Sunday, September 28, 2008

Natural Selection: Darwin's Dangerous Idea


Charles Darwin’s theory of evolution by natural selection is both a useful and a “dangerous” idea. This is because Darwin’s theory provides a naturalistic foundation for biology, but also undermines the idea of purpose in nature and the concept of human free will. Darwin proposed two ideas in the Origin of Species: “descent with modification” (what we now call “evolution”) and natural selection (the process by which evolution proceeds). In Darwin’s theory, natural selection is both the “engine” of evolution and the explanation for the origin of adaptations.

Darwin’s theory is based on three pre-conditions:
Variation (i.e. differences between individuals in populations),

Inheritance (from parents to offspring), and

Fecundity (i.e. the tendency for all organisms to produce more offspring than are necessary to replace themselves).

These three pre-conditions entail the following outcome:
Natural Selection: Some individuals survive and reproduce more often than others, and as a consequence their heritable characteristics become more common over time.

Central to Darwin’s theory is that it is not necessary to assume that evolution by natural selection has any purpose. Like all natural processes, it proceeds via natural laws that are combinations of chance and necessity alone.


Daniel Dennett, a prominent philosopher of science, said this about Darwin's theory of evolution by natural selection:

Darwin's work has had an enormous impact on society, perhaps more than that of any other scientist. His books have changed the world, and will continue to change it for the foreseeable future. His ideas have revolutionized science, and not just biology. By the end of the 20th century, it had become clear to intellectuals in both the sciences and the humanities that the idea of evolution by natural selection could be applied almost without limit to understanding not only life on Earth, but also much of the human condition itself.

And therein lies the “danger” to which Dennett alludes, because Darwin’s theory of evolution undermines not only the creation stories central to the world’s dominant religious, it also undermines the very idea of purpose in nature and the concept of human free will. Even atheists have trouble with Darwin’s ideas, especially his idea of natural selection. In this lecture, I intend to show how Darwin’s ideas apply to an understanding of human behavior, and why such ideas might be “dangerous,” and at the same time indispensable to understanding where we have come from and why we do what we do.

Darwin’s most famous and most important book was entitled: On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. At first glance, this seems a typically long-winded Victorian book title, but on closer examination it neatly separates Darwin’s “one long argument” into its two separate but interrelated parts:
An argument for the “transmutation of species” – what Darwin called “descent with modification” and we now refer to as “evolution,”


An argument for how evolution happens – natural selection – and how it produces the seemingly purposeful adaptations we see in living organisms.

At the time of its publication the most immediately controversial idea that Darwin presented in the Origin of Species was the idea that species could change over time. In particular, Darwin implied (but did not explicitly state in the Origin) that humans had evolved from “lower forms of life” (ape-like primates, to be exact).

Although in the Origin Darwin merely implied that "Light will be thrown on the origin of man and his history,” both his critics and his supporters immediately began arguing about the evolutionary origins of humans, an argument that Darwin finally addressed twelve years later in The Descent of Man and Selection in Relation to Sex. And not just humans: According to Darwin’s theory, the diversity of all organisms, living and extinct, could be explained as the result of descent with modification, which Darwin suggested could also be used to revise the taxonomy of life on Earth.

Almost as controversial at the time of the publication of the Origin, and growing ever more so since then, was Darwin’s proposed mechanism for such evolution – natural selection – according to which,
there was no reference to supernatural forces whatsoever, especially in the first edition,


blind and purposeless natural forces – variation, heredity, fecundity, and unequal survival and reproduction – were all that were necessary to explain the extraordinary diversity and adaptive perfection of living systems.


What makes his accomplishment all the more surprising is that Darwin was not much of a scholar. Indeed, as a young man, Darwin showed almost no interest in academics. Gertrude Himmelfarb, a historian of science who wrote a book about Darwin and the Origin of Species asked:

“Why was it given to Darwin, less ambitious, less imaginative, and less learned than many of his colleagues, to discover the theory sought after by others so assiduously?"

According to his autobiography, Darwin studied latin, greek, and mathematics in primary school, but without distinguishing himself as a good student. His greatest interest as a boy was in collecting things – stamps, rocks, birds’ eggs, etc. – and in taking long walks in the country.

His primary interests in college were hunting and collecting beetles. His father, a prosperous physician, wanted him to become a doctor (his older brother, Erasmus, was already a doctor), and so sent him to Edinburg University to study medicine with his brother. However, Charles couldn't stand to witness surgery being performed on children without anesthetic, and so he quit medical school. His father then recommended that he become a country parson, so Darwin entered Cambridge University, where he earned a degree in theology. He was, at best, an indifferent student, earning what amounted to a "gentleman's C".

