50:50 is the magic ratio.

topic of the day 052 & video of the day 047

why are there as many males as females?

Youtube channel: It's Okay to be Smart

What, who, when, how, why? Then, so?

Evolution is all about reproduction.

The equal ratio of sex in nature is so common that nobody would question why is it so.

You know that when male (XY) and female (XX) chromosomes shuffle and combine, there's a 50% probability of either sex. As I've mentioned in the previous entry in this segment, getting your genes from one generation to another is the point of evolution, so why don't they have more females to benefit that since sperms are cheap to produce and males make a lot of them. Since childcare has always been done by females even in nature, this would lean towards that.

Let's use that same example given by Joe:

In a population of a mysterious species, in which out of 10 babies born, 9 are female and 1 is male.

Every male will get to mate nine times as often as every female.

>If every mating results in 2 new babies, then in the next generation, each male's genes are carried on by 18 offsprings but each female 'on average' get their genes into just 2.

Fitness in biology means makes-the-most-offspring, and the fitness score for this population is every male has 9 times the fitness of every female.

> On the flip side, if the genetic changes due to evolution and it produces 9 male kids for every female kid, those offsprings would have a huge advantage. One of these males would produce roughly 4 and a half times more grandchildren than a male without the change.

This mutation would spread in a few generations and you'd have more males than females in this population, hence defeating the purpose of the advantage eventually.

Now, any parent with a mutation for extra female offspring would be favoured to move back the sex ratio towards female, as this process would be repeating again and again through time as the population seesaw from one side to another ... until a mutation arises that results in an equal amount of offspring.

This magic 50:50 sex ratio is the only one that is evolutionarily stable.

It's not only just about the number of offspring, it's also about the time and resources parents invest in their offspring, with the magic ratio resulted from equal investment in their male and female children.

If investment is unequal, like if males require twice the parental care of females, you'll expect to see twice as many females than males. Here each female has less chance to pass on their genes, but they cost half as much so it evens out in that department. Take Australian brushtail possums for example.

There are many exceptions to the rule but it is extraordinary how the 50:50 rule transcends species and how universal it is, from species where the parents care for their babies to those who don't, those who have many partners or just one, and even in species where one sex dies a lot more than the other.

This just shows the power of natural selection to shape the characteristic of a population. 

If love is a battlefield, at least each side is evenly matched.

~ Joe Hanson, a Ph.D. biologist ~