Darwin's Ideas
One of the biggest problems facing natural philosophers of Darwin’s day was the conundrum of the multiplicity of species. Why were there so many different species of organisms? As paleontology emerged, new fossils showed that the species in existence in the present were not the same as those that had lived in the past. How had these new species arisen? What had happened to the earlier species? The Biblical account of a single act and location of creation could not answer these questions. Some people suggested that there had been multiple acts of creation at different times and places, but this was almost as heretical as denying the involvement of a deity in the first place. Other people suggested that perhaps the new species had arisen out of the old ones through a process of transmutation.
Although Darwin is often credited (or reviled by opponents) as the creator of the theory of evolution, this is by no means correct. The concept of evolution had been around for decades; even the idea of human descent from ape-like ancestors predated Darwin. The mutability of species—the idea that species could change over time or in different locations—was an idea that had been seriously considered and investigated since the 18th century. Darwin’s own grandfather, Erasmus Darwin, a philosopher, physician and member of the Lunar Society of Birmingham, argued in the 1770s that different species had evolved from a common ancestor.
In 1801, Jean-Baptiste Lamark put forth the first real theory of evolution by suggesting that organisms responded to their environments and passed along traits to their offspring. In this way, with each successive generation of giraffes stretching to reach the highest leaves, the necks would gradually elongate into what we see today. Although Lamarkian evolution is ridiculed today because we know that it is impossible for children to inherit acquired traits, his observation that evolution occurs in relation to the environment was not wrong, only the mechanism he proposed. Evolution and transmutation were discussed in both scientific and public circles throughout the first half of the 19th century, although they were not part of the official curriculum at Cambridge University where Darwin studied. There is no evidence that Darwin was an evolutionist before he began his trip on the Beagle, but he was almost certainly aware of the theory.
While Darwin did not invent the theory of evolution, he was fundamentally important to its success because he discovered the mechanism by which evolution works: he answered the "how" question. Darwin, and his younger colleague Alfred Russell Wallace, independently realized that evolution works through natural selection—the survival of the fittest. It was Darwin’s genius to devise a theory that integrated two common observations. The first is variation; no two individuals of a given group are exactly alike. The second is culling. Everyone has observed predation: cats hunting mice, for example. Culling affects a local population by eliminating some individuals, thereby allowing the others more access to resources, thus giving them a better chance to survive and reproduce.
Darwin and Wallace were both influenced by Thomas Malthus's Essay on the Principle of Population, which described how population is kept in check by the weaker or less fit individuals succumbing to disease and predators, while only the strong survive. Darwin and Wallace realized that evolution might be caused by certain traits possessed by individuals, giving them an advantage at surviving the culling process. Where Darwin’s genius, and subsequent fame, surpassed his colleague, was in the meticulous evidence he provided that made the theory of evolution by natural selection not only plausible, but probable.
The theory can be broken down into three inferences or deductions based on four observable facts:
Fact #1: All populations of organisms tend to increase at a geometrical rate. This means that each generation tends to have many more offspring than the environment can support.
Fact #2: In spite of this tendency toward growth, populations of organisms tend to remain stable in number. This means that many individuals either do not survive or are not able to reproduce.
Deduction #1: There is a struggle for existence among members of a population.
Fact #3: No two individuals in a given population are exactly alike (= variation). These variations are copious and, in general, qualitatively insignificant. In a single generation, most variations are very minor, such as hair color or height.
Fact #4: Most of these variations are inheritable.
Deduction #2: Natural selection or "survival of the fittest": those individuals with favorable variations tend to survive in greater numbers.
Deduction #3: Natural selection leads, over many generations, to the production of new species.
To go back to the giraffe example, Darwin would explain the long neck by stating that sometime in the distant past a young giraffe was born with a slightly longer neck than others of his generation. Because he could reach the higher leaves, he ate better and was able to produce more offspring than his rivals. Those of his offspring who inherited his long neck were also more successful in surviving and reproducing than others of their generations. Each new giraffe born with an even longer neck was still more successful, thus leading to a gradual elongation of the necks of the entire species.
