Darwin's Theory

From History of Evolution

Contents 1 Introduction 2 Darwin's Theory 2.1 Common Descent 2.1.1 Biogeography 2.1.2 Taxonomy 2.1.3 Vestigial Organs 2.1.4 Embryology 2.2 Natural Selection 2.2.1 Variation 2.2.2 Fecundity and the Struggle for Existence 2.2.3 Heritability of Variation 2.3 Evolution as an Algorithm 3 Difficulties of the Theory 4 Summary

Introduction

By the mid-19th century the background for Darwin's work was set in both theology and science. Paley's natural theology and its elaboration in the Bridgewater Treatises put a high value on the study of nature in order to elucidate God's plan and even the attributes of God. As David Quammen put it, "Studying nature was considered a path to piety" (p. 30). It was perfectly respectable for clergymen to study natural history because they were studying God's creation. The religious view of the day also emphasized human exceptionalism -- the view that there is a wide and uncrossable gap between humans and the other animals. That view was dealt a major blow by the Origin.

While there was still considerable sentiment for a static earth, developments in geology had brought attention to dynamics and had established that the earth was considerably older than the Biblical 6,000 years. Geologists had also established that the evidence did not support a single global (Noachian) flood as the cause of the earth's geological features, and that considerable change had occurred over earth's history. In biology the view of species fixity was still powerful, but there were intimations that biological suites of organisms had changed over time and a prominent French naturalist, Lamarck, had ventured a theory of biological change in the face of strong resistance from his peers.

The outlines of Darwin's story are well known, and we will not rehearse them here. Excellent book-length biographies are available, most notably [ Janet Browne's definitive two-volume work Voyaging and The Power of Place. Darwin's Autobiography, written for the private reading of his family and edited by his grand-daughter, is on the web. The Darwin Correspondence Project has a good brief biographical sketch covering the period up to Darwin's return to Great Britain from his voyage aboard HMS Beagle, and John van Wyhe, editor of The Complete Work of Charles Darwin Online, has a brief biography on that site.

Darwin's Theory

For our purposes here the central issue is the structure of Darwin's theory of evolution. While he first outlined his theory in an 1842 manuscript revised in 1844 (see especially pages 87ff), the first definitive published statement of his theory was in On the Origin of Species published in 1859, a year after his and Alfred Russell Wallace's papers were read before the Linnean Society (to deafening silence).

Darwin became a transmutationist in the few years following his return from the voyage of the Beagle, and indeed, there are indications of earlier thoughts along those lines. For example, in his Ornithological Notes made in 1835 while still in the Galapagos, he wrote

If there is the slightest foundation for these remarks the zoology of Archipelagoes — will be well worth examining; for such facts [would inserted] undermine the stability of Species. (Some place the writing of this in 1836 or 1837 after Darwin's return to Great Britain, but Barlow argues persuasively that Darwin wrote it in Sep-Oct 1835 while still aboard the Beagle in the Galapagos Islands.)

In On the Origin of Species Darwin offered two main hypotheses: Common Descent and Natural Selection.

Common Descent

Darwin argued that all living things are descended from one (or a few) common ancestor(s) in the distant past. Early intimations of his theory of common descent are in his Notebooks, and the logo of this wiki uses the drawing from his Transmutation of Species Notebook started in 1837, soon after he returned from the voyage of the Beagle (from p. 255):

That figure clearly shows the branching structure generated by common descent. I am entranced by the phrase "I think" at the top of the figure.

Darwin offered several lines of evidence to support common descent.

Biogeography

Darwin starts his book off with biogeography:

WHEN on board H.M.S. 'Beagle,' as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to me to throw some light on the origin of species—that mystery of mysteries, as it has been called by one of our greatest philosophers. (p. 1)

Biogeography (the distribution of forms of life across territory) and one aspect of paleontology (the geographical distribution of past forms of life relative to current forms) were the initial impetus for his studies. Darwin observed that similar species tended to occur in adjacent geographical areas. For example, one of the specimens he brought back from the voyage of the Beagle was that of a new species of rhea, a flightless bird found in southern Patagonia, that was morphologically very similar to an already known species that occupied an adjacent area. In the Galapagos Islands Darwin collected three species of mockingbirds that occupied three different islands, suggesting that they were descended from a common ancestor and had adapted to the varying conditions on the three islands. Similar patterns of geographical island-by-island variation were observed in Darwin's famous finches, iguanas, and tortoises.

