Bacterial genomes are much, much simpler than animal genomes. They reproduce very quickly, and even if many bacteria suffer from a problematic (or fatal) mutation, others will be OK, and some may even have adaptations to changing conditions.

Animals have much more complex genomes. They are more at risk from mutations, and they cannot reproduce as quickly. There is more incentive to "get it right", if you will. With asexual reproduction, selection occurs at the level of the genome, meaning you are as good as your "worst" gene. You can't get rid of harmful mutations, and it seems like (complex) species that get stuck in asexual reproduction go extinct.

With sexual reproduction, selection can occur at the level of the gene, allowing beneficial mutations or recombinations to spread through a population AND correcting (many) harmful mutations.

The dropout regularization method for neural networks was sort of inspired by this observation, i.e. that recombination (combining genes of more than one individual) makes the resulting structures more robust [1]. The idea is basically that it avoids overfitting to the evolutionary context. This has two effects: the fitness cannot change very quickly in response to single mutations, and, conversely, it avoids that genes co-adapt such that the entire genome becomes inflexible at exploring alternatives.

To achieve the same without sex, mutations would would have both break up the co-adapted genes without breaking their functions and also find a good alternative mutation. With sex, it does not matter if co-adapted genes are broken up, because they are forced to evolve to deal with it. (This is kind of similar to the concept of learning to learn in neural network research.) Also, if a parent gene is unfit, it is possible that the child can escape this mutation by receiving that part of the genome from the other parent instead by a 50% chance.

[1] https://arxiv.org/abs/1207.0580

Bacteria reproduce asexually, yes, but they have developed an even more effective means of introducing variation than sex: horizontal gene transfer.

That sex introduces lots of variation is undisputed, however, and the mechanism of sexual selection accelerates the process over that of the random drift in asexually reproducing species.

horizontal gene transfer is basically what sex is, unless you do it standing up, then I guess it'd be vertical gene transfer.

You're taking it back a step to asking "why sexual reproduction"?

The original question is... assuming that we have mandatory sexual reproduction, why do we have a 1 to 1 ratio?

Because upper vertebrates normally can't exert influence on the sex of the offspring before birth. Your DNA will have an X chromosom at least. In principle [1], you can have either a second X or an Y. 50% probabilities. 1:1.

Only reptile, amphibious, fishes and it seems that very primitive mammals also can do this. Our system to get rid of the excedent is to make males sillier, much more prone to take life-threatening risks than females.

1 [Those aren't our only options, we have the so called supermales (one X, two Y), and superfemales (XXX) but they are not common and some combinations ("1.5 X") are sterile]

> Because upper vertebrates normally can't exert influence on the sex of the offspring before birth. Your DNA will have an X chromosom at least. In principle [1], you can have either a second X or an Y. 50% probabilities. 1:1.

That's not a sufficient explanation, as there could be a different ratio during the sperm cell generation process or selection during insemination or from the egg cells themselves. Y and X sperm cells could easily be handled differently at any point of the whole process.

Humans have a ~1:1.05 F:M ratio so evolution really can alter the ratio. (Though this ratio varies by population.) https://en.wikipedia.org/wiki/Human_sex_ratio

I could have sworn i have read about a species of mice that do not have Y at all, but still produce males.

Bacterial have something far better than sex - horizontal gene transfer [1]. They can share genes with any organism.

1. https://en.m.wikipedia.org/wiki/Horizontal_gene_transfer

Snails are all hermaphrodites. No complicated ratios, sexual reproduction, equality and interesting sex.

Clones don't work well with the evolution. All of the individuals are exactly the same, so they are very prone to a whole population being wiped out at once. The key of evolution are mutations that promote survival of the best adapted. And the bacterias are different from what I understand because they reproduce at extremely fast rates. The more complex creatures could do the same but it would take centuries to do the same number of cycles and they (as a population) just wouldn't survive that long. Mixing genes is thus a better way for bigger creatures.

Corals are an example of animals that can do both. They reproduce asexually through fission, which keeps clones in local similar conditions where they can thrive.

While sexual reproduction occurs through spawning, where genetically diverse offspring travel far and wide on currents to possibly different habitats and niches.

The idea here is that asexual reproduction is a benefit to the species during relatively stable conditions, while sexual reproduction works best to populate new areas or recover from population disasters.

It'd be really interesting to get to see this happen on a wide scale sometime soon. It'd be better not to have to, as well...

Because it leads to overfitting. Sexual reproduction is just more robust to the environment changes.