ELI5: How has evolution ended up with such clear differences in species and not a blend of everything?
I've just watched a video about thresher sharks, and they are a species of shark who have evolved to have a really really long tail, that they use as a weapon during hunting. No other shark has anything like this as far as I know? And it made me wonder how has that evolved to get to that state, but no other breed has?
Putting my question simply, how does a shark go 'I think I'll use my tail as a weapon' and it eventually turns into a thresher shark, and a basking shark for example goes 'I think I'll just swim around with my mouth open and eat whatever goes in'. BUT, nothing goes 'Sometimes I'll use my tail, and sometimes I'll just have my mouth open' and we end up with a long tailed, big mouthed shark?
Explanations kept purely to sharks would be greatly appreciated but not necessary.