The Origin of Species: Natural Selection
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Text length: 2,700 words
Excerpts from The Origin of Species, Chapter IV
By Charles Darwin (1809-82)
, London: John Murray, 1859
Inheritance, mutation, and selection drive evolution - these forces lead to the emergence and preservation of well-adapted traits and fitness over time in a changing environment
Progress may happen naturally - even without intentional direction or planning, complex adaptation to a specialized niche can arise given the capacity for variation and selection
Changes in the environment precipitate change - when conditions change, greater variability is an asset in adapting successfully to the new circumstances
Keywords:
selection, variation, struggle, battle, life, death, existence, extinction, environment, climate, inheritance, mutation, advantage, niche, individual, species, offspring, accumulation, change, evolution, complexity, isolation, mixing, competition, chance, time, unconscious, improvement, specialization, natural, England, Britain, 19th century, 1800s
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Summary
Variation among individuals of a species occurs frequently in nature and can be beneficial to the species through the mechanism of natural selection: "the preservation of favourable individual differences and variations, and the destruction of those which are injurious."
Natural selection operates at the level of the individual. Those individuals who are best adapted to the environment survive to pass on their traits to successive generations. As conditions change, traits that are advantageous in the new environment will be preserved and inherited. Over time, accumulated variations can change the nature of a species or give rise to differentiated new species. Factors such as the amount of variation in a population and the degree of isolation or intermingling of populations affect the rate of speciation.
Natural selection gives rise to increasingly complex beings. However, this process is not deliberate 'progress' toward a goal. Simpler forms can coexist with more complex ones if they are well adapted by selection to their particular niches.
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Survival of the fittest: countless variations
How will the struggle for existence... act in regard to variation? Can the principle of selection, which we have seen is so potent in the hands of man, apply under nature? I think we shall see that it can act most efficiently. Let the endless number of slight variations and individual differences occurring in our domestic productions, and, in a lesser degree, in those under nature, be borne in mind as well as the strength of the hereditary tendency. Under domestication, it may be truly said that the whole organisation becomes in some degree plastic. But the variability, which we almost universally meet with in our domestic productions, is not directly produced, as Hooker and Asa Gray have well remarked, by man he can neither originate varieties, nor prevent their occurrence he can preserve and accumulate such as do occur. Unintentionally he exposes organic beings to new and changing conditions of life, and variability ensues but similar changes of conditions might and do occur under nature. Let it also be borne in mind how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to their physical conditions of life and consequently what infinitely varied diversities of structure might be of use to each being under changing conditions of life. Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should occur in the course of many successive generations?
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Variation and natural selection
If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) 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 least degree injurious would be rigidly destroyed. This preservation of favourable individual differences and variations, and the destruction of those which are injurious, I have called Natural Selection, or the Survival of the Fittest. Variations neither useful nor injurious would not be affected by natural selection, and would be left either a fluctuating element, as perhaps we see in certain polymorphic species, or would ultimately become fixed, owing to the nature of the organism and the nature of the conditions.
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Adaptations in response to change
We shall best understand the probable course of natural selection by taking the case of a country undergoing some slight physical change, for instance, of climate. The proportional numbers of its inhabitants will almost immediately undergo a change, and some species will probably become extinct. We may conclude, from what we have seen of the intimate and complex manner in which the inhabitants of each country are bound together, that any change in the numerical proportions of the inhabitants, independently of the change of climate itself, would seriously affect the others. If the country were open on its borders, new forms would certainly immigrate, and this would likewise seriously disturb the relations of some of the former inhabitants. Let it be remembered how powerful the influence of a single introduced tree or mammal has been shown to be. But in the case of an island, or of a country partly surrounded by barriers, into which new and better adapted forms could not freely enter, we should then have places in the economy of nature which would assuredly be better filled up, if some of the original inhabitants were in some manner modified for, had the area been open to immigration, these same places would have been seized on by intruders. In such cases, slight modifications, which in any way favoured the individuals of any species, by better adapting them to their altered conditions, would tend to be preserved and natural selection would have free scope for the work of improvement.
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Small modifications accumulate over time
We have good reason to believe . . . that changes in the conditions of life give a tendency to increased variability and in the foregoing cases the conditions have changed, and this would manifestly be favourable to natural selection, by affording a better chance of the occurrence of profitable variations. Unless such occur, natural selection can do nothing. Under the term of "variations," it must never be forgotten that mere individual differences are included. As man can produce a great result with his domestic animals and plants by adding up in any given direction individual differences, so could natural selection, but far more easily from having incomparably longer time for action. Nor do I believe that any great physical change, as of climate, or any unusual degree of isolation to check immigration, is necessary in order that new and unoccupied places should be left, for natural selection to fill up by improving some of the varying inhabitants. For as all the inhabitants of each country are struggling together with nicely balanced forces, extremely slight modifications in the structure or habits of one species would often give it an advantage over others and still further modifications of the same kind would often still further increase the advantage, as long as the species continued under the same conditions of life and profited by similar means of subsistence and defence. No country can be named in which all the native inhabitants are now so perfectly adapted to each other and to the physical conditions under which they live, that none of them could be still better adapted or improved for in all countries, the natives have been so far conquered by naturalised productions, that they have allowed some foreigners to take firm possession of the land. And as foreigners have thus in every country beaten some of the natives, we may safely conclude that the natives might have been modified with advantage, so as to have better resisted the intruders.
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