Learning about evolution from species who thrive without sexual reproduction
nautil.us
nautil.us
> 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.
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.
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.
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.
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.
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.
(The interplay of Eusociality and genetics is one of the most interesting wiki dives I ever did, highly recommended it)
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
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?
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
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...
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.
This seems to be a hybrid species that resulted from the sexual reproduction of two different species many thousands years ago.
From the 3 sets of DNA, 2 are from the ancestral mother and 1 from the ancestral father, and using this trick, each generation manages to keep the whole DNA information of the original two without loosing it, like it happens in the sexual reproduction, and also provide enough genetic variation in newborns to allow evolution.
So I would say that basically, each individual is the child of that ancestral pair that generated this lineage.
(They also look very sexy -- to several different species -- allowing them a wide pool of partners to "embrace eternity" with.)
https://en.wikipedia.org/wiki/Address_space_layout_randomiza...
addendum. The subtitle ("What we can learn about evolution from species who thrive without sexual reproduction") is more descriptive.
In general I tend to agree title is not well fitted and the subtitle does better:
> What we can learn about evolution from species who thrive without sexual reproduction.
One can only speculate that their editor came from a publication with a habit of writing clickbait titles.