I'm an A level student looking into the fine tuning of various constants. Physicists explain the extensive effects that would happen if these constants were to be changed/different and hence, how this affects the probability of life existing. What I fail to understand is why, if these constants were to be different, life wouldn't adapt to these changes. If gravity was stronger, then wouldn't the general muscle mass/stability of life be greater through evolution in order to withstand a greater force? Or am I looking at it from the wrong perspective? Some clarification on this would be appreciated.
The variation you are talking about here would still be considered relatively 'fine-tuned', in the following sense:
If the strength of gravity was stronger by such an amount such that the processes that govern the formation of stars, planets, complex molecules, and life were relatively unchanged (in that they still take place in a recognizable fashion), then the strength of gravity must be quite similar to what we observe. If this were the case, yes, there is no reason that life might not develop to be a bit tougher.
However, such a difference would have to be very small indeed. Arguments about fine-tuning are based on the observation that even relatively small changes to certain constants would be enough to drastically change the make-up of the universe.
For example, Paul Davies notes that if the strong force were 2% stronger than it is, hydrogen would fuse to form diprotons as opposed to helium as it would be energetically favorable. This would drastically alter structure formation in the early universe, leading to a today where planets do not even exist, let alone weak or strong animals on them. I should note here that the 2% figure quoted by Davies may not be accurate, but this is the idea at play here.
In short, the problems from fine-tuning start to occur far before life would ever develop in the first place.