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Temperature-based determination existed before genetic one. Mammals, birds, and some other animals including many insects evolved genetic determination because the temperature of their egg was too stable to serve as a random way to assign sex. If the eggs have varying temperatures, temperature-based sex determination is the simplest way.

Hermaphrodism existed before non-hermaphrodite species. When sex was first evolved in the first eucharyote, they evolved into hermaphrodite species. Male and female sexes evolved later. Most likely because male animals were more successful at forcing their mate to do the female role which is more energy demanding, so they could breed more.

The question should be, what benefit does a determined sex have. Being a hermaphrodite doubles you're chances in a partner.

My guess is, the more complex the organism the harder a sex change is, so it's only beneficial in certain circumstances. Snails for example move very slowly and their senses aren't the best, so it's good if you can make children with every other individual of your species that you meet.

Sex change can often be found in fish, ocean is vast and it can be dangerous to venture out of your part of the reef, so it's great if one of your buddies can become a female when your original female dies. Or you're a male moray eel and never get to make little morays because your the runt of the reef, your genes would be lost, but at the end of your life you become a female and bam youre genes get to live on.

Then you have species with no need for sex, prospering with only female members.

Also male and female isn't answered with a simple XX or XY chromosome pair. There are species of frogs where in the north XX are female and XY are male, but in the south XX are male and XY are female and in the middle 50-50 chance. Sex isn't determined by only one chromosome. It's far more complicated and can differ between species.

Most of the time the answer to such more general questions is because it works and isn't a deadly disadvantage. To get a more specific answer you'll have to look at a more specific scenario (aka a specific species or environment).

As I recall from school, all reproduction is asexual or sexual. But it gets complex because asexually reproducing bacterium are able to laterally transfer genetic material. And then there are viruses out there incorporating themselves into genomes. And so it appears to me that the very basis of reproduction has and-ifs and it just gets more sophisticated from there.

For example, environmental factors are able to affect genetic expression as you observe with reptiles.

However, if you take a planaria (flatworm) and mechanically split it in the proper manner, you can get two individuals.

And then we toss in hermaphrodism: plants don't stop and go ahead and fertilize themselves because their genetics provides for two sets of sexual organs. The angiosperms have been very successful at an evolutionary level as we are able to witness by so much green.

>Why do some animals have sex determination which is not genetically determined?

So your Q is about "environmental sex determination" and the mechanism is genetic. The organisms have the genetic ability to express all sexual phenotypes and the egg of a reptile makes a genetic decision to express a certain set of genes based on an environmental cue if hot or cold. It's just a game time decision.

https://en.m.wikipedia.org/wiki/Environmental_sex_determination

> Most of the time the answer to such more general questions is because it works and isn't a deadly disadvantage.

I didn't really think about this but yeah, it makes a lot of sense.

> Also male and female isn't answered with a simple XX or XY chromosome pair. There are species of frogs where in the north XX are female and XY are male, but in the south XX are male and XY are female and in the middle 50-50 chance. Sex isn't determined by only one chromosome. It's far more complicated and can differ between species.

Learning something new everyday it is!

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>Temperature-based determination existed before genetic one.

Specifically, there is a hypothesis that the very first amniotes (reptiles, birds, and mammals are all amniotes) used temperature-dependent sex signalling, and only evolved genetic sex signalling later. There is some good support (phylogenetically) that the first amniotes did not have sex chromosomes but used temperature instead...and that sex chromosomes independently evolved multiple times within the amniotes.

The best model for why any species would adopt temperature-determined sex is probably the Charnov-Bull model. Simply put, the model predicts that in some species, the temperature at which they develop and hatch has a different effect on males and females, so temperature-dependent sex signalling gives you the best possible fitness in your males and females. For example, maybe a species lives in a place with cold winters, and lays eggs early in the spring. And let's say that species is best served by more females surviving the winter than males. If females develop in colder eggs, then eggs laid earlier in the springtime nesting season will all be female. That head start means the females will be bigger in the fall when the first frost hits. Males might then develop later in the nesting season and be smaller when the first frost hits, and a few more males may die during the winter, but the species as a whole preserves more females overwinter this way and improves its odds of surviving.

Of course, both can be true; Charnov-Bull could be the reason why the first amniotes used temperature as a sex selector, and part of the reason that some species have kept it to this day...but obviously many later evolved sex chromosomes (including us warm-blooded mammals).

Wow, that's a pretty solid model.

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