Evolutionary Biology

Charles Darwin II: Natural selection

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Did you know that Darwin's experience with his ten children fueled his thinking about evolution? He theorized that some human behaviors, such as a young child's selfishness, were based upon instincts that were adaptations. These natural differences that always exist among individuals are at the heart of the principle of natural selection as the engine of evolutionary change.

How Charles Darwin came to understand evolution is a fascinating and important story. In our Charles Darwin I module, we focused on how he arrived at an alternative to the idea that each species was uniquely created and unchangeable. Here we look more closely at how Darwin came to propose the mechanism of evolutionary change, which he called "natural selection." Natural selection is the force that promotes changes in a species over generations. It is also the force that produces new species from the changes that accumulate in a population over long periods of time.

Darwin learned the importance of natural selection in bits and pieces as he developed his scientific skills and credentials. He lived during one of the most interesting times, the heyday of Great Britain's Victorian era, from 1809 to 1882, when the sciences, and openness to questioning the status quo, were growing cultural forces. He had a long, productive, brilliant career, and was almost famous even before returning from his five-year voyage around the world on the H. M. S. Beagle. Luckily, his correspondence, diaries, and personal workbooks, as well as the writings of his relatives, friends, colleagues, and rivals, document Darwin's adult life extensively. They tell us that every facet of the man reflected his passion for the patterns of evolution, its rules and consequences. Once he fully grasped how it worked, Darwin's life became so steeped in thinking about evolution that today we might call his fascination an obsession.

Darwin builds on the ideas of others

Figure 1: The title page of Darwin's most famous book, On the Origin of Species by ^~Means of Natural Selection.

Although we properly credit Darwin with being the founding father of evolutionary theory, one of his own great gifts was being able to spot a good idea and synthesize information from many fields of knowledge. Darwin's success was due, in part, to having learned from others, just as the great physicist Isaac Newton claimed to have stood "on the shoulders of giants."

Thus, to develop the concept of evolution by natural selection, Darwin did not have to invent the idea that animals and plants were adapted to their environment because that was already recognized in the late 1700s. He did not have to buck the Biblical story of a seven-day creation because the father of modern geology, Charles Lyell, had already shown that Earth's history extended over at least millions of years, not the thousands implied by the Bible. Darwin did not even have to come up with the idea of natural selection by himself – it was inspired by someone else! Another Englishman, Thomas Robert Malthus, who was a clergyman and an economist, wrote Essay on the Principle of Population in 1798. Malthus argued (from an economic standpoint) that human population growth, if it were not reigned in by disease, starvation, war, and other factors, would naturally expand beyond our capacity to produce the food we need to sustain it. In other words, societies of people also are locked in a "struggle for existence." In his autobiography, Charles Darwin acknowledges this thought as the beginnings of natural selection:

In October 1838, that is, fifteen months after I had begun my systematic inquiry, I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes on from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The results of this would be the formation of a new species. Here, then I had at last got a theory by which to work.

Charles Darwin, 1876

Comprehension Checkpoint
Darwin is credited as being the first person to recognize that plants and animals adapt to their environment.

Darwin learns from research and life experience

Figure 2: Down House - Charles Darwin's home and research laboratory.

After his famous five-year voyage around the world on the Beagle, most of Darwin's life was spent at his home on the outskirts of London, which he used as a base of scientific operations. His efforts involved much more than writing about big ideas like natural selection. He worked hard to build his knowledge of all manners of animals and plants from the ground up, learning lessons from many diverse research projects that were always underway in the Darwin household. Many of them might seem small and trivial, but they left him with enormous insight and they added up to a vast body of experience that earned Darwin a great reputation among the public, as well as among scientists from many different fields. Darwin pioneered studies of barnacles, coral reefs, hybridization between species, orchid fertilization, human origins, animal behavior, and other topics that are now basic to oceanography, botany, genetics, ecology, geology, and psychology.

Figure 3: Annie Darwin (March 2, 1841 to April 23, 1851), the second child and eldest daughter of Charles and Emma Darwin.

