If mermaids were real and using real science in fantasy

My WIP features traditional (human torso, fish tail ect) mermaids. However, I want for the world that mermaids live in to somewhat make sense from a scientific perspective. Lately, I've been scouring the internet for information about what mermaids would look like if they were real. From the articles I've read, I have learned how much evolution has played into the way living creatures look today. So fellow AbsoluteWriters do you put as much effort into how evolution has played into the appearance and abilities of the species in your work? Also, when reading a novel that is set on Earth are you able to suspend disbelief if the appearance and abilities of the species in the novel make no sense scientifically?

If mermaids were real and using real science in fantasy