bison by the explorers, is taller and heavier, with larger and longer horns. These bison lived in Canada, fed on trees as well as grass, and traveled in smaller herds than did the plains bison.
The modern bison bison arose 4,000 years ago showing
adaptations for grassland grazing and social herd behavior.
(Illustrations adapated from North American Bison, Jerry McDonald, 1981)
The Plains Bison, Bison bison bison, is the most recently evolved and most highly adapted to favor social herd behavior. The smaller horns are rotated to the side, and the front skull is thickened so that buttinsg in males has replaced hooking and goring, which can lead to injury and death.
Ideas for Research Projects
Loss of Biodiversity Due to Non-native species
House cats in Wisconsin alone may kill 19 million songbirds each year, according to a University of Wisconsin study by Stanley Temple and John Coleman
Non-native species are a major cause of biodiversity loss around the world. When a species is brought to a new area accidentally or on purpose it may compete with native species for food, sunlight, shelter, space, or other special features of the habitat. It may also prey on native species that have not evolved defenses against the new predator.
The house cat is an example of a very hardy and adaptable predator that has established wild populations wherever it has been released. Birds that nest on the ground and small mammals are particularly at risk.
Non-native species can also carry diseases and parasites to which the native species have little or no immunity. Europeans brought smallpox, tuberculosis and other human and livestock diseases to the American continents. A succession of epidemics swept through South, Central, and North America, drastically reducing the populations of Native Americans who had little or no immunity to the “old world” diseases.
The brown tree snake, brought accidentally to Guam following World War II, is an incredible example of an exotic species that took over a new area because it had no natural predators or diseases. The snakes have exterminated native species of birds and small mammals on Guam and now threaten Hawaii and other islands. The snakes slither unseen into airplanes and are transported across the ocean to new habitats.
Find out about non-native species in your area
- Where did they come from?
- How do they change the local ecosystems?
- Do they compete with or prey on native plants or animals?
- Is anything being done to reduce their impact?
VIII. A Theory That Changed Our Perspective
Since Darwin and Wallace’s time, the idea of natural selection has frequently been distorted to justify political ideas such as Nazism, Communism, and Free Enterprise. However, research into the evolution of species has revealed that the notion of “survival of the fittest by tooth and claw” is incomplete at best. Animal behavior studies have revealed many examples of social evolution and mechanisms for the evolutionary benefits of cooperation. Humans are an obvious example of a species which has thrived through sharing resources and skills. Wolves, bison, gorillas, musk oxen, and honey bees, are a few examples of other species that have successfully used a strategy of promoting the welfare of the group.
The great diversity in humans has undoubtedly enabled our species to colonize many challenging environments. From far flung Pacific islands, to the Arctic, deserts, and dense forests, human populations reflect special physical and cultural adaptations to their environments. Judging from the social nature of humankind, inherited behaviors that promote learning, communication, and cooperation appear to have been favored. It appears that diversity, both physical and behavioral, has been important to humans in providing options for survival when environments change.
Unfortunately, our physical and social diversity has tended to pit culture against culture, resulting in wars, slavery, and genocide. It is only in recent times that combined ethical, political, and scientific wisdom is helping us to see our diversity as a great strength. We have much to learn from cultures that developed in challenging environments.
Wallace was an early champion of native cultures that were being destroyed by European colonization. He was also a champion of social reform and better working conditions for the poor. He spoke out frequently throughout his life about the injustices perpetuated by England’s colonial systems.
Using his growing reputation as the co-developer of the Theory of Natural Selection to provide income from lectures and articles, Wallace continued to study and travel around the world. Observing the destruction of the tropical forests, he was concerned about the future of the environment and wrote, “To pollute a spring or a river, to exterminate a bird or beast should be treated as moral offenses and as social crimes.”
Public concern for protecting Earth’s biodiversity has continued to grow with our understanding of the processes that have shaped the planet and all life forms. A significant social effect of the Theory of Natural Selection is that humans are now seen as a part of the process of evolution, compelling us to view our relationship with nature in a more humble way. In the next chapter we will investigate the link between natural selection and inheritance that has implications for our survival in a changing world.
Visit the Shape of Life website – http://www.shapeoflife.org – to find free videos and animations about the many varied lifeforms on Earth.