Not a proof
From my biology book:
Overall, natural selection has apparently favored organisms whose mix of membrane lipids ensures an appropriate level of membrane fluidly for their environment.
This is not a proof but, rather, a description; there is no need to mention natural selection at all!
These kinds of descriptive stories are very common when reading about evolution. They seem to think that if you can make up a story that matches the data and that fits in with the theory, then, somehow, this provides evidence that evolution is true.
And thus, there seems to be overwhelming proof for evolution (by "proof" I mean very likely probabilistically). But much of this proof is merely these manufactured unproven stories.
A more accurate statement would be:
Overall, the mix of membrane lipids in the cells of organisms have an appropriate level of membrane fluidly for their environment. Without this, the cells could not survive.
But this does not mean that an intelligent designer created all of these optimized biological structures. If he did, then he is very mean-spirited to have created conscious creatures who suffer as a normal part of life.
Proponents of intelligent design wish to teach it in schools instead of evolution. If they got their way, the quote in my biology textbook would be:
Overall, the intelligent designer has apparently designed organisms whose mix of membrane lipids ensures an appropriate level of membrane fluidly for their environment. This, with no concern for whether such designs result in conscious suffering but, rather, only in whether the design functions as a viable biological machine.
Since the intelligent designer cares about the organisms he designs (whereas natural selection has no such concern), the outcome of experienced conscious existence needs also to be mentioned. What good is it to have an intelligent designer who doesn't care about the consequences of his designs?