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Alex Bäcker's Wiki / Why female eggs are all produced by birth while sperm are produced continuously over a lifetime
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Why female eggs are all produced by birth while sperm are produced continuously over a lifetime

Page history last edited by Alex Backer, Ph.D. 13 years, 4 months ago

The reason sperm don't last forever is that it'd be awfully heavy to carry 270 liters of them around.


By making them all at once rather than over a lifetime, ova are likely to undergo less mutations with respect to the carrier's genome, thanks to less mitotic events in between. Thus, by not being produced monthly but rather released monthly, ova are genetically more similar to their mother's genome, which is a conservative strategy. For females to follow a more conservative strategy makes sense for two related reasons, both of which boil down to the number of gametes produced by each sex.


First, because the female can only produce one litter every gestation period at most, it is the most selective of the sexes in terms of reproductive strategy. Males, in contrast, can have a much higher number of offspring and each offspring has a much lower opportunity cost (it does not stop its father from conceiving more). Thus, in general terms, males will impregnate more or less any healthy young female that will let them, whilst females are more picky about the genes they will spend a good portion of their lives rearing.


Second, sperm undergo intense competition, with only one or few sperm achieving fertilization of an egg out of more than 100 million in each ejaculate. In contrast, the egg ovulated each month is a female's only chance at reproduction for the month --for the next many months if she's successful and gets pregnant.


Thus, females cannot afford to risk a highly mutant egg. Females who produce eggs more similar to themselves will get more of those eggs to survive. Males who produce mutant sperm run much less risk, in contrast, as most of those severely deficient will be weeded out by the competition prior to conception. Thus, the need for variation in evolution is best fulfilled by sperm, which undergo much heavier selection than ova do, eliminating all but the fittest. In other words, the abundance of sperm makes it safer to experiment with genomes that are more different than their progenitors' in sperm than it is in females, whose single ovum released each month must be fit enough to sustain a healthy life through reproduction or else face a severe selective penalty.


A prediction of this theory is that the average genetic distance between an animal and his dad is higher than between it and his mother, or that most genetic innovations happens in sperm. This can of course be tested.

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