I find it wordy to repeat
trait X is a trait that confers fitness.
I wonder if I can instead write
trait X is useful.
If not, is there an adjective that describes this property of conferring fitness?
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A trait variant that is associated to a higher fitness than another variant is said to be beneficial or sometimes adaptive. On the other hand, a trait variant that is associated to a lower fitness is said to be deleterious or sometimes maladaptive.
"beneficial / deleterious" vs "adaptive / maladaptive"
The terms are very similar but I would tend to think that there are tendencies of using either "beneficial / deleterious" or "adaptive / maladaptive" depending upon the specific context.
These terms can also be used when talking about genetic variants and not only phenotypic trait variants. In evolutionary genetics, the terms "beneficial / deleterious" are used more often than "adaptive / maladaptive".
Also, in general, the terms "adaptive / maladaptive" are often used in absence of variation for the trait in question in the population while the terms "beneficial / deleterious" are most often used when there is variation in the population.
trait X is a trait that confers fitness does not make much sense as is explained here.
I'm not sure "conferring fitness" is the correct way of thinking about things, since fitness is typically a complex combination of traits, but I think the adjective you are looking for is "adaptive" - though this adjective implies that it has been selected as well as that it has contributed positively to fitness, for any inheritable trait that has been around through a reproductive cycle both would necessarily be true.
From https://www.merriam-webster.com/dictionary/adaptive one of the example sentences is:
Webbed feet is an adaptive trait in aquatic animals.