"Natural Selection"  or "The Theory of Natural Selection" 
adaptation, reproduction, speciation, species, survival, natural selection, sexual selection
"Natural Selection constitutes a natural process of evolutionary change; that results in the survival (of the fittest) and reproductive success (sexual selection) of individuals or groups; best adjusted to their environment; leading to the perpetuation of their genetic qualities and traits; culminating in speciation" 
The theory of Natural Selection is relatively simple, but often misunderstood. 
To explain Natural Selection, let's look at beetles.
» Beetles come in a variety of species with a variety of traits (shapes, sizes and colors). [6|7]
» Due to environmental pressures (i.e. weather, predators, etc) "not all individuals get to reproduce to their full potential" . For instance, green beetles tend to get eaten more often than brown beetles. This is known as "differential reproduction" .
» The surviving brown beetles have brown baby beetles because this trait has a genetic basis. This is known as "heredity" .
» The end result is that the more advantageous trait, brown coloration, which allows the beetle to have more offspring, becomes more common in the population. If this process continues, eventually, all individuals in the population will be brown.
Natural Selection is NOT Evolution [8|11]
It must be noted that a belief in the evolutionary process of Natural Selection, does NOT necessitate a belief in Old Earth Theory. A belief in Natural Selection is compatible with Young Earth Creationism.
Natural Selection is but one scientific theory, often misrepresented as conclusive and/or supporting evidence of Evolutionary Theory.
The Theory of Natural Selection, however, was developed long before the Composite Theory of Evolution.
Several ancient philosophers expressed the idea that nature produces a huge variety of creatures, apparently randomly, and that only those creatures survive that manage to provide for themselves and reproduce successfully; well-known examples include Empedocles (490–430 BC) [1|12|28] and his intellectual successor, Lucretius (99–55 BC) [1|13|29], while related ideas were later refined by Aristotle (384–322 BC) [1|14|30].
The famous Arab scholar, Al-Jahiz (781-868/869) , argued that environmental factors influence animals to develop new characteristics to ensure survival. [1|15|16|17]
The Persian Muslim scholar and polymath, Abu Rayhan Biruni, described the idea of artificial selection (selective breeding) arguing that nature works in much the same way. [1|19]
Similar ideas were later expressed by the Persian polymath, Nasir al-Din Tusi [1|23], and the North African polymath, Ibn Khaldun [1|24].
Such classical arguments were reintroduced in the 18th century by French mathematician and philosopher, Pierre Louis Maupertuis (1698–1759) [1|25|26], and others, including Charles Darwin's grandfather, the physician, physiologist and abolitionist, Erasmus Darwin (1731–1802) [1|27].
Natural Selection is NOT Unnatural
Natural Selection leads to Adaptation through Reproduction, but Adaptation also leads to Natural Selection through Survival. Entities are either genetically predisposed to survival and reproduction due to genetic changes (as a result of Genomic Mutation, Genetic Recombination and/or Genetic Drift), or not. They cannot adapt themselves to circumvent Natural Selection.
Much of the confusion, however, comes from the wording of scientists like Al-Jahiz, who in describing the struggle for existence, argued that "environmental factors influence animals to develop new characteristics to ensure survival" [1|15|16|17].
Al-Jahiz did not mean to suggest that animals actively and consciously attempted to adapted to their environment, but rather that their survival was insured due to the random genetic development of needed characteristics (the result of Genomic Mutation, Genetic Recombination and/or Genetic Drift).
THE FORMATION OF NEW SPECIES
There's this raging argument surrounding the issue of whether or not the process of Natural Selection creates "new species".
According to "Potholer", Natural Selection "can create new species" [3|4]. According to Douglas Futuyma, Natural Selection "sometimes does but not always" . According to Creation Scientist, Ken Ham, new species do appear because of Natural Selection.  But according to Michael Suttkus it is impossible. 
Most of the existing evidence for the evolution of new species, has been for small-scale change within a species: house sparrows from all over North America are still classified as house sparrows, the same species as was introduced in the 1850s. 
The amounts of artificially selected change in pigeons and other domestic animals borders on the species level, but to decide whether the species barrier has been crossed we need a concept of what a biological species is. 
How do you define a new specie?