Further, examination of the fossils that Darwin shipped back to Great Britain showed that they were morphologically similar to living species. Fossil ground sloths and armadillos from South America showed anatomical affinities with living species in the same areas.

Finally, suites of organisms occupying island groups resembled those of the nearest large land masses. The organisms occupying the Galapagos resemble those of western South America, and those of the West Indian Islands in the Caribbean resemble those on the nearby American continent:

The most striking and important fact for us in regard to the inhabitants of islands, is their affinity to those of the nearest mainland, without being actually the same species. Numerous instances could be given of this fact. (Origin, p. 397)

Darwin also noted that the further an island is from a major land mass, the less likely it is to have large mammals which would have difficulty migrating long distances over water. He inferred that island groups were populated by migration from the nearest large land masses, not by special creation.

Darwin took all this to mean that

If the difficulties be not insuperable in admitting that in the long course of time the individuals of the same species, and likewise of allied species, have proceeded from some one source; then I think all the grand leading facts of geographical distribution are explicable on the theory of migration (generally of the more dominant forms of life), together with subsequent modification and the multiplication of new forms. We can thus understand the high importance of barriers, whether of land or water, which separate our several zoological and botanical provinces. We can thus understand the localisation of sub-genera, genera, and families; and how it is that under different latitudes, for instance in South America, the inhabitants of the plains and mountains, of the forests, marshes, and deserts, are in so mysterious a manner linked together by affinity, and are likewise linked to the extinct beings which formerly inhabited the same continent. (Origin, p. 408)

and

On my theory these several relations throughout time and space are intelligible; for whether we look to the forms of life which have changed during successive ages within the same quarter of the world, or to those which have changed after having migrated into distant quarters, in both cases the forms within each class have been connected by the same bond of ordinary generation; and the more nearly any two forms are related in blood, the nearer they will generally stand to each other in time and space; in both cases the laws of variation have been the same, and modifications have been accumulated by the same power of natural selection. ('Origin, p. 410)

Taxonomy

In Chapter XII of Origin Darwin argued that the fact that organisms can be classified in a nested hierarchy (whjich can be represented graphically as a branching tree) implies common descent. The tree diagram reproduced above tells me that Darwin had common descent nailed as early as 1837. He wrote

Naturalists try to arrange the species, genera, and families in each class, on what is called the Natural System. But what is meant by this system? Some authors look at it merely as a scheme for arranging together those living objects which are most alike, and for separating those which are most unlike; or as an artificial means for enunciating, as briefly as possible, general propositions,—that is, by one sentence to give the characters common, for instance, to all mammals, by another those common to all carnivora, by another those common to the dog-genus, and then by adding a single sentence, a full description is given of each kind of dog. The ingenuity and utility of this system are indisputable. But many naturalists think that something more is meant by the Natural System; they believe that it reveals the plan of the Creator; but unless it be specified whether order in time or space, or what else is meant by the plan of the Creator, it seems to me that nothing is thus added to our knowledge. Such expressions as that famous one of Linnæus, and which we often meet with in a more or less concealed form, that the characters do not make the genus, but that the genus gives the characters, seem to imply that something more is included in our classification, than mere resemblance. I believe that something more is included; and that propinquity of descent,—the only known cause of the similarity of organic beings,—is the bond, hidden as it is by various degrees of modification, which is partially revealed to us by our classifications. (Origin pp. 413-414)

Darwin is here arguing that descent from predecessors is the only variable known to produce resemblances like those seen in time (from fossils) and space (from biogeography), and that the very fact that early taxonomists, who had no evolutionary agenda, were able to classify organisms into a hierarchical scheme of nested classes supports the hypothesis of common descent. Moreover, descent with modification explains the rules of thumb that taxonomists had empirically found to be useful:

All the foregoing rules and aids and difficulties in classification are explained, if I do not greatly deceive myself, on the view that the natural system is founded on descent with modification; that the characters which naturalists consider as showing true affinity between any two or more species, are those which have been inherited from a common parent, and, in so far, all true classification is genealogical; that community of descent is the hidden bond which naturalists have been unconsciously seeking, and not some unknown plan of creation, or the enunciation of general propositions, and the mere putting together and separating objects more or less alike.