In later years he said that he had forgotten virtually everything he had learned while at Cambridge, with the exception of two books by William Paley:
A View of the Evidences of Christianity, in which Paley tried to dismantle David Hume's attack on miracles


Natural Theology, in which Paley presented the argument for design in nature.

So, how could Darwin have written the most important book in biology, if not all of science? There are several aspects of Darwin's character that could explain this apparent anomaly:
Darwin was an extraordinarily avid collector, especially of natural objects. For example, he once tried to capture three beetles (for his collection) by popping one in his mouth and one in each hand. Unfortunately, the one in his mouth exuded an acrid fluid, causing him to drop and lose all three beetles.

Darwin was very deeply interested in geology, as a result of a course he had taken in Cambridge. It was not uncommon for students in training as parsons to take courses in geology and natural history, as many of them were also amateur naturalists. Darwin showed an unusual interest in geology, which impressed his teacher, Professor Sedgwick, to take him on many field trips throughout England to study the various landforms and rock formations.

Darwin was also very interested in botany and "natural history" (what biology was called in those days). His botany teacher, Professor Henslow, thought so much of his talents that he recommended that Darwin be appointed as ship's naturalist for the voyage of HMS Beagle (a post Henslow himself had been offered, but turned down due to family responsibilities).

Darwin jumped at the chance to serve as ship's naturalist aboard the Beagle. However, the Beagle's captain, Robert Fitzroy, felt that Darwin was insufficiently qualified for the post, and hired someone else instead. However, Captain Fitzroy did consider Darwin for the position of "gentleman's companion" (someone with whom he could dine and talk who would be of his own social class). At first Fitzroy was reluctant, based on the shape of Darwin's head - Fitzroy was a believer in phrenology, the divining of a person's character and intellect by studying the shape and bumps on a person's head. However, Fitzroy eventually relented, and Darwin was taken on as Fitzroy's companion and amateur naturalist.

Darwin's five-year voyage aboard the Beagle transformed him from an indifferent student to a passionate and highly skilled naturalist. He had a natural talent for collecting specimens and recording his observations. He wrote volumes of notes and sent thousands of specimens back to museums in England. His noted eventually became the basis for his first book, A Journal of the Voyages of HMS Beagle, which he wrote with the encouragement of Captain Fitzroy upon his return to England, and which established his reputation as a naturalist among his colleagues.

However, Darwin was still essentially a creationist when he returned to England after the voyage of the Beagle. Upon his return, he started several notebooks in which he pondered the things he had observed while on the voyage. Within two years, he had become convinced that “descent with modification” had occurred, but he was at a loss as to how it happened. The crucial turning point in Darwin's thinking came on the evening of 28 September 1838, when he read Malthus' Essay on Population. This essay gave him the key to his theory: it suggested a mechanism by which "descent with modification" could occur. It was this mechanism that Darwin eventually called "natural selection" and which he made the basis of his theory of evolution.

Darwin's Conditions for Natural Selection

Understood correctly natural selection is not itself a mechanism. Rather, it is the outcome of the operation of three mechanisms:
Variety: There are always variations between the characteristics of the members of any population of living organisms.
These variations need not be extreme, as illustrated by the relatively large changes that animal and plant breeders have accomplished, using relatively slight differences in physical appearance and behavior among domestic animals and plants.

Heredity: The different variations noted above must be heritable from parents to offspring. Darwin couldn’t propose a mechanism for such inheritance, as none was known at the time. Instead, he simply appealed to the common sense and experience of his readers, counting on them to grant that variations (however acquired) are generally heritable from parents to offspring.

Fecundity: Living organisms have a tendency to produce more offspring than can possibly survive.

Among those individuals that survive, those that also reproduce pass on to their offspring whatever characteristics made it possible for them to survive and reproduce. This was the missing piece in his theory that Darwin got from his reading of Malthus’ essay on population.

Given these three pre-conditions, the following outcome is virtually inevitable:
Non-random, unequal survival and reproduction.
Survival and reproduction are almost never random. Instead, individuals survive and successfully reproduce at least partly as a result of their characteristics. It is these characteristics that provide the basis for evolutionary adaptations.

Darwin had no empirical (i.e. "observational") evidence for natural selection. Instead, he used imaginary examples and analogies to animal and plant breeding. In particular, he began to study the processes of animal and plant breeding very intensively, culminating in an essay that he wrote on the subject in 1842, which he later revised in 1844, and finally published as part of the Origin of Species in 1859.