Let us now imagine that among the short-necked giraffes from the original generation there were some who survived by migrating to a new area and began grazing on low bushes. By the time their distant cousins have developed the long necks, these short-necked giraffes may have headed in a different direction, maybe stronger molars to eat the tough branches of the bushes. If they were once again to inhabit the same lands, they would look very different, have different diets, and probably not even recognize each other as potential mates (if their DNA was still similar enough to even allow them to mate). The long-necked giraffes may have developed courtship patterns that involve neck intertwining—something that the short-necked giraffes could not do. In this way, from a common ancestor, two species arose through environmental factors that favored particular variations.
Darwin had begun to think about how this "branching" from a common trunk would result in a multiplicity of species as early as 1827, when a sketch in his notebook shows a branched evolutionary tree. A tree is not the best metaphor, however, as it implies constant upward growth. Darwin's "tree" looked more like a bush grown in outer space, with branches going in all directions, none favored over any other. It is important to realize that Darwin did not believe that evolution was heading toward some advanced, progressed state. Many people wrongly believe that the theory of evolution implies some sort of grand progress from a primitive state to an advanced state. This allows us to continue to believe that humans are the most evolutionarily "advanced" species. However, Darwin recognized that evolution will cause species to develop in any empty ecological niche, whether it requires variations that appear more or less complex from our viewpoint. Thus snakes, which used to have legs, have lost them in order to fill the niches they have adapted to. (The late biologist Steven Jay Gould argued that the reason there are fewer cases of evolution toward simpler states such as single-celled organisms, is because those niches are already full.)
Darwin's first public presentation of his ideas on natural selection came in On the Origin of Species by Means of Natural Selection, or The Preservation of Favoured Races in the Struggle for Life (called On the Origin of Species) published in November 1859. As important and influential as this book was, Darwin considered it a mere abstract to his final volume(s) that he intended to publish. He left out the matter of human evolution entirely, and did not address selection for variations that seemed to have no useful purpose, such as the peacock’s tail.
More than a decade later Darwin finally dealt with these topics in The Descent of Man, and Selection in Relation to Sex published in 1871. Because the idea that humans had evolved from an ape-like ancestor had been around for a long time before Darwin published the Descent, he wanted to go far beyond simply proving the simian ancestry of humanity. Although he did give a great deal of evidence showing the similarities of humans with other animals, especially through anatomy and embryology, the focus was on showing how human mental traits could have arisen through the evolutionary process. He pointed out that all the qualities thought to be uniquely human could be found in varying degrees in other species, undermining the idea that humans must have arisen through some means separate from the rest of the animal kingdom.
In many ways, the most controversial aspect of the book at the time was not the idea of simian ancestry, but Darwin’s assertion that variations in race were entirely superficial. The bigotry of the time led many people to believe that human races were separate species who had likely been created separately and in a distinct hierarchy. Darwin dismissed this concept and showed how the minor variations in skin color and hair could easily arise through the evolutionary process. (He argued that most of the other differences were purely cultural and due to a difference in the state of civilization.)
In order to explain the process by which cosmetic characteristics such as hair color or decorative feathers might become chosen traits, Darwin introduced the concept of sexual selection. Unlike traits such as speed or claws that might allow an individual to better survive predation, sexual selection favors traits that lead to greater reproductive success. If peahens are more likely to choose a peacock with an enormous tail, those peacocks with such tails are more likely to reproduce. The peacock with the most elaborate tail, which would be a hindrance when trying to escape a predator, becomes a benefit when attracting a mate. In this case, evolution ends up finding a balance between traits that promote survival (small, inconspicuous, fast) and traits that promote reproduction (showy, elaborate). Some traits may appeal to both natural and sexual selection, since being the best hunter in the pack may attract mates as well as ensure survival. Sexual selection was a very important concept introduced by Darwin, and he spent a good portion of the Descent explaining it and giving evidence for it in nature.
Darwin worked on a number of other topics and wrote other books which continue to be influential today. However, none compare to On the Origin of Species or The Descent of Man in terms of influence, importance, and controversy. His ideas on natural selection and sexual selection were revolutionary in the same way as heliocentrism and have impacted the world no less than that shift in worldview. For a brief overview of how his ideas continue to be relevant today, go to Darwin's Impact.
