Determining Effects of Natural Selection on Mermaids

Question

Okay, after a lot of thought, I decided that mermen wouldn't exist in my setting. Why? Because in various IRL situations, balance is necessary for proper function. If men represent land and mermaids represent the sea, the two together represent a perfect balance. (There is still human women, I didn't see any reason to remove them from the setting.) However, this comes with a price.

Due to the constraints of their environment, mermaids will be stuck at pre-medieval levels of technology, as they can't work metal, create fire...in essence, they lack the resources and quite possibly the motivation to develop anything truly advanced. ("We've been doing this for years, why would we change it?" AKA "don't mess with success.")

In other words, part of my premise is the idea that mermaids will survive with primitive technology (coral branches as improvised spears, for instance), focusing instead on the social aspects of life (relationships, altruism, cooperation). This means they'll have an intricate culture, lots of social rituals, and an emphasis on teamwork and relationships but lousy tech.

This also means that mermaids will likely put even more emphasis on increasing desirability than regular people do-firstly, they are only part human, which could very well hurt their chances with potential mates, and secondly, with their society's focus on relationships, plus the fact that their survival literally depends on this and failure threatens the whole species, it seems obvious mermaids will undergo intense sexual selection.

My question is, How Will Mermaids Be Influenced by Sexual Selection?

Specifications:

1. I really don't want or need a ton of detail, just generalities are fine. You can even sum up by saying "this will inevitably lead to a Hollywood phenotype among mermaids," if that's accurate.

If you don't feel such a sum-up is possible, feel free to use the following list of traits and explain how natural selection will influence them:

a. Hair length/color b. Scale color/appearance c. Skin color (maybe texture?) d. Fin appearance e. Fishtail form (appearance, not function) f. General appearance

2. The best answer will be realistic; I am a fictional realist, I put realism into my fiction, so input on realistic mermaid traits will be appreciated. Just know that mermaids are basically magic-born chimeras, female humans altered by an ancient cataclysmic event into the half-fish creatures they are now, so they will have both skin and scales at first, barring natural selection.

I say 'barring natural selection' because such a topic as mermaids having scales and skin or just scales is exactly what Spec #2 concerns, namely, realism. I'm all for traditional mermaid depictions, but I understand that may not be possible when natural selection is applied. Anyway, thank you and good luck!

Answer 1

This question seems impossible to answer since any evolutionary process that results in "all-female species that can inter-breed with humans" is not a process as happens in the real world. Such a process is usually referred to as 'Fictional And Narrative SElective Reverse VanIty Contrived Evolution'

Assuming this is already covered somehow (i.e. by the species being able to reproduce asexually but gaining some benefit from incorporating human DNA, like more variation/healthier offspring), I'd argue that there isn't a tremendous amount of pressure on the mermaids to evolve towards being 'more attractive' than human females. The reason being that they don't really compete with human females in the first place.

Assuming 'standard' mermaids that live in the water most of the time and can breathe both air and water (or hold their breath for prolonged periods), most of their encounters will be with ships full of sailors* which are overwhelmingly male for all of human history. Females onboard exist as passengers and in more modern times as crew, but for most of human history, most seafaring was done by adolescent young males who were at sea for months at a time. If they encounter a group of actual mermaids, not the manatees mentioned in a comment, it does not take a lot for mating to happen. Especially since the men would not need to, or be able to, stay around to raise the offspring. Cultural and moral barriers wouldn't really be able to hold the hormones at bay.

The question is whether the humans would trust the mermaids enough to get close enough for this to happen, since traditionally mermaids are considered a danger to seafolk for various reasons. So the mermaids primary objective would be to 'be helpful' to sailors, and there's a lot that intelligent, aquatic mammals can do in order to be helpful to ships in the age of sail at least. Provide navigational aid, supply fresh fish, perhaps give information on other ships in the area or warn them of shoals and other dangers to ships in the area. To that end they might develop organs that help them navigate (perhaps some kind of internal compass so they always know North?) or otherwise. Presumably, they'd usually flock around areas dangerous for ships to warn them of the danger - or perhaps wait until a ship has run aground so they can take their pickings then.

*Given the nature of the question, I request extra points for not putting the other word there.

Answer 2

You wrong about technology. Mermaids can use radioactive materials in the place of fire.

https://m.youtube.com/watch?v=nvYCUSvW7LA&t=548s

As for traits sexual attraction tends to evolve towards health, fertility and survival.

So for the human half probably be what most humans find attractive. Large breast,(fertility) larger ass (survival), birthing hips (fertility), long hair (health)and so on.

For the fish we would see large muscular tails that can give the mermaids more power when moving through the water. Perhaps brighter colors that can be more easily camouflaged in coral.

Since there competing with human woman there would probably evolve to be much more fertile and more likely to have twins or triplets.