Natural selection

:For the computer game, see Natural Selection (computer game).

Mechanisms of natural selection

In Chapter 4 of The Origin of Species, Darwin wrote:

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:It may be said that natural selection is daily and hourly scrutinising, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until the hand of time has marked the long lapses of ages, and then so imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they formerly were.

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What makes one trait more likely to succeed is highly dependent on environmental factors, including the species' predators, food sources, abiotic stress, physical environment, and so on. When members of a species become separated, such as geographically, they face different environments and tend to develop in different directions. After a long period of time, their traits will have developed along different paths to such an extent that they can no longer interbreed, at which point they are considered separate species. This is why a species will sometimes separate into multiple species, rather than simply being replaced by a newer form of the species (from this fact Darwin suggested that all species today have evolved from a common ancestor).

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Additionally, some scientists have theorized that an adaptation which serves to make the organism more adaptable in the future will also tend to supplant its competitors even though it provides no specific advantage in the near term. Descendants of that organism will be more varied and therefore more resistant to extinction due to environmental catastrophes and extinction events. This has been proposed as one reason for the rise of mammals. While this form of selection is possible, it is more likely to play an important role in cases where selection for adaptation is continuous. For example, the Red Queen hypothesis suggests that sex might have evolved to help organisms adapt to deal with parasites.

Related Topics:
Extinction events - Mammals - Red Queen - Sex

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Natural selection can be expressed as the following general law (taken from the conclusion of The Origin of Species):

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• IF there are organisms that reproduce, and
• IF offspring inherit traits from their progenitor(s), and
• IF there is variability of traits, and
• IF the environment cannot support all members of a growing population,
• THEN those members of the population with less-adaptive traits (determined by the environment) will die out, and
• THEN those members with more-adaptive traits (determined by the environment) will thrive

The result is the evolution of species.

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This is a continuing process—it accounts for how species change, and can account for both the extinction of one species and the creation of a new one. The formulation does not rule out selection occurring at all biological levels (e.g., gene, organism, group), and the particular process of introducing new traits does not matter.

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Darwin did not maintain that natural selection was the only mechanism of evolution, however, as he pronounced in the introduction to The Origin of Species: "I am convinced that has been the most important, but not the exclusive means of modification."

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