Origin, Chapter 9, part II

Darwin recognized that only a fraction of the number of living beings leave behind a fossil record. He realized, for example, that organisms with “wholly soft” bodies were not likely to be preserved, and that shells and bones will decay and disappear if they are deposited where sediment is not accumulating, and that organisms living in shallow oceans were more likely to be preserved that those living on land.

Even buried remains might not survive to become fossils if their remains are later “dissolved by the percolation of rainwater”. These factors along with the fact that only a small portion of the surface of the earth had been explored in Darwin’s day all contributed to what Darwin termed, “the poorness of our palaeontological collections” and the imperfection of the geological record.

Origin, Chapter 9, On the imperfection of the geological record

This is a geologist's favorite chapter because the data (or lack of data?!), that is, fossils and the rocks they are found in, play the starring role; they are the source of the "most obvious and gravest objection which can be urged against my theory" as Darwin wrote, because of the apparent lack of transitional lifeforms predicted by his theory of incremental, gradual change.

Darwin was well aware of this "problem" with his theory and used this chapter to head off anticipated objections.

The apparent lack of transitional forms was not the only problem Darwin addressed in this chapter. Communicating to the general public the immensity of geological time was another challenge. Darwin's scenario of slow, gradual change required vast expanses of time and he advised his readers that if they did not acknowledge "how incomprehensibly vast have been the past periods of time" they should "at once close this volume."

Always the experimentalist, Darwin suggested a line of inquiry/observation one could pursue to illustrate the phenomenon: "A man must for years examine for himself great piles of superimposed strata, and watch the sea at work grinding down old rocks and making fresh sediment, before he can hope to comprehend anything of the lapse of time, the monuments of which we see around us."

Happy Origin Anniversary!

As celebrated as Darwin's On the Origin of Species by means of Natural Selection is for its impact on science and society, it almost was not written. Darwin kept his ideas on the trasmutation of species confined to his notebooks and conversations with close friends and colleagues because he was aware of the firestorm of public opinion that would result from the clash of his ideas with prevailing Victorian philosophy of special creation and immutability of species.

It was only after Alfred Russel Wallace wrote Darwin with his own nearly identical views of the origin of species that Darwin’s friends pushed him to publish. Darwin considered the resulting 490-page book only an “abstract” of a larger work to come.

The first printing of the Origin sold out on its publication date, November 24, 1859 .

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Origin, Chapter 8, Hybridism

In this chapter Darwin addressed the potential problem that sterile hybrids posed to his idea of common descent and evolution by means of natural selection.

Hybrid crosses, for example, a horse with a donkey, generally result in sterile offspring, in this case, the mule (shown here), a situation which would seem to be an evolutionary dead-end. The prevailing view of Darwin's time was that species were independently created and separate and that the phenomenon of sterile crosses existed to (quote) "prevent the confusion of all organic forms."

How could all organisms have a common ancestry, and how could natural selection act, given the phenomenon of sterile hybrids? Darwin acknowledged that the sterility of hybrids could not possible be of any advantage to them and "could not have been acquired by the continued preservation of successive profitable degrees of sterility." He then described a host of hybrid crosses that show a variety of responses and makes the case that sterility is not universal, and the variation in hybrid response counters the idea that hybridism was specially endowed to prevent "confusion of all organic forms" thus opening the door to other explanations.

Origin, Chapter 7, Instinct

In this chapter Darwin continues his preemptive explanations and justifications for natural selection in relation to anticipated criticisms, this time of the criticism that instinct—a form of behavior—would not be amenable to natural selection. Not so, said Darwin.

Darwin wrote, “It will be universally admitted that instincts are as important as corporeal structure for the welfare of each species, under its present conditions of life. Under changed conditions of life, it is at least possible that slight modifications of instinct might be profitable to a species; and if it can be shown that instincts do vary ever so little, then I can see no difficulty in natural selection preserving and continually accumulating variations of instinct to any extent that may be profitable. It is thus, as I believe, that all the most complex and wonderful instincts have originated.

He argued that instincts are subject to natural selection and are as important as the “corporeal structure” of the organism, and as such was subject to natural selection.

