"Survival of the fittest" is a phrase which is shorthand for a concept relating to competition for survival or predominance. Originally applied by Herbert Spencer in his Principles of Biology of 1864, Spencer drew parallels to his ideas of economics with Charles Darwin's theories of evolution by what Darwin termed natural selection.
Although Darwin used the phrase "survival of the fittest" as a synonym for "natural selection", it is a metaphor, not a scientific description. It is not generally used by modern biologists, who use the phrase "natural selection" almost exclusively.
An interpretation of the phrase to mean "only the fittest organisms will prevail" (a view common in social Darwinism) is not consistent with the actual theory of evolution. Any organism which is capable of reproducing itself on an ongoing basis will survive as a species, not just the "fittest" ones. A more accurate characterization of evolution would be "survival of the fit enough", although this is sometimes regarded as a tautology.
In The Man Versus The State Spencer used the phrase in a postscript to justify a plausible explanation for why his theories would not be adopted by "societies of militant type." He uses the term in the context of societies at war, and the form of his reference suggests that he is applying a general principle.
Thus by survival of the fittest, the militant type of society becomes characterized by profound confidence in the governing power, joined with a loyalty causing submission to it in all matters whatever.
In the first four editions of On the Origin of Species, Darwin used the phrase "natural selection", and preferred that phrase. However, Spencer's Principles of Biology drew parallels between his economic theories and Darwin's biological ones and made first use in print of the phrase "survival of the fittest". Darwin agreed with Alfred Russel Wallace that this phrase avoided the troublesome anthropomorphism of "selecting", though it "lost the analogy between nature's selection and the fanciers'." It was used by Darwin in the 5th edition of The Origin published on 10 February 1869, in a secondary header of Chapter 4 about natural selection and at several places in the text, mostly using the phrase "Natural Selection, or the Survival of the Fittest". He gave full credit to Spencer, writing "I have called this principle, by which each slight variation, if useful, is preserved, by the term natural selection, in order to mark its relation to man's power of selection. But the expression often used by Mr. Herbert Spencer, of the Survival of the Fittest, is more accurate, and is sometimes equally convenient." At this time the word "fittest" would have primarily meant "most suitable" or "most appropriate" rather than "in the best physical shape".
In modern times, however, the phrase is widely used in popular literature as a catchphrase for any topic related or analogous to evolution and natural selection. It has thus been applied to principles of unrestrained competition, and it has been used extensively by both proponents and opponents of Social Darwinism. Its shortcomings as a description of Darwinian evolution have also become more apparent (see below).
Evolutionary biologists criticize how the term is used by non-scientists and the connotations that have grown around the term in popular culture. The phrase also does not help in conveying the complex nature of natural selection and modern biologists prefer and almost exclusively use the term natural selection. Indeed, in modern biology, the term fitness measures reproductive success and is not explicit about the specific ways in which organisms can be "fit" as in "having phenotypic characteristics which enhance survival and reproduction" (which was the meaning that Spencer had in mind).
"Survival of the fittest" is sometimes claimed to be a tautology. The reasoning is that if one takes the term "fit" to mean "endowed with phenotypic characteristics which improve chances of survival and reproduction" (which is roughly how Spencer understood it), then "survival of the fittest" can simply be rewritten as "survival of those who are better equipped for surviving". While this is not exactly a tautology (we might imagine a benevolent deity or experimenter that would consistently favour the poorly adapted, and destroy well-adapted creatures, so that "survival of the fittest" might actually not occur), this is not a very informative statement: it simply reduces to a statement that the game of Life is not rigged in favour of the poorly adapted, which is not controversial. Furthermore, the expression does become a tautology if one uses the most widely accepted definition of "fitness" in modern biology, namely reproductive success itself (rather than any set of characters conducive to this reproductive success). This reasoning is sometimes used to claim that Darwin's entire theory of evolution by natural selection is fundamentally tautological, and therefore devoid of any explanatory power.
