Okay, so what do I mean by 'mermaid' here? A half-fish, half-woman creature, AKA a typical, standard mermaid.
Why did this happen?
Long ago, there was an event called the Trifold Clash involving a certain magical structure called the Source Gem. This Gem takes essence from those it contacts and uses it to transfer traits from the creature it last contacted to the next creature it contacts, and the energy released in the aforementioned Clash caused an awful lot of essence to go around.
This resulted in a lot of otherwise impossible hybrids and chimeras, including mermaids. However....
Magic Has Rules
Despite their being magical creatures, mermaids still play by the rules of genetics. In other words, natural selection is still a thing for them. What I'm wondering is how this would change their 'final form,' their design. Restated, my question is How Would Natural Selection Influence Mermaid Physiology?
- What I'm looking for is a realistic mermaid concept, based off the 'traditional' mermaid portrayal but advanced by natural selection into an optimal form. Yes, some parts will be mostly (if not totally) handwaved, like breathing air and water (covered by this question.)
For example, please take first the chimerical design of the typical mermaid: normal skin above the waist and scaly skin below said waist. It seems obvious natural selection would change this, no? But, seeing as my mermaids will start out like this, is it more likely that they'll simply develop tougher skin (like a dolphin's) on their upper half or end up covered entirely in scales?
Second, the fishtail. Normal depictions of mermaids have just a caudal fin, but this seems highly unlikely; fish have more fins than that, for good reason, so why not mermaids? As logical as that is, I lack the knowledge necessary to determine which fins they'd gain (result) and why that'd happen (to explain why).
Basically, I'm asking how natural selection/evolution would adapt a population of 'typical' mermaids over time, and what the final result would be.
Please Note: I have decided (for now) that mermaids will be a female-only species, viable with regular humans (this bit is handwaved, don't worry about it), which means that things like long hair will be akin to a peacock's grandiose tail. As this will reasonably impact answers, I felt it best to add this now.
Also Note: Yes, mermaids are as intelligent as humans, just with a 'fishy' influence. This means they have extremely good long-term memory and spatial awareness, which is good for navigation, and is a prime factor behind their highly social, close-knit and cooperative culture.