As usual, Darwin supported his arguments with a catalog of examples of instinctive behavior from cats to dogs to birds to the hive-making of bees to demonstrate
variation in instinctual behaviors present in modern groups, variation that would be available for natural selection to act upon.

Darwin ended this chapter with some of the nastier instincts in the animal kingdom, like juvenile cuckoos ejecting siblings from the nest, and concludes that ‘it is far more satisfying “ to look at these unappetizing behaviors not as specially endowed or created instincts, but as small consequences of the general law, leading to the
advancement of all organic beings, namely, multiply, vary, let the strongest live and the weakest die.”

Origin, Chapter 6, continued

Another difficulty Darwin faced in advocating a theory of species origin by descent from a common ancestor was the Victorian era mind-set that species were specially created and immutable or unchanging and without transitions between forms.

To support his ideas Darwin pointed to living animals that show a range of modifications of a shared structure. Using birds as an example, Darwin described the diverse development of the wing, from fully functional for flight in air, to their use as fins in an aquatic habitat, as with penguins, to functionally of no use, like the wings of the flightless kiwi.

These examples served to show the range of transitions present within a living group of animals and laid the groundwork for thinking about the cumulative effect of variations as natural selection acted over the immensity of geologic time.

Image from http://farm4.static.flickr.com/3272/2592965662_0bef2d19e2.jpg?v=0

Origin, Chapter 6, a whale of a difficulty

Darwin wrote, “It has been asked by the opponents of such views as I hold, how, for instance, a land carnivorous animal could have been converted into one with aquatic habits; for how could the animal in its transitional state have subsisted?”

To address this Darwin cited examples of modern animals that posses transitional habits, like mink, which are at home on land and in water, and examples of animals with present adaptations that are ill suited to their present habitat, like upland geese that rarely or never go near the water, yet have webbed feet.

Again, 150 years of research has borne out Darwin’s hypotheses, and the evolution of the whale is the prime example of a land carnivorous animal that converted to an aquatic habit.

Image of whale evolution from http://darwiniana.org/whale1.gif

Origin, Chapter 6, yet more difficulties

Darwin anticipated by 150 years arguments that modern day advocates of intelligent design would make that “organs of extreme perfection and complication”, like the eye, could not have been made through natural selection, but must have been specially created. Actually, the idea of intelligent design is not new, but goes back at least to British Philosopher William Paley’s 1802 book entitled Natural Philosophy, with which Darwin was familiar. In the Origin, Darwin presents a scenario for the evolution of the eye, from a light-sensitive nerve imparting some advantage to the animal, through intermediate stages gradually to a more complex light sensitive organ. Since publication of the Origin, research on the evolution of the eye has in fact filled in details of this transition, confirming Darwin’s hypothesis.

Origin, Chapter 6, Difficulties on Theory

This chapter is a preemptive strike against anticipated criticisms of Darwin’s ideas.

One obvious potential flaw in Darwin’s theory of common descent was the apparent lack of transitional forms linking different groups in the fossil record. Darwin himself asked, “Firstly, why, if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms?” Darwin’s response to this question was to invoke the imperfection of the geological record, the fact that fossilization is the exception rather than the rule. Darwin wrote, “The crust of the earth is a vast museum, and the natural collections have been made only at intervals of time immensely remote.”

Darwin’s theory of gradual change made the prediction that transitional forms should exist, and this prediction has been borne out many times in the 150 years since publication of the “Origin” by new fossil discoveries.

Image: Archaeopteryx, whose discovery just after publication of the Origin lent support to Darwin's theory.

Origin, Chapter 5, Laws of Variation

Darwin recognized that heritable variation in individuals was the key to evolution through natural selection. The “conditions of life” as Darwin called it, were the natural processes that selected which variations would be successfully passed on to the next generation.

Darwin’s ideas about selection have often been misrepresented as invoking chance, and evolution portrayed as a random process. Nothing could be further from the truth. Darwin addresses this in the first paragraph of Chapter 5, stating that viewing variation as due to chance is “wholly incorrect but serves to acknowledge plainly our ignorance of the cause of each particular variation.”

