The only thing I would like to add is that you should either skip or change the phrase 'beneficial trait'.

If changed, I'd say 'a trait that is selected for'.

In some cases we can predict/suspect that certain traits will be selected for in nature, but those are usually isolated cases. In general it is impossible to make predictions.

Lets take the example 'size'. There are of cource many genes involved in determining the size of an individual, but cases are known where a small mutation can dramatically change the size of an individual. Darfism is a famous example, but there are more.

In any case, there are advantages to being big and there are advantages to being small. So a 'big' gene is not inherently 'beneficial', neither is a 'small' gene (leaving aside many disadvantages dwarfism causes aside from just a smaller size).
So a trait is only beneficial when placed in an organism, in a population, in an ecological system.

Hence there is basically no way of saying that a specific trait is beneficial. One can test a specific trait in a specific organism, population and ecological system and expect the results to replicate the next time a similar trait is introduced in the same way, but then it's still problematic to call it 'beneficial', because it is not inherently beneficial, but only 'preferably selected in the specific situation'.