system
Given the following post, respond as if you're commenting on Reddit:
human
Why can bull sharks survive in salt and fresh water but other species of fish can't? Why haven't most other fish evolved beyond this stage?
gpt
Some other species of fish can — salmon and some eels would go extinct, for example, if they didn't transit from salt to fresh water to breed — and it requires major changes in body chemistry to do it. It's a biologically expensive process, and there are better alternatives for the majority of fish, which is why the majority tend not to do it; in other words, for most fish there's a biological imperative to specialise in either salt or fresh water, so they don't have to put in that big investment in changing how their cells handle changing osmotic gradients. When there's no pressure to evolve, because of that big biological demand and the ability to get everything you need from your current environment, there's less likely to be evolution in that "direction".