The question lacks a simple answer that would satisfy all biologists. We can take two of the most important species concepts and see for each what the evidence for the evolution of a new species is.
» The Concept of Reproduction defines a species as a set of organisms that interbreed among themselves but do not breed with members of other species. Humans (Homo sapiens) are a separate reproductive species from the common chimpanzee (Pan troglodytes). Any human can interbreed with any other human (of appropriate sex), but not with a chimp.
» The Concept of Phenotypic Appearance defines a species as a set of organisms that are sufficiently similar to one another and sufficiently different from members of other species. Again, humans and common chimpanzees belong to different species, and they are clearly distinct in phenotypic appearance.
The process of Natural Selection do NOT necessitate any support for an Old Earth Theory (of "millions of years"), but rather fit perfectly well within the parameters of Young Earth Creationism.
If you have variation, differential reproduction, and heredity, you will have evolution by natural selection as an outcome. It is as simple as that.
Among those who support Natural Selection you will find: Ken Ham [3|8|9|10|11] and myself.
Among those who deny Natural Selection you will find: Michael Suttkus  and Psycheout .
1. Natural selection (Wikipedia, the free encyclopedia; 9 September 2010)
2. Survival of the fittest (Wikipedia, the free encyclopedia; 9 September 2010)
3. 7 -- The Theory of Evolution Made Easy (YouTube; 9 September 2010)
4. The evidence for evolution - Can the formation of a new species be observed? (Blackwell Publishing; 9 September 2010)
5. Theory of Natural Selection (Biology Online; 1 January 2000)
6. Natural selection (Understanding Evolution; 9 September 2010)
7. Beetle (Wikipedia, the free encyclopedia; 9 September 2010)
8. Michael Suttkus. Ken Ham's Unnatural Selection (No Answers in Genesis; 6 December 2002)
9. Natural Selection Exhibit in the News (Answers In Genesis; 20 March 2009)
10. Ken Ham (Wikipedia, the free encyclopedia; 9 September 2010)
11. Exhibits (Creation Museum; 10 September 2010)
12. Empedocles. On Nature. Book II
13. Lucretius. De rerum natura. Book V
14. Aristotle. Physics. Book II, Chapters 4 and 8
15. Conway Zirkle. Natural Selection before the "Origin of Species" (Proceedings of the American Philosophical Society 84 (1); 1941) p.71-123
16. Mehmet Bayrakdar (Third Quarter, 1983). "Al-Jahiz And the Rise of Biological Evolutionism", The Islamic Quarterly. London.
17. Paul S. Agutter & Denys N. Wheatley (2008). Thinking about Life: The History and Philosophy of Biology and Other Sciences. Springer. p. 43. ISBN 1402088655
18. Al-Jahiz (Wikipedia, the free encyclopedia; 10 September 2010)
19. Jan Z. Wilczynski. "On the Presumed Darwinism of Alberuni Eight Hundred Years before Darwin" (December 1959) Isis 50 (4): 459–466. doi:10.1086/348801
20. Speciation (Wikipedia, the free encyclopedia; 10 September 2010)
21. Douglas Futuyma. Natural Selection: How Evolution Works (Action Bioscience; 2004)
22. EJ Hill. Summary Definition: Defining Natural Selection (Hermanus, SA: Ejays)
23. Farid Alakbarov. A 13th-Century Darwin? Tusi's Views on Evolution (Azerbaijan International; Summer 2001) 9 (2).
24. Franz Rosenthal and Ibn Khaldun. Muqaddimah, Chapter 6, Part 5 and 29.
25. Pierre Louis Maupertuis. Derivation of the laws of motion and equilibrium from a metaphysical principle (Original French text)" (1748) Histoire de l'academie des sciences et belle lettres de Berlin 1746: 267–294.
26. Pierre Louis Maupertuis (Wikipedia, the free encyclopedia; 10 September 2010)
27. Erasmus Darwin (Wikipedia, the free encyclopedia; 10 September 2010)
28. Empedocles (Wikipedia, the free encyclopedia; 10 September 2010)
29. Lucretius (Wikipedia, the free encyclopedia; 10 September 2010)
30. Aristotle (Wikipedia, the free encyclopedia; 10 September 2010)
31. Psycheout. Heliocentrism is an Atheist Doctrine (Blogs 4 Brownback; 18 May 2007)
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