Darwin began the Origin of Species by arguing that the various breeds of domesticated pigeons are analogous to the products of natural selection. He pointed out that all domesticated pigeon breeds are descendants of the wild rock dove (Columba livia). He went on to note that, although "[t]he diversity of the breeds is something astonishing…", they are not separate species. However, they are at least as different from each other as natural species are in the wild.

From his interviews with pigeon breeders, Darwin concluded that pigeon breeders of his time believed that all of the various breeds of pigeons were derived from separate kinds of pigeons that existed in the wild. That is, that no evolution or selection had taken place to produce such breeds. Darwin concluded otherwise: that all 700+ breeds of pigeons had been derived from the wild rock dove by means of artificial selection. He asserted that pigeon breeders were denying the evidence right in front of them: that their choices of breeding pairs were shaping the breeds that exist.
Darwin asserted that most of the artificial selection done by animal and plant breeders was probably done unconsciously, by breeders choosing desirable traits among their domesticated animals and plants.

Natural Selection and the Problem of Purpose in Nature

Darwin had two aims in writing the Origin of Species. The first was to convince his readers of the reality of "descent with modification" from common ancestors. He was largely successful in this aim.

To accomplish the first aim, Darwin presented an overwhelming mass of evidence from animal and plant breeding, animal behavior, paleontology, biogeography, comparative morphology, classification and taxonomy, and embryology, much of it newly acquired by naturalists from England and other European countries. Many of his readers were avid naturalists themselves, and followed Darwin’s arguments and evidence to their obvious conclusion: that species had indeed changed over time.

In terms of the types of logical arguments we discussed in the previous lecture, Darwin’s argument for descent with modification was essentially based on inductive reasoning. He presented multiple, independent, yet similar cases of observable phenomena, all pointing to the same conclusion: that species had gradually descended from previously existing species over long periods of time. Although the evidence could not absolutely prove that evolution had occurred, it was sufficient to convince most of the scientists of his time.

Darwin’s second aim – which was much more important to Darwin himself – was to convince his readers that natural selection was the cause of the "beautiful adaptations" that largely define species. He was mostly unsuccessful in this aim.

To accomplish his second aim, Darwin was forced to use an argument from analogy, because he could not point to any real-world examples of natural selection in action. In the first chapter of the Origin of Species, he essentially argued that "breeds" under domestication are analogous to "species" in the wild insofar as both are shaped by selection. In other words:
Natural selection is analogous to artificial selection.

As I discussed in the previous chapter, arguments by analogy – while they are often used – are logically very weak. They depend fundamentally on the validity of the analogy, and are susceptible to subversion if another equally compelling analogy is presented. Historically, this was precisely what happened to Darwin’s proposal of natural selection as the “engine” of descent with modification.

To understand why, recall our discussion of the concept of inference from the previous chapter, in which we considered an example of what looked like a house fire. Recall that you didn't actually witness the house fire. All you observed were its effects. What you are doing when you make a guess like this is inferring that an event that you have not actually observed has, in fact, taken place.

This is precisely what the theory of evolution does, and when you apply the theory to the natural world, you are using essentially the same reasoning that you would use to decide whether a house fire has happened along the road to work. Lacking direct evidence for natural selection, Darwin argued that its operation could be inferred from observable phenomena. He argued that natural selection is analogous to the artificial selection by which animal and plant breeders had developed the various characteristics of domesticated animals and plants. He also argued that most of this artificial selection had been conducted unintentionally (i.e unconsciously) by animal and plant breeders, thereby suggesting that natural selection could also operate without intentions or purposes.

In the Origin, Darwin tried to convince his readers of two propositions: that descent with modification had occurred, and that natural selection was the driving force behind it. Darwin’s argument for descent with modification was based on inductive reasoning. He presented multiple, independent, yet similar cases of observable phenomena, all pointing to the same conclusion: that species had gradually descended from previously existing species over long periods of time. most scientific arguments are grounded in inductive reasoning, he was largely successful in convincing other scientists that descent with modification had occurred.

Darwin’s argument for natural selection was largely based on analogy, primarily with artificial selection. Since natural selection was a new idea, there was virtually no actual evidence for or against it when the Origin was published. Instead, Darwin presented an essentially logical argument for natural selection, based on a few largely imaginary examples, and then encouraged his readers to accept the idea on that basis. In doing so, Darwin asked his readers to infer that natural selection was the most likely cause of the evolutionary changes he cited in his argument for descent with modification.

The great strength of Darwin’s argument for descent with modification was the huge amount of evidence from natural history that he marshaled to support it. Arguments by induction, while not absolutely conclusive, are extremely powerful, especially in science. The greatest weakness of Darwin’s argument for natural selection was the lack of empirical evidence he could site to support it. Arguments by analogy, while extremely common, have almost no logical force or validity, especially in science.

