Losing Biodiversity—chapter 3:
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.
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.
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.
Map of the voyage of the H.M.S. Beagle
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. 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.
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.
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:
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.
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.
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.
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."
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.
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.
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 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.
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.
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 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.
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.