What is natural selection? Is it a law on the same level as the law of gravity or the laws of thermodynamics? Is it a theory on the same level as relativity or quantum physics? Is it a process, like combustion or decomposition? Does it create force like electricity? Does it have creative power?
Evolutionary scientists have repeatedly put natural selection into all these categories, but in actuality natural selection is not a power, force or law, it has no creative abilities, no measurable quantities, and no physical properties.
You can't observe it directly. You can't determine with certainty what will be selected. It is a compilation of after the fact observations. An extraordinary feature of natural selection is that it does not actually select anything. Selection, i.e. the ability to make a choice, is a function of intelligence. Natural selection makes no decisions, or choices. So how does something that can't choose make a choice?
Basically any organism fit enough to survive and reproduce has been selected. Natural selection is considered to be the opposite of artificial selection, or intelligent selection. The prediction that natural selection makes is that the fittest will survive. This is a tautology, or truism, because a population that survives will contain the most fit. Natural selection however explains nothing about the cause of changes within an organisms genes. Even so it is used as a generic term to explain any and every structure, system or behavior of organisms. This is based on the structures perceived enhancement to its survival, even going so far as to assign it the power to initiate genetic changes.
Evolutionist have misused this term since the time of Darwin. He made the claim that "natural selection modifies" many times. At times evolutionists assign to it almost supernatural creative powers, giving it the attributes of genius and insight, the abilities to test, experiment and probe. It has been given the godlike powers of decision and creativity.
When a farmer selects the best sheep for breeding he is using intelligent, or artificial selection, the farmer has a goal and a reasoning process to hopefully reach that goal. The scientist trying to engineer new tools from living organisms also has a goal, by selecting only those organisms that fit his criteria he is making an intelligent selection. When they state that these things are comparable to natural selection they demonstrate a problematic logical error in evolutionary thinking. That is, evolutionists have not been able to deal with the true role of natural selection and on rare occasions admit that natural selection has no creative role. They do this because the true engines of genetic change cannot drive evolution forward.
The concept of natural selection was introduced by a Bible believing scientist, Edward Blythe, in the late 1830s. He correctly saw it as a process of conservation, not creation. In Darwin's proposition that evolution was driven by a combination of both variation and natural selection. Variation, one of the actual change agents, was and still is given a secondary role. The cause of variation was not even understood at the time. Darwin, unable to uncover the cause of variation eventually returned to Lamarckian views of evolution through acquired characteristics. Mutation the only possible creative mechanism for evolution was unknown. Interestingly another Bible believing scientist, Gregor Mendel, discovered the true nature of variation several years before Darwin published "The Origin of the Species." But his work, unfortunately, did not come to light until the early 20th century.
In order to define natural selection we need to ask a few questions. What is being selected, how is it being selected and when is it selected. None of these questions have a straightforward answer, which confirms the fact that natural selection is not what it is often claimed to be.
Consider these two definitions from Biology textbooks.
"Natural selection, however, can act on genetic variation only when it is expressed in the phenotype" 
"A population is the smallest unit that can evolve. Natural selection involves interactions between individual organisms and their environment, but individuals do not evolve" 
The gap between the definition of each organism in the DNA and the point where natural selection actually takes place, at the phenotypic level is enormous. Evolutionists often speak of natural selection acting at all levels of organization, even lower than the cellular level. This clearly disagrees with a basic understanding of biology.
First, what is being selected? Natural selection can be said to select the phenotype. The phenotype is the body of the organism containing all its cells, organs, systems and functions. The genotype, coded on the DNA, the actual instructions to build the phenotype is not affected or selected by natural selection. Natural selection does not affect the genetic makeup of an organism in any way. Now, natural selection can be said to change genetic percentages within of a population of organisms. The available genetic variation within a species is known as the gene pool. But again, it is not natural selection that made any of the genetic variation available nor does it control any circumstances that may give an organism an advantage.
Secondly, how does natural selection select? Selection is not a conscious choice, it is the result of survival, and survival is based on fitness. For Darwin Survival of the fittest and natural selection were synonymous. The term "fitness" lacks a straightforward definition and is not entirely based on the qualities of the organism. A gecko for instance is not going to be fit to survive in the Antarctic. The organism must be adapted to its environment. No organism can be prepared for every environmental change, natural disasters or other changes in the environment, such as the introduction of a new predator, can reduce the population of any organism that we would normally call fit.
Natural selection basically "chooses" the organisms that are left over after the "unfit" are gone. We can therefore say, that everything that is currently alive is selected. Things that that prevent selection are, lack of resources, disease, predation and environmental change. It must be noted that none of these are agents for genetic change. The genetic code is very stable, but even then it is always in a state of flux and generally degrading. Exposure to radiation and harsh chemicals can accelerate this. The genetic code is normally unaffected by environmental changes.
Thirdly, when is the selection made? Since selection is based on survival, and survival of a species is based on how effectively it passes on its genetic information. Selection then has to take place at the time of birth. When a new organism is born, the selection has been made; the genes have been passed on successfully. Its will be selected again if it successfully reproduces.