Excerpts from Darwin's "On the Origin of Species"

Excerpts from Darwin's "On the Origin of Species"*

Leandro Nunes de CastroLnunes@mackenzie.br

@lndecastro

Faculdade de Computação e Informática &

Programa de Pós-Graduação em Engenharia Elétrica

Laboratório de Computação Natural (LCoN)

www.mackenzie.br/lcon.html

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* Darwin, C. R. (1859), The Origin of Species, Wordsworth Editions Limited (1998).

Summary

PART 1

Introduction

1. Variation Under Domestication

2. Variation Under Nature

3. Struggle for Existence

4. Natural Selection

5. Laws of Variation

6. Difficulties on Theory

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Introduction

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• Born in England in 1809

• Started medicine at the Edinburgh University in 1827 and Theology at Cambridge in 1829

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From 1831 to 1836 Darwin joined the HMS Beagle expedition to the Southern Hemisphere, Brazil and Galapagos Archipelago

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• By the 1850’s, Darwin’s research had turned into a massive ongoing book, Natural Selection.

• Alfred Russell Wallace, a young socialist naturalist collecting and researching in the Malay Archipelago, had been communicating with Darwin, and sending him specimens.

• He had also sketched out his own theories of the evolutionary mechanism, which Darwin, recognizing their similarity to his own, had complacently encouraged Wallace to develop.

• On 18 June 1858, a twenty-page letter arrived from Wallace, containing – in effect – an outline of the natural selection theory.

• Incapable of acting dishonorably, Darwin proposed to help gain due public recognition for Wallace’s work, while having to confront the painful truth that his own dilatory perfectionism had led to him being upstaged.

• A delicate compromise was thus reached: Wallace’s letter and relevant extracts from Darwin’s work were presented together at a meeting of the Linnean Society of London on 1 July 1958.

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Darwin x Wallace

1. Variation UnderDomestication

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“*w+hen we look to the individuals of the same variety or sub-variety of our older cultivated plants and animals, one of the first points which strikes us, is that they generally differ much more from each other than do the individuals of any one species or variety in a state of nature”. (p. 8)

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Discussions about domestic breeding

“But I am strongly inclined to suspect that the most frequent cause of variability may be attributed to the male and female reproductive elements having been affected prior to the act of conception.” (p. 9)

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Genetics was still unknown

“Seedlings from the same fruit, and the young of the same litter, sometimes differ considerably from each other, though both the young and the parents, as Müller has remarked, have apparently been exposed to exactly the same conditions of life; and this shows how unimportant the direct effects of the conditions of life are in comparison with the laws of reproduction, and of growth, and of inheritance; for had the action of the conditions been direct, if any of the young had varied, all would probably have varied in the same manner.” (p. 10)

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Contrasting with Lamarck

“*a+ny variation which is not inherited is unimportant for us.” (p. 12)

“Perhaps the correct way of viewing the whole subject, would be, to look at the inheritance of every character whatever as the rule, and non-inheritance as the anomaly”(p. 13)

“…by crossing we can get only forms in some degree intermediate between their parents…” (p. 17)

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Importance of inheritance

“…as a general rule, I cannot doubt that the continued selection of slight variations…will produce races differing from each other…” (p. 28)

“…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.”

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Natural Selection

2. Variation Under Nature

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What is a species

“Nor shall I here discuss the various definitions which have been given of the term species. No one definition has yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species.” (p. 36)

“Those forms which possess in some considerable degree the character of species, but which are so closely similar to some other forms, or are so closely linked to them by intermediate gradations, that naturalists do not like to rank them as distinct species…Practically, when a naturalist can unite two forms together by others having intermediate characters, he treats the one as a variety of the other, ranking the most common, but sometimes the one first described, as the species, and the other as the variety.” (p. 38)

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Variety

“I was much struck how entirely vague and arbitrary is the distinction between species and variety.” (p. 39)

“…I look at the term species, as one arbitrary given for the sake of convenience to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms.” (p. 42)

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Species vs Variety

3. Struggle for Existence

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“I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man’s power of selection.” (p. 49)

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Natural Selection

“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.” (p. 50)

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Struggle for existence

“*t+here is no exception to the rule that every organic being naturally increases at so a high rate, that if not destroyed, the earth would soon be covered by the progeny of a single pair.” (p. 51)

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Struggle for existence

“The elephant is reckoned to be the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase: it will be under the mark to assume that it breeds when thirty years old, and goes on breeding till ninety years old, bringing forth three pairs of young in this interval; if this be so, at the end of the fifth century there would be alive fifteen million elephants, descended from the first pair.” (p. 51)

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Geometrical increase

4. Natural Selection

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“…can we doubt…that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? On the other hand, we may feel sure that any variation in the last degree injurious would be rigidly destroyed. This preservation of favourablevariations and the rejection of injurious variations, I call Natural Selection. Variations neither useful nor injurious would not be affected by natural selection, and would be left a fluctuating element, as perhaps we see in the species called polymorphic.” (p. 65)

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Darwin’s belief in Natural Selection

“…I think it inevitably follows, that as new species in the course of time are formed through natural selection, others will become rarer and rarer, and finally extinct. The forms which stand in closest competition with those undergoing modification and improvement, will naturally suffer most.” (p. 85-86)

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Extinction

“I believe…that the more diversified the descendants from any one species become in structure, constitution, and habits, by so much will they be better enabled to seize on many and widely diversified places in the polity of nature, and so be enabled to increase in numbers.” (p. 87)

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The importance of diversity

5. Laws of Variation

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• Darwin believed that differences in character were a result of variation in the reproductive organs. But he strongly believed that these and other changes are not a result of the direct effect of the environment.

“How much direct effect difference of climate, food, etc., produces on any being is extremely doubtful. My impression is, that the effect is extremely small in the case of animals, but perhaps rather more in that of plants.” (p. 103)

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Environmental influence

• Use and disuse law.

• Acclimatization.

• Correlation of growth.

“Our ignorance of the laws of variation is profound” (p. 129)

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Some Laws of Variation

Difficulties on Theory

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1. Is it possible that an animal having very specific structure and habits could have been formed by the modification of some animal with wholly different habits?

2. How can organs of extreme perfection and complication be generated by natural selection?

3. Why are there organs of little apparent importance?4. Can instincts be acquired and modified through natural

selection?5. How can we account for species, when crossed, being

sterile and producing sterile offspring, whereas, when varieties are crossed, their fertility is unimpaired?

6. Why, if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms?

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Questions pointed out by Darwin

“In North America the black bear was seen by Hearne swimming for hours with widely open mouth, thus catching, like a whale, insects in the water. Even in so extreme a case as this, if the supply of insects were constant, and if better adapted competitors did not already exist in the country, I can see no difficulty in a race of bears being rendered, by natural selection, more and more aquatic in their structure and habits, with larger and larger mouths, till a creature was produced as monstrous as a whale.” (p. 141-142)

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Darwin’s support of Natural Selection

PART 27. Instinct 8. Hybridism9. On the Imperfection of the Geological Record 10. On the Geological Succession of Organic Beings11. Geographical Distribution 12. Geographical Distribution – Continued13. Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs 14. Recapitulation and Conclusions

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Summary

Thanks!

Leandro Nunes de Castro

Lnunes@mackenzie.br

@lndecastro

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