However, the expression "survival of the fittest" (taken on its own and out of context) gives a very incomplete account of the mechanism of natural selection. The reason is that it does not mention a key requirement for natural selection, namely the requirement of heritability. It is true that the phrase "survival of the fittest", in and by itself, is a tautology if fitness is defined by survival and reproduction. However, natural selection is not just survival of the fittest. Natural selection is the portion of variation in reproductive success, that is caused by heritable characters (see the article on natural selection).
If certain heritable characters increase or decrease the chances of survival and reproduction of their bearers, then it follows mechanically (by definition of "heritable") that those characters that improve survival and reproduction will increase in frequency over generations. This is precisely what is called "evolution by natural selection." On the other hand, if the characters which lead to differential reproductive success are not heritable, then no meaningful evolution will occur, "survival of the fittest" or not: if improvement in reproductive success is caused by traits that are not heritable, then there is no reason why these traits should increase in frequency over generations. In other words, natural selection does not simply state that "survivors survive" or "reproducers reproduce"; rather, it states that "survivors survive, reproduce and therefore propagate any heritable characters which have affected their survival and reproductive success". This statement is not tautological: it hinges on the testable hypothesis that such fitness-impacting heritable variations actually exist (a hypothesis that has been amply confirmed.)
Skeptic Society founder and Skeptic magazine publisher Dr. Michael Shermer addresses this argument in his 1997 book, Why People Believe Weird Things, in which he points out that although tautologies are sometimes the beginning of science, they are never the end, and that scientific principles like natural selection are testable and falsifiable by virtue of their predictive power. Shermer points out, as an example, that population genetics accurately demonstrate when natural selection will and will not effect change on a population. Shermer hypothesizes that if hominid fossils were found in the same geological strata as trilobites, it would be evidence against natural selection.
Critics of evolution have argued that "survival of the fittest" provides a justification for behaviour that undermines moral standards by letting the strong set standards of justice to the detriment of the weak. However, any use of evolutionary descriptions to set moral standards would be a naturalistic fallacy (or more specifically the is-ought problem), as prescriptive, moral statements cannot be derived from purely descriptive premises. Describing how things are does not imply that things ought to be that way. It is also simplistic to suggest that evolutionary "survival of the fittest" implies treating the weak badly, as social behaviour cooperating with others and treating them well improves evolutionary fitness.
It has also been claimed that "the survival of the fittest" theory in biology was interpreted by late 19th century capitalists as "an ethical precept that sanctioned cutthroat economic competition" and led to "social Darwinism" which allegedly glorified laissez-faire economics, war and racism. However these ideas predate and commonly contradict Darwin's ideas, and indeed their proponents rarely invoked Darwin in support, while commonly claiming justification from religion and Horatio Alger mythology. The term "social Darwinism" referring to capitalist ideologies was introduced as a term of abuse by Richard Hofstadter's Social Darwinism in American Thought published in 1944. This claim is also an example of the appeal to consequences fallacy – even if the concept of survival of the fittest was used as a justification for violence, this has no effect on the truth of the theory of evolution by natural selection.
The famous anarchist philosopher and scientist Peter Kropotkin viewed the theory of survival of the fittest as supporting co-operation rather than competition. In his book Mutual Aid: A Factor of Evolution he set out his analysis leading to the conclusion that the fittest was not necessarily the best at competing individually, but often the community made up of those best at working together. He concluded that "In the animal world we have seen that the vast majority of species live in societies, and that they find in association the best arms for the struggle for life: understood, of course, in its wide Darwinian sense – not as a struggle for the sheer means of existence, but as a struggle against all natural conditions unfavourable to the species. The animal species, in which individual struggle has been reduced to its narrowest limits, and the practice of mutual aid has attained the greatest development, are invariably the most numerous, the most prosperous, and the most open to further progress."
Applying this concept to human society, he presented mutual aid as one of the dominant factors of evolution, the other being self assertion, and concluded that "In the practice of mutual aid, which we can retrace to the earliest beginnings of evolution, we thus find the positive and undoubted origin of our ethical conceptions; and we can affirm that in the ethical progress of man, mutual support not mutual struggle – has had the leading part. In its wide extension, even at the present time, we also see the best guarantee of a still loftier evolution of our race."