The chance of random variation acted on by selection achieving a particular goal identified in advance is minuscule. The chance that the same mechanisms will achieve some lasting changes that we can look back on and call “evolution” is a near-certainty. (It’s the distinction between the chance of me winning the lottery – unfortunately, minuscule – and *someone* winning the lottery, which nearly always occurs, week by week.)

I realize that you use this only as shorthand, but it is indistinguishable from a hybrid between the teleology of Lamarckism and the inevitability of orthogenesis. Given that Lamarckian (mis)understanding of evolution is very common among students and non-experts, I would stay far away from anything that will only exacerbate confusion.

What really happens is still what I described. Why did gazelles evolve longer legs? Because undirected (“random”) mutations that produced longer legs were preserved in the population while mutations causing shorter legs were eliminated as a result of a selection pressure imposed by fast-running predators.

More specifically, the genetic composition and associated phenotypic characters of the population changed because longer legs, which resulted from a mutation(s), allowed those mutant alleles to be passed on more often than the alleles associated with shorter-legged, slower-running phenotypes.

Gazelles evolved longer legs “because”, not “so that”.

For example, once an ur-gazelle type committs to “running away” as their primary response to predators, any change that makes them better runners (other things being equal) is more likely to stabilize, while anything that interferes with running, must offer a drastic advantage to be retained.

At that point, I’d say it’s fair to say, e.g., “they evolved longer legs in order to run faster”. Their prior evolution has bequeathed them a mandate that constrains future developments, to the degree that “being a good ur-gazelle” now implies “running fast”. Rephrasing that in turns of “purpose” may not be strictly equivalent, but it’s good enough for casual discussions.

(I’m ignoring questions of “good enough”, because in the long term that tends to be a moving target.)

In this case, “because” implies “progress from” (and is therefore evolutionary or developmental), while “so that” implies “progress toward” (and is therefore vitalist or teleological).

I like the way you put it….