The theory of evolution by natural selection is precisely that. Theory is the way scientists understand their subject and natural selection is the theory that most generally explains biology. Science as a whole is in fact an effort to understand the natural world using formal thinking about observations. This is a quintessentially human activity, as valid and noble as any expression of art.
Theory not only gives understanding, it is understanding. Without understanding we do not truly have science; biology would be far less a science without natural selection because that theory explains and gives order to all the jumble of facts we obtain from observing the natural world.
Natural selection stands alongside other great theories of science such as Einstein's relativity and the electronic theory of chemistry.
But what are its practical consequences?
We can imagine consequences that arise from the application of science which depends on the understanding a theory gives - for example nuclear power would be impossible without theoretical physics. As well as this we can also imagine the social and moral consequences of people knowing the theory (or their misunderstanding from knowing it falsely).
This can best be explained by looking at another great theory developed, like natural selection, from the enlightenment thinking of the Scottish philosophers. What I am referring to is the classical model of economics, most associated with Adam Smith. In order to understand the consequences of trade, early economists realised that they needed to simplify and formalise their observations. They constructed a highly abstract mathematical model in which people were perfectly selfish and greedy - this was a purely technical procedure intended only for economic theory.
Now these abstractions have been taken up as proscriptions - telling people how to behave in order to maximise economic efficiency - the theory has been transformed from a scientific tool into a dogma, with terrible social and ethical consequences.
This dogmatisation has two steps: first the application of the theory beyond its bounds (and in so doing violating it) and second the transformation of its axioms and conclusions into normative proscriptions - changing from 'assume x' to x ought to be so.
What then, can we say about the theory of evolution?
Direct practical applications are not so common but remember that theory is primarily there for the scientists, not for engineering solutions and it is certainly not justified by practical use. The justification is the understanding it gives to scientists. Having said that, there are examples of technology, in the new medicine for example and in my own work on fisheries ecology, that lean heavily on understanding evolution by natural selection.
Perhaps then, the main effects of the theory for non specialists lies in the social and moral consequences of knowing it or misusing it.
A proper understanding of evolution tells us that there is not a linear progression of organisms from the lowly to ourselves at the top, a line of ever improving design. Indeed evolution shows the diversity of life being the result of gradually filling in the space of all possible design solutions and shows each species to be well suited to its circumstances, rather than a step on some evolutionary ladder. This shows the popular idea of a hierarchy of organisms to be a fallacy. The theory also tells us that however creation occurred, it was a process beyond the present capabilities of humankind. We now know it is a lot easier to destroy a species than to create one. This fact certainly has ethical implications - for believers and non-believers alike. Indeed, here we find agreement between evolutionists and the ethics of most faiths that is in sharp contrast with the false ethics of neo-classical economics.
A notable example of the misuse of the natural selection theory is called 'social Darwinism', though examination of it quickly reveals that it is in fact a restatement of classical economics and conflicts sharply with natural selection. Its outcome is a society in which only the most aggressively selfish people prosper (or even survive), leading to a dramatic fall in diversity - precisely the opposite result to that predicted by natural selection. Much of the abuse of the theory arises from sloppy thinking, for example "survival of the fittest" does not imply only the fittest should survive, but has been taken as justification for the destruction of all but the 'fittest' - an ideology at the core of Naziism and a consequence of the right wing neo-liberalism beloved of so many American self-styled Christians.
I believe it is no accident that these are the same people responsible for attacks on evolutionary theory. I am compelled to comment, as a Christian, that it is very sad that active and creative fellow Christians are spending their time on such academic, futile and ultimately counter-productive campaigns. I have yet to find a single example of someone put off God because they have been told that evolution by natural selection is the best scientific theory we have to explain the origin of species. I know several who are put off by right-wing 'evangelists'. Any scientific theory about the origin of species is irrelevant to Christ's core principle: love God and love one-another.
Contrast this with neo-classical economics which is about how we live, and how we relate to one another. The application of economic theory regulates what most of us do as work, it orders priorities in public life, determines the levels of poverty and wealth and is a major factor in power relationships among people. Christ had plenty to say about such matters. The theory of evolution simply provides a scientific explanation of natural phenomena which may be useful to some sorts of biological research. In scale it is a speck to the economic plank.
If the assumptions of neo-classical economics were correct, then it would all be a very serious violation of Christ's teaching, happily they are not. If classical economic theory remained as it was intended, a mere mathematical model of market place behaviour, then the only sin would be one of poor modelling. The really serious assault on Christian values comes from the modern practice of using these abstract assumptions as normative injunctions: that we should be selfish, that we should consume without restraint and that we should not concern ourselves with the consequences of our economic behaviour. To follow these precepts is indeed a sin.
Where then is the Christian outcry against the modern economic ideal? Where are the pamphlets and public meetings railing against the delusion of the market? Where are the preachers seeking to open people's minds to alternative, Christ-centred economic theories? They have yet to be found.
In summary, the theory of evolution by natural selection is useful and successful in enabling scientists to understand biological observations, but has had little practical effect other than as a victim of misrepresentation. The worst offenders are those right-wing religious fundamentalists who attack evolution. They do this to distract us from Christian criticism of their real faith. It is a faith in the dog-eat-dog economic competition that leads to violations against people, society, nature and Christ. I hope that biologists will make common cause with true believers to lead humanity in respecting nature, leaving room for its diversity to flourish as God intended.