Only thing I can think of off the top of my head is that the temperature is linked to food availability, when food is available its fine to make a bunch of females since their off spring wont have to compete for food, but if food is scarce then its better to make males, the ones best suited for survival will end up getting to mate, so its as if nature is using the food scarcity as a filter to get rid of bad genes

It doesn't have the intentionality of A/B testing, but it does allow for genetic divergence and drift, and it is subject to survivorship bias. Instead, it's survival by brute force calculation, with a high degree of casualty.

Edit: Changed a word to avoid redundancy.

The way I understood it, crocodiles evolved before sex determining chromosomes like X and Y, or sex determining ploidy as seen in some plants and formics.

There's no reason for them to change the way that they do things, do they haven't.

[removed]

It could be that most mutations do nothing bad, or that they end up being harmful in time, such as cancer. Here is one paper on the topic:

https://www.sciencedaily.com/releases/2022/06/220608112504.htm

And yes, that was my point. Evolution tends to work slowly, over millions of years, due to the low chance of a beneficial mutation occurring, and then also being selected for in a given environment. And, since environments change, what was once beneficial, may end up becoming harmful over time.

[removed]

it's not uncommon for traits to evolve across "fitness valleys". That is, a trait with positive fitness that requires multiple generations to evolve with intermediate generations having negative fitness.

With a large population and a lot of time, random variation makes it possible to evolve across fitness valleys.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2711507/

[removed]

it's not uncommon for traits to evolve across "fitness valleys". That is, a trait with positive fitness that requires multiple generations to evolve with intermediate generations having negative fitness.

With a large population and a lot of time, random variation makes it possible to evolve across fitness valleys.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2711507/

it's not uncommon for traits to evolve across "fitness valleys". That is, a trait with positive fitness that requires multiple generations to evolve with intermediate generations having negative fitness.

With a large population and a lot of time, random variation makes it possible to evolve across fitness valleys.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2711507/

That's not quite true. I work on mathematical models of evolutionary processes and it's not uncommon for traits to evolve across "fitness valleys". That is, a trait with positive fitness that requires multiple generations to evolve with intermediate generations having negative fitness.

With a large population and a lot of time, random variation makes it possible to evolve across fitness valleys.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2711507/

No, it needs to be not worse than. Better is nice, but it's not necessary.

The selection process is often not about gaining an advantage, it's about not having a disadvantage. 'As good as' is usually just fine for evolution.

>Why does temperature determine the sex of certain egg laying animals like crocodiles?

well explained . I have also go through the topic TSD. It is also true that a lot of work has left to be done on this mechanisms .

[removed]

So you asked why/how temperature-dependent sex determinism works and a lot of people mentioned greater whys and whats but not a lot about the the how.

And to be honest, we don't really know the exact microbiological pathways. There are three main theories https://www.sciencedirect.com/science/article/abs/pii/S0306456518304170 as to how this happens. https://www.ncbi.nlm.nih.gov/books/NBK9989/ But the main thing that scientists agree on is that there is an organ(s) that measures/monitors the environmental temperature, and then that sends signaling molecules that allow the gonads to develop as male or female, and then the gonads will produce additional sex determining hormones that will influence how the rest of the body will develop. One such base hormone (or a critical hormone in the signaling path) is aromatase. When scientists injected aromatase inhibitor into developing lizards, those lizards became male.

So the following theories are as such.

1. The yolk contains steroids (cholesterol based signaling hormones) that are released at certain temperatures.
2. The brain releases neurotransmitters as a general signaling hormone that are released at certain temperatures.
3. The gonads themselves react to the environment temperature epigenetically. This is the theory that is the biggest departure from the others as it doesn't specifically target a separate messenger hormone (even though one may or may not be present).