Darwin’s argument for evolution by natural selection was a naturalistic argument. Like virtually all scientific arguments and explanations, it was based on the assumption that the only valid causes for observed effects were those that were entirely limited to natural objects and processes. Central to the naturalistic stance in science is the assumption that natural processes are not intentional; that is, they have no purpose and are not assumed to be the result of intelligent design. is one reason why scientists adopted Darwin’s views so readily:

In the Origin of Species, Darwin presented the first fully naturalistic explanation for the evolution of life on Earth.

However, this is also why the theory of evolution by natural selection has met with such intense opposition by non-scientists, and especially religious believers; it ignores (and therefore implicitly negates) any possibility for intentional design or purpose in nature.

This conflict between the naturalistic viewpoint shared by virtually all scientists and the “intentional stance” so common among non-scientists (and especially religious believers) is nowhere more intense than in the field of evolutionary psychology. As we will see, evolutionary psychologists assume that much of human behavior is motivated by drives (and even physiological mechanisms) that are largely unconscious, and may even oppose what we perceive to be our conscious desires and intentions.

As just one example (which will be discussed in more detail in a later lecture), it has been widely observed that when couples divorce, the husband often remarries a woman much younger than himself, with whom he then has several children. Furthermore, this often happens despite the husband’s lack of obvious intention to start a new family with his new wife. Social scientists often explain this behavior as stemming from economic or social causes, especially the economic disparity between men and women, which makes it much more likely for men to be able to support a new family than their ex-wives.

However, this explanation completely ignores the underlying motivations for this pattern of behavior, motivations that become clear only when one views this behavior from an evolutionary perspective. That is, the tendency for men to remarry women younger than themselves is most likely an evolutionary adaptation that is the result of the increased reproductive success that is the result of this behavior.

The idea that behaviors, including human behaviors, are evolutionary adaptations that are the result of natural selection is the basis for the science of evolutionary psychology. In the next lecture, therefore, we will take a closer look at what adaptations are and how they can be identified and distinguished from characteristics that are not the result of unequal, non-random survival and reproduction.

Essential Reading:


Supplemental Reading:

Barlow, N. (ed.) (1958) The autobiography of Charles Darwin, 1809-1882, with original omissions restored. Collins. Available online here.

Darwin, C. (1845) Journal of researches into the natural history and geology of the countries visited during the voyage of H.M.S. Beagle round the world, under the Command of Capt. Fitz Roy, R.N. 2d ed. John Murray. Available online here.

Darwin, C. (1871) The descent of man and selection in relation to sex, 1st ed. John Murray. Available online here.

Himmelfarb, G. (1959). Darwin and the darwinian revolution. Doubleday.

Malthus, R. T. (1798) An essay on the principle of population. J. Johnson. Available online here.

Questions to Consider:

1. Darwin is often credited with founding the science of biology with his publication of the Origin of Species in 1859. Why is this the case, and do you agree?

2. Darwin’s theory of “descent with modification” was accepted by nearly all scientists within a decade of the publication of the Origin of Species in 1859. However, his proposed mechanism of natural selection was not nearly so widely accepted. Why not, and has this situation changed significantly today?





ragzy said...

I thoroughly enjoyed that post. My favorite part was Daniel Dennett's comment about Darwin's idea being the greatest in human history. I agree with him to the fullest! Never before has an idea been as bold as Darwin's because the implications and the magnitude of this "dangerous idea" was unlike anything in recorded history. It forced human beings to look in the mirror and say, "maybe I'm not special after all."

Joshua kocsar said...
This comment has been removed by the author.
Joshua kocsar said...

parden me for being a skeptic but how can a slowly evolving animal fuction if it has a half evolved organ? can a mose trap work if its missing half the parts? Darwin's ideas are flimsy at best and it's such problems like these that realy make me shake my head.

ragzy said...

Ragzy says: It's apparent that you have a very limited understanding of evolutionary biology. There is no such thing as "half an eye" or "half a brain." I'm not trying to come off offensive, but you really need to study the intricacies of historical biology and physiology to understand the genetic workings of an organism.

Coming from an anti-evolutionary background myself, it took me many years to come to grips with the truth of evolution, but it's been truly the most amazing epiphany I have ever experienced, and when the light bulbs start going off for you too, trust me, you will be amazed as well. There is no need for a God when you see how life has evolved from simple beginnings. And to let go off this afterlife may be hard to do, but it absolutely makes a person value each and every day to the utmost.

Again, I just ask of you to dive into, with an open mind, evolutionary studies and you will find so many more answers than the Bible can provide. That is, if you are into science at all.