Darwin formed his ideas about natural selection before the discovery of what we now realize is the genetic basis for variation; research in the 150 years since publication of the Origin largely confirm Darwin’s hypotheses.

image: http://www.anselm.edu/homepage/jpitocch/genbio/mitosisnot.html

Origin, Chapter 4, Natural Selection

Darwin devoted this chapter to explaining his ideas about natural selection.

To do this Darwin included a diagram, the only illustration in the original editions of the book. The diagram shows gradually diverging lines representing the offspring of a species becoming more and more distinct through successive generations as natural selection acts on the variation inherent in each individual. Only variations which are in some way profitable to the individual will be preserved or naturally selected, that is, passed on to the next generation.

Through time the gradual accumulation of differences in successive generations results in descendants that are distinct from the ancestral forms. Darwin envisioned this process acting slowly through tens of thousands of generations, and his model for evolution became known as gradualism.

The copy of the diagram shown here is from http://friendsofdarwin.com/docs/origin-1/diagram/

Origin, Chapter 3, The struggle for existence

In this chapter Darwin set out to answer the question of how adaptations arise. His explanation was that adaptations result from what he called the “struggle for existence”. He wrote: “Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection…”

Illustration: The diversity of bird beaks is a prime example of adaptation to different life habits.

Origin, Chapter 2, Variation under Nature

In Chapter 2 Darwin took the principles of selection he described in Chapter 1 for domesticated plants and animals and applied them to the rest of the world, what he termed the natural world. Darwin realized that to describe the variation present in nature and the origin of new species one needs to have a clear understanding of the concept of species and a useful working definition of the word species, and Chapter 2 is devoted to his exposition on the subject. Modern biologists can appreciate the difficulties Darwin recognized in attempting to define species, for even today there is considerable discussion among scientists on what is known as “the species concept”. Darwin’s words from 150 years ago still ring true today when he wrote, “No one definition has as yet satisfied all naturalists…”

A good website for an overview of the species concept can be found at

http://evolution.berkeley.edu/evosite/evo101/VA1BioSpeciesConcept.shtml

Origin, Chapter 1, Variation under domestication

In the first chapter Darwin builds his case for the origin of species by starting with familiar, observable changes brought about by breeding domestic animals. Darwin recognized that the variation inherent in all living things was the key to these changes, and discussed selection as the mechanism of for producing new breeds. Darwin himself took up the popular Victorian hobby of pigeon breeding to test his ideas, and in this chapter he traces the history of pigeon breeding from 3000 B.C., giving credit to Chinese and Romans for recognizing the importance of selection. In summary Darwin wrote, “Over all these causes of Change I am convinced that the accumulative action of Selection, whether applied methodically and more quickly, or unconsciously and more slowly, but more efficiently, is by far the predominant Power.”

Read the Origin online at http://darwin-online.org.uk/EditorialIntroductions/Freeman_OntheOriginofSpecies.html

On the Origin of Species....Introduction

Darwin uses an introduction to briefly trace the evolution of his thinking on the origin of species and to sum up his main conclusions. The orthodox thinking of Darwin’s time was that species were independently created and immutable, that is, unchanged, since their creation. Darwin challenged this view, which he himself had held until amassing countless observations to the contrary through his travels on the Beagle and experiments he conducted in the years following the voyage. Darwin saw patterns in living things that pointed to common descent and that indicated change or what was then called transmutation of species through time. And he suggested a mechanism for this change, which he called Natural Selection.

150 Years of "On the Origin of Species..."

One hundred and fifty years ago, Charles Darwin published The Origin of Species by means of Natural Seletion. In honor of this anniversary, GeoLog Blog will post bite-sized summaries of each chapter from the first edition.

Darwin begins the Origin with a preface, a review of research and thinking on the subject of the origin of species from ancient times to his contemporaries of the mid nineteenth century. Contrary to widespread misunderstanding today, Darwin did not invent the idea of evolution and in this preface he traces the development of ideas about species changes through time from Aristotle to Lamark to his own Grandfather, Erasmus. Throughout the Origin Darwin was scrupulous in giving credit where credit was due and he supported his own observations and conclusions with copious reference to the research of others.

For a link to the Origin, go to:

http://darwin-online.org.uk/EditorialIntroductions/Freeman_OntheOriginofSpecies.html

The Origin was published on November 24, 1859.