And while it's unknown specifically how these pathways form. The generic biomolecular pathway is pretty straightforward. Temperature can affect the shape of proteins directly. This is how some of your temperature receptors can figure out if things are cold or hot. A secondary method of temperature monitoring can happen by monitoring side effects of temperature. Chemical reactions vary based on temperature; generally reactions are faster the hotter it is. And by measuring the activity of certain chemical reactions (measuring the concentration of end/by-products, etc.) cells can determine the temperature. Then, that temperature protein can start a signal chain cascade to produce male hormones or female hormones depending on the environment.

And this is where the three theories take over. Scientists don't quite agree on where this temperature monitoring happens and which signaling cascades are involved.

But once those signals reach the gonads, they will influence the development into juvenile male or female gonads, which will then start to produce additional sex hormones that will further cement the development into male and female reptiles.

One thing that is important to note is that all fetuses start without sex differentiation. Embryos do not care about XY chromosomes, sex hormones, etc first. until a few weeks in. Male or female, all fetuses need lungs, heart, brains, intestines etc. This undifferentiated fetus stage is what allows fertilized reptile eggs to become either male or female based on these hormones. Fun fact, in humans, all babies start off development as females.

Hormone based sex determinism is actually very common and is one of many many many sex mechanism in even humans. One specific case is Androgen Insensitivity Syndrome. While it is a complex condition that affects many people, its base mechanism is that one of the sex determining hormones acts like the aromatase that I talked about in lizards. This hormone, androgen, is what allows human fetuses to halt development of female anatomy and promote male anatomy. Those affected by AIS are genetically male, but possess a lot of female anatomy and do not possess male anatomy because androgen could not act as a sex differentiating hormone. Due to mammalian XY genetics, however, their gonads are still proto-testicles and do not produce secondary sex hormones that promote the development of female anatomy. Nearly all people with AIS do not have uteruses or a fully formed vaginal cavity and are considered infertile. I hope I have not done the AIS community an injustice with my bullet overview of a complex topic.

The main point I want to get across is signal pathways, signal pathways, signal pathways. Complex multicellular organisms need a way to talk to itself and signal pathways is the method.

TLDR: Temperature -> temperature-sensitive protein -> signal hormone(s) -> sex differentiation -> additional signal hormones -> male or female.

[removed]

I cannot wait for the day I can replace my internal sinus structure with something more cyborg-y than the reconstruction I already had done once, which is gonna have to get done again (deviated septum due to allergies leading to infections, swelling, etc...) I feel so bad for brachiocephalic animals because I know how it feels having a smashed up nasal passageway that doesn't work properly, combined with asthma issues. I'd take a giant hook nose and my general skull structure elongating in that area by a half inch, if it'd mean no chronic dryness and other problems, but I know that any biological solution is still going to have the allergies pitfall. So cyborg it is! Maybe I'll get the upgraded model with higher sensitivity to certain smells (custom tuneable of course, nobody wants to smell themselves on the toilet)

You can also have a later mutation that randomly takes advantage of a past mutation that was neutral or even somewhat detrimental to confer a significant advantage that wouldn't happen alone.

Yes. Evolution doesn't do A/B testing. If it survives and breeds lucky you.

Something I haven't seen discussed here is the answer to your title when taken as the whole question; a not insignificant amount of water is "lost" to space every second in the form of H2 leaving our atmosphere. Said H2 can't be combined into watermolecules anymore, at least not on earth. Some molecules get enough kinetic energy (temperature) to simply shoot out of our orbit, others get taken away by solar winds.

[removed]

[removed]

Not sure what you’re getting at here, can you clarify? What does this have to do with temperature-dependent sex determination?

Something like a virus could be considered “efficient” in that it has a small genome because it hijacks host machinery to replicate, but it can’t be “self-contained” due to this reliance on a host.

But genuinely, what are you trying to say here?

> Most mutations are not good...

Based on what exactly? A single mutation isn't likely going to do much unless it happens in the correct place.

> It may be something more akin to flipping 50 or 100 heads in a row.

Which is rather easy if you've got millions of years to do it.

It might not even be that it worked better, but just that it worked well enough in context of how well-adapted the species was at the time overall.