To qualify for a patent an invention has to be both novel and inventive, something that a selection invention may, at first glance, not appear to be. However, appearances can be deceptive.
To assess the novelty it is essential that nothing has ever been published about an invention in the past. However, if a document refers to the use of a metal object, and you specifically use copper, then that more generic description (metal) will not affect the novelty of the invention.
The same applies, for example, to the description 'acids' in a publication about a chemical reaction. If you specifically use acetic acid for your invention, then this invention will in fact be novel. This is because you have selected a specific substance from a generic group. When it comes to the inventiveness criterion, the use of acetic acid will need to have an unexpected advantage. Otherwise the use of acetic acid will not yet be patentable.
Selection from two lists
The term selection invention is used in particular when the choices you have made have already been described in detail, for example when two substances react to form a product. Suppose that alcohol is used as the solvent and kitchen salt as the excipient to bring about the reaction while an earlier publication sums up a whole list of solvents and a whole list of excipients with alcohol in the one list and kitchen salt in the other. In that case the entire invention will have been described almost word for word. Despite this your invention will nevertheless be novel because you have made a choice from two lists. In order to be inventive as well, your selection must have an unexpected advantage.
Selection inventions are possible in every sector. We do, however, mainly tend to see them in chemistry and Life Sciences. This is because of the unpredictable nature of chemical processes. In the example above it is not unthinkable that by opting for alcohol and kitchen salt the yield of the product may unexpectedly double. Your choice is then not an obvious one and is therefore patentable.
Tangible mechanical engineering
A similar assessment of choices in mechanical engineering, for example, which is very tangible, is far less likely to result in a patent. Suppose you find the use of metal connections in a publication and you specifically use an aluminium blind rivet for your invention. It is then obvious that your connection can be made quickly and will result in a light product. In order to qualify for a patent you will have to do better than that.