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The phrase "survival of the fittest" is often used to describe what Charles Darwin called "natural selection". These exact words were not coined by Darwin but by a contemporary of his, Herbert Spencer.

Spencer used the concept, if not the words, in works on economics and philosophy prior to Darwin's publication of his "On the Origin of Species." After reading Darwin's book, Spencer wrote "Principles of Biology" stating, ""This survival of the fittest, which I have here sought to express in mechanical terms, is that which Mr. Darwin has called 'natural selection', or the preservation of favoured races in the struggle for life."

In later editions of "On the Origin of Species", Darwin started using the phrase, crediting Spencer. Darwin felt that the phrase had some advantages over his original "natural selection" while giving up some meaning.

Contrary to popular usage, survival of the fittest doesn't necessarily imply that the strongest or smartest organisms will survive. It only states that the organisms best fitted to ecological niche in which they live will be more likely to reproduce and pass on their genes. Subsequent offspring of those that best utilize their unique ecological place are likewise more likely to reproduce, spreading their genetic advantage throughout a population.

On his explorations to the Galapagos islands, Darwin observed differences between finches in various ecological niches. These observations lead to the development of his theory of natural selection.

Noting the differences of some 13 species of finches on the isolated islands helped him to develop his ideas about natural selection. All of the species appeared to be descendents of a single type of bird but differed in the ecological niche to which they were adapted. Approximately of the same size and coloration, the greatest differences were in the size and shape of their beaks.

Some of the species have large, heavy beaks which are ideally adapted to cracking hard seeds and small nuts. Other species' beaks are perfectly suited to eating insects and picking ticks off of the famous Galapagos tortoise. There are species that have adapted to using small twigs and cactus spines as tools to dig insects and larvae out of trees, much like a woodpecker. And, there is even a species that pecks at the flesh of sea birds and drinks their blood.

Darwin theorized that each of these species evolved from a single species of birds that somehow arrived on the islands and became isolated from the parent

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