Here Darwin is describing what nowadays is called cladistics, the classification of organisms in terms of their common lines of descent, their shared ancestry. Darwin used the fact that organisms can be classified in a nested hierarchy as support for common descent, while nowadays common descent is so well established that it is a basis for classifying organisms in cladistics. Creationists sometimes argue that doing so illegitimately employs an "evolutionist presupposition," but that's merely special pleading. Using the now well-established fact of common descent as a basis for classifying organisms is no less 'presuppositional' than is using the relative masses of objects to classify them according to their gravitational effects.

Vestigial Organs

Darwin described a number of examples of vestigial organs, organs that no longer appear to have function, like rudimentary wings in flightless birds and the rudiments of pelvic and hind limb bones in snakes. He argues that these are the remains of formerly functional organs that, through disuse under changed conditions of life, have atrophied.

It's of interest that in this section (Chapter XIII) Darwin introduces what is now a powerful means of evolution, co-option and change of function. He wrote

An organ serving for two purposes, may become rudimentary or utterly aborted for one, even the more important purpose; and remain perfectly efficient for the other.

...

... an organ may become rudimentary for its proper purpose, and be used for a distinct object: in certain fish the swim-bladder seems to be rudimentary for its proper function of giving buoyancy, but has become converted into a nascent breathing organ or lung. Other similar instances could be given. ((Origin, p. 451-452)

Particularly at the genetic level this is a powerful mechanism of evolution. Gene duplication followed by co-option and divergent evolution of one of the duplicated copies to perform a new function is a ubiquitous phenomenon.

Embryology

Darwin argued that

Descent being on my view the hidden bond of connexion which naturalists have been seeking under the term of the natural system [of taxonomy]. On this view we can understand how it is that, in the eyes of most naturalists, the structure of the embryo is even more important for classification than that of the adult. For the embryo is the animal in its less modified state; and in so far it reveals the structure of its progenitor. In two groups of animal, however much they may at present differ from each other in structure and habits, if they pass through the same or similar embryonic stages, we may feel assured that they have both descended from the same or nearly similar parents, and are therefore in that degree closely related. Thus, community in embryonic structure reveals community of descent. (Origin p. 449)

Darwin's argument is not 'ontology recapitulates phylogeny' -- that the embryonic forms of organisms go through the same stages as the evolutionary history of the organisms -- but rather that similarities of embryonic forms across species reflect common ancestry.

Natural Selection

Darwin's "long argument" depends on several fundamental premises.

Variation

Variability among organisms in a population is the raw material of evolution. The first premise for Darwin's argument is that the organisms in a population -- a species -- vary, one from another. Darwin began the Origin with one of the most revolutionary aspects of his new theory, two chapters on variation, Variation Under Domestication and Variation Under Nature. In doing so Darwin wrenched attention away from archetypes and Platonic ideal forms and species uniformity, and focused it on distributions of varying organisms, on populations. Before Darwin it made sense to speak of the horse, the dog, the pigeon. After Darwin one must speak of populations of horses, dogs, pigeons, and the distribution of features across those populations. Evolution is a process that takes place in populations through time, not in individual organisms. It is change through time (generations) of the distribution of the variability of traits in a population.

It is hard to overstate the importance of this shift on Darwin's part. He is here abandoning the notion that a species is a static entity with fixed attributes -- an Aristotelian set -- and is taking the position that a species is a fuzzy cloud in morphospace, an entity with fuzzy boundaries. He offers in evidence the variability in morphology within species, even to details of the wiring of the nervous system of insects:

I am convinced that the most experienced naturalist would be surprised at the number of the cases of variability, even in important parts of structure, which he could collect on good authority, as I have collected, during a course of years. It should be remembered that systematists are far from pleased at finding variability in important characters, and that there are not many men who will laboriously examine internal and important organs, and compare them in many specimens of the same species. I should never have expected that the branching of the main nerves close to the great central ganglion of an insect would have been variable in the same species; I should have expected that changes of this nature could have been effected only by slow degrees: yet quite recently Mr. Lubbock has shown a degree of variability in these main nerves in Coccus, which may almost be compared to the irregular branching of the stem of a tree. (Origin pp. 45-46)

Recall that Darwin had spent eight years dissecting and classifying barnacles prior to publishing Origin. He was intimately -- even painfully -- familiar with the variability that characterizes species and with the difficulty of ascertaining where one species leaves off and a new variety begins, or where one species ends and another begins.

