Suppose male births are less common than female.
A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring.
Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them.
Therefore the genes for male-producing tendencies spread, and male births become more common.
As the 1:1 sex ratio is approached, the advantage associated with producing males dies away.
The same reasoning holds if females are substituted for males throughout. Therefore 1:1 is the equilibrium ratio.
1:1 is really only a theoretical balance point. 1:1.05 is better balanced because boys and men have a higher rates of death. (every year except right before puberty ~10 and 11 oddly enough.) https://www.ssa.gov/oact/STATS/table4c6.html
PS: Men are around 3x as likely to die at 20 than woman. And the oldest woman lived 6 years longer than the oldest man. The 10 oldest living people are all women. https://en.wikipedia.org/wiki/Oldest_people
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.
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.
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.
The original question is... assuming that we have mandatory sexual reproduction, why do we have a 1 to 1 ratio?
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]
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.
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.
Changes to average beach temperatures can skew sex ratios in hatchlings. (Will find for refs but mobile right now)
Could this mechanism of rebalance work for turtles, even though the mortality of hatchlings and juveniles might be most important to the survival to breeding age, and subsequent numbers of offspring?
(The interplay of Eusociality and genetics is one of the most interesting wiki dives I ever did, highly recommended it)
I believe this would be advantageous in species with longer gestational periods where you'd want to put as much of the fitness test up front and not wait until the babies are born to eliminate the weaker offspring.
Its these kinds of lines that make it very hard to take this analysis seriously.
Ouch
Incidentally, the cost of males is only a 50% overhead when the males contribute nothing to raising the offspring. In some species, the energetic/resource cost is dominated by caring/feeding/raising long after birth, which males may contribute to as much as females.
A lot of mammal species eliminate excess males using other methods - mainly bloody fights to the death, which also tends to result in the remaining males being the strongest ones.
The mechanism is much more complicated than a 50/50 chance of inheriting some chromosome.
The second answer is that there are indeed cases the environment can influence the chances for either male or female in several species. For example, mostly for reptiles, https://en.wikipedia.org/wiki/Temperature-dependent_sex_dete...
> Chris Jankowski • 2 hours ago
> The author says that sex "imposes a huge cost on a species, and that cost is called “males.”" Well, if the males are such a cost to a species, then why nature does not respond by heavily skewing the sex ratio of the offspring towards females. Males produce sperm which is cheap compared with the cost of producing eggs. A male could fertilize eggs of many - tens or hundreds of females. So the ratio could be 1 to 10 or even 1 to 100. But I do not know of a species that would do this.
> Some marsupials - possums and kangaroos create some skew, but is is slight and in response to environmental conditions or is more sequencing oriented depending on the age of the breeding female.