What is evolution by natural selection?

Evolution by natural selection if defined as simply change in a species over time has been tested and proven to be true.

Species change due to environmental pressures. The fish living in the underground rivers and lakes of Death Valley have changed (evolved) by losing the genes that produce eye tissues.

Bacteria have evolved resistance to antibiotics by losing the genes that produce the proteins that the chemicals attacked.

The peppered moths in England have changed from being primarily white to being primarily black and back to being primarily white due to environmental changes. This is an example of evolution due to natural selection as defined as change within a species.

If by evolution people mean changes within a species due to natural selection acting on existing variations within a species as a result of environmental changes or pressure evolution would be considered a law. This can be referred to as the law of adaptation.

However when most people use the word evolution it refers to Darwin's or Neo Darwinian's theory of decent through modification of all living things from a "simple" single cell, to the diversity of life present on the earth. This concept remains a scientific theory due to lack of scientific evidence.

Darwinian evolution requires the movement from the simple to the complex and the random chance accumulation of new unique complex information. Over 100 years of experiments with mutations have failed to provide direct empirical evidence that supports the theory of decent through modification.

More recently mathematical studies of the DNA as an information code based on Shannon's laws of information have indicated that any mutations could result in an increase in information needed for Darwin's theory of descent with modification to be true.

Though there is substantial indirect evidence of descent through modification until there is direct evidence of the types of changes required by Darwinian evolution natural selection as a mechanism for descent with modification will remain a theory.