As noted, variability is the raw material on which natural selection works. It provides a range of differing alternatives, and the differential survival and reproductive success of those alternatives produces adaptive evolution. Some variants are more successful in a given environment than others, and over generations the more successful variants come to dominate the population.

Fecundity and the Struggle for Existence

Darwin observed that in general, more offspring are born than can be supported by enviromental resources. He got this thought from reading Thomas Malthus' Essay on the Principle of Population, which argued that humans, like plants and other animals, would over-reproduce if left unchecked. From this Darwin inferred that there is a struggle for existence (Struggle for Existence:

I should premise that I use the term Struggle for Existence in a large and metaphorical sense, including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny. Two canine animals in a time of dearth, may be truly said to struggle with each other which shall get food and live. But a plant on the edge of a desert is said to struggle for life against the drought, though more properly it should be said to be dependent on the moisture. A plant which annually produces a thousand seeds, of which on an average only one comes to maturity, may be more truly said to struggle with the plants of the same and other kinds which already clothe the ground. (Origin pp. 62-63)

and

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. (page 64)

In that struggle, even a small advantage can make the difference between winning (surviving and reproducing) and losing (not surviving and/or not reproducing).

Darwins' remark that there must be a struggle for existence "one individual with another of the same species" brings to mind the story about two men, Harvey and Bob, who encounter a grizzly bear while hiking in the wilderness. Harvey immediately leaned down to tighten his shoe laces. Bob remarked, "That won't do any good -- you can't outrun a grizzly." Harvey replied, "I don't have to outrun the grizzly. I just have to outrun you." The toughest competition is among conspecifics because they use the most similar environmental resources.

In this context it's important to remember that "fitness" is not a fixed property of an organism; it's a property of an organism in a specified environment. The same variation may be relatively fit in one environment and relatively unfit in another. Moreover, it's always relative fitness that counts, i.e., fitness is measured relative to the competition. In the Bob and Harvey story, how fit Harvey is depends on how fast Bob is.

Heritability of Variation

Darwin had no satisfactory theory of inheritance. While he knew that traits were passed on from parents to offspring, he had no idea how that occurred. Well after publication of Origin he offered an unsuccessful proposal, pangenesis, in Animals and Plants Under Domestication. At other times he seemed to accept Lamarck's proposal that characteristics acquired during the lifetime of an organism could be inherited by offspring. Though Mendel published his experiments on inheritance in peas in 1866, Darwin was unaware of it and never did find a satisfactory theory of inheritance. Nevertheless, he knew that traits were somehow inherited, and that was sufficient for his purposes.

Evolution as an Algorithm

Given those premises -- a fecund population with varying characteristics and heritable variation in an environment with at least one limiting resource, then evolution, defined as a change in the distribution of traits across the population, is guaranteed to occur. It is an utterly automatic process, and requires no direction or plan. Fitness, defined as relative reproductive success in a given environment, varies among the members of a population, and over generations the reproductively more successful variants come to dominate the population. Anyone who understands compound interest knows how small differences in interest rates add up to large differences in assets over time. Natural selection is similar: small differences in fitness add up to large differences in numbers through generations. (In the letter linked in the next paragraph Darwin discusses why he emphasizes small changes over long times in evolution rather than "jumps" -- saltations.)

Natural selection is a conservative sorting process: it preserves favorable variations while discarding unfavorable variations. In fact, in a letter to an Irish naturalist Darwin said he should have used the phrase "natural preservation" rather than "natural selection" because the latter is prone to misunderstandings. Darwin used "natural selection" on an analogy with artificial selection by humans in plant and animal breeding, and that was (and still is) misconstrued to suggest some sort of active or intelligent selecting agent in nature. In fact, the 'selecting' agency is just the differential reproduction of more vs. less advantaged variants in some environment. It is purely automatic.

Difficulties of the Theory

Darwin was well aware of the potential problems of his theory, and scrupulously described them in Origin. See here for a summary.

Summary

At a stroke Darwin made two grand claims that contradicted the conventional wisdom of his day: All living things are descended from one (or a few) common ancestor(s), and the diversification of the original ancestral lineage into all the varieties of living things was a purely mechanical affair, generated by natural selection operating on the variability within and between populations.

Written as a direct counter to Paley's Natural Philosophy, Darwin's theory could not fail to produce a substantial reaction, and it did so.

(REFERENCES AND RESOURCES TO BE ADDED)

Quammen, D. 2006. The Reluctant Mr. Darwin. New York: Norton.