With his interest in the behaviors of organisms, even Darwin's family life provided lessons about evolution. While he was a doting father to all of his ten children, he also studied them carefully for clues about how nature gave way to nurture. From watching them he theorized that some human behaviors, such as a young child's selfishness, were based upon instincts that were adaptations, while other behaviors were learned, shaped by culture. The death of one of his daughters, Annie, at the age of ten was also a painful reminder to Darwin that all species are captives of their environment and undergo a "struggle for existence" during each generation. Disease was an environmental hazard to all individuals, a potential obstacle to their success. Some individuals were better able to cope with disease than others, just as some are better able to escape predation. Some won and some lost; some grew up and others did not; some lived to have many children while others had few or none. These natural differences that always exist among individuals are at the heart of the principle of natural selection as the engine of evolutionary change.

Comprehension Checkpoint
In Darwin's family life, he found

Fundamentals of natural selection

The idea of natural selection rests on several key points:

  • More individuals are born to a species in every generation than actually live to reproduce.
  • All individuals differ in structure and behavior, and many of these variations are inherited.
  • Some individuals have a greater ability to survive and reproduce than do others because their inherited traits are better adapted to the conditions of the environment than the other traits present in different individuals of the same population.
  • Because the rate at which offspring are produced in every species is greater than the rate at which the environment can provide food, shelter, and other needs, individuals who carry the advantageous traits will come to outnumber those without them, causing a shift in the common characteristics of the species over time.

That shift is a change, an evolutionary adjustment that takes place across generational time. The process behind it is natural selection. Darwin chose the term because the process works much like "artificial selection," the methods people have long used to produce and maintain the breeds of animals and plants that we live with. Both rely on differential reproduction to have an effect. That is, both promote the reproduction of certain members of a population with a desirable set of characteristics. For example, dogs are commonly bred to be protective but not overly aggressive. Eventually, those traits become established as key features of the breed or population.

Figure 4: Dogs with different traits. The collie (left) is frequently used to herd sheep and other livestock. They are bred for their thick coats, which protect them from injury and from intemperate weather while they work, and for their intelligence. The Dalmatian (middle) is similarly intelligent, and is bred for its distinctive black and white coat. The dachshund (right) is a short-legged, elongated dog, originally bred to chase rabbits and other small game living in burrows.

image ©Corel Corporation

Unlike artificial selection, natural selection is ever present, ongoing, long-term, and utterly beyond human control or prediction. After all, there is no telling what new disease might pop up to threaten a population, how severe a drought might be to limit the food supply during a bad summer, or if the predator from the next valley should decide to swim the river and hunt in a new territory just when vulnerable babies are being born. At the same time, there is no telling how well a species can resist the disease, how many nearly starved individuals are able to travel a long distance to the next food-rich plateau, or how clever some individuals might be in protecting their kids from the new carnivore that is tracking them.

Archaeologists have shown that artificial selection of animals and plants has been going on for at least 10,000 years. But Darwin knew that the Earth was far older than that – at least millions of years old – thus a lot of change can accumulate in a species through natural selection. In some years, food may be abundant and disease rate low, so the environment exerts less of a "pruning" effect on individuals. A species' total population size may then grow unchecked. However, this means that more individuals who are less fit for lean times will survive, and selective pressure, the forces that shape reproductive success, will be greater when conditions shift. So, it is difficult to tell what types of traits will be favored by natural selection in the long run.

Comprehension Checkpoint
Selective pressure weeds out traits that do not help a species to

Natural selection applied to the scientific method

The scientific method has its own ways of pruning, as lesser ideas are separated from good ones that explain the data in better ways. The idea of natural selection has survived many tests and challenges as progress in many fields leapt far beyond what was known in Darwin's day. One might have guessed, for example, that the principle of natural selection would fail when we finally learned the basics of heredity decades after Origin was published: Darwin didn't have a clue how traits were passed down across the generations. Yet the theory still stands. For every decade that passes, it only becomes stronger as genetics, molecular biology, geology, paleontology, and other disciplines continue to explain phenomena new and old without having to invent another evolutionary mechanism to replace natural selection.

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