Masturbation and Evolution: Or Don't Cry Over Spilled Sperm

youtube.com

Newsweek had an interesting article (Why Masturbation Helps Procreation)

observing something that should fall under the Duh category, but unfortunately remains controversial thanks to fundamentalist nonsense propagated by people like Christine "I'm Not a Witch" O'Donnell.   

As Newsweek kindly put it, "O’Donnell’s mid-’90s crusade against autoeroticism wasn’t based on science."

 Those who adhere to the "every sperm is sacred" idea have absolutely no idea how many sperm are produced each day (about 100 million) or how many are present in each ejaculate (about 50-200 million). If a man were to have several young wives and impregnate them all several times, only the tiniest fraction of his spermatazoa would ever end up becoming people; there is no sperm shortage, in other words, far from it, and only one healthy sperm is needed to fertilize an egg (so it's the proportion of healthy sperm in each ejaculate, not the number, that matters).

What first stimulated Charles Darwin's interest in what became known as natural selection (versus artificial selection practiced by animal breeders, gardeners, and farmers) is that so many more seeds are produced than could ever lead to offspring, and so many more fertilizations occur than could ever lead to a surviving reproducing organism, and that the odds against any seed therefore passing on its genetic material into the next generation asymptotically approaches zero (he didn't quite use the mathematical language, but the idea is the same). 

Nature is shockingly wasteful with its output of seed in all species.  Only a tiny minority of seeds ever become offspring.  Most offspring of most species die young (including ours until recent advances in hygiene, nutrition, and medicine).   Those individuals who don't die possess some advantage, often randomly acquired, over those who do. Those traits, if genetically based, will more likely pass to the next generation if the survivors reproduce and their offspring survive. 

It's all quite brutal ("red in tooth and claw" as Tennyson put it) and frankly depressing when you think about it too much, but on balance none of us should be here.  The fact that we are is so shockingly, freakishly small.  

Let us consider the odds of one particular sperm and one particular ovum from our parents coming together in the year of our birth.  (This actually narrows the odds considerably, since either of our parents could have chosen another partner or none at all, and it only looks at one year, but the odds against us being here even with these constraints are astronomically high.)  

Since a conception requires a sperm and an ovum coming together, the number of possible combinations can be computed by multiplying the total number of sperm produced by our father in a single year (100 million per day x 365 days = 36.5 billion sperm per year) times the total number of eggs made ready for fertilization by our mother in a single year (let's assume 12)  leading to 438 billion possible sperm-ovum combinations!  Out of those 438 billion possibilities came the "you" that is reading this right now.   

But even this is understating the odds, since when she is born, your mother had all of the eggs she would ever ovulate (whereas your father's testicles were busy producing sperm every day of his adult life).   How many eggs?  Perhaps 2 million.  But as part of nature's plan, about 11,000 of those potential brothers and sisters of yours die every month until she reaches puberty, when she has about 300-400,000 eggs remaining.   But even after she reached reproductive age, your mother was losing about 1,000 eggs per month in a natural pruning process. 

So just as the someone reaching into a bag to grab a ball at random has to count all of the balls in the bag to determine the odds of pulling out any single ball, to determine your odds of being fertilized from the particular egg that became you, you really should multiply the 36.5 billion sperm by the total number of maternal eggs, any one of which could have been ovulated at the the time your father's sperm came swimming upstream, beating out all the competition.   Multiplying 36.5 billion (the annual sperm production) times 300,000 (your mother's total egg supply) and you get an astonishingly tiny 10.95 million billion to 1 (that's 10.95 x 10 to the power of 16).   Those are the odds of you being you.

On average, none of us exists. It's absolutely mind-boggling, but another reason that those who get misty-eyed about frozen embryos or stem cells do not understand the astonishing wastefulness that is not only an inherent part of reproduction, but the absolutely critical importance of that wastefulness to genetic variation as well as viability of offspring.