Darwin didn't miss much, I think we all agree, and came up with a lot given the limited science of his day.   One thing he missed, that by this time tomorrow will be the source of outrageous titles from every schlock science publication in existence, was that sexual selection that goes on even after actual sex.

Confusing?  It's not so difficult to understand.   Some female critters are trampy and have sex with more than one guy, for example (what, you think other parts of the animal kingdom don't have Jenna Jameson?)  so there's sperm competition but there are also other factors having to do with the internal workings of the female body (i.e. that magical place), so let's catalog a few of post-copulatory sexual selection's greatest hits:

* Male genitalia that “sing” during copulation

* Seminal fluid that invades the female's nervous system and influences her behavior

* Male genitalia that explodes or breaks, leaving parts behind  

* 'Dirty talk' during animal sex

Have you noticed nature seems to have it in for male genitalia?

It was almost 100 years after Darwin's seminal(*) book on sexual selection, "The Descent of Man and Selection in Relation to Sex", that a researcher in Biological Reviews(1) stated that sexual selection occurred after the fact too.  How did Darwin miss it?  Well, he wasn't perfect, so it may have made logical sense (an animal that can remove the sperm of a prior suitor has a reproduction advantage, for example) but he didn't have any way to scientifically know it.

By 1983, 'cryptic female choice' was the highlight of an article in The American Naturalist(2) but it didn't get a lot of traction until the following decade.   Now there's a whole article in PNAS about it - rodents, flatworms, spiders, primates, you name it and there's some interesting male genitalia to talk about and some sexually antagonistic coevolution going on.

But it isn't just animals, post-copulatory sexual selection is happening in plants also.   Aren't many plants hermaphroditic, you ask?  Sure, but that means sexual selection has to avoid self-pollination and postcopulatory selection can help avoid strange hybrids.

It may seem counterintuitive to humans that a fly starts conversation and foreplay after sex has begun but nature has a plan.  
Male insects and spiders use virtually all parts of their bodies as they tap, slam, squeeze, bite, lick, rub, shake, gently rock, or twist the female, coat her with liquid, cover her eyes with semitransparent colored plates, wrap her symbolically in weak silken lines, feed her, wave at her, and sing to her 
It sounds downright sexy.  For her.  For the male, it is a lot more work.

Darwin's omission?  Maybe.  Or maybe he knew he needed to leave a few things for future biologists and he didn't want postcopulatory sexual selection concern to be one more way to add pressure to date night.

Article: William G. Eberhard, 'Postcopulatory sexual selection: Darwin's omission and its consequences', Published online before print June 15, 2009, doi: 10.1073/pnas.0901217106


(*)  HA HA
(1) Parker GA (1970) Sperm competition and its evolutionary consequences. Biol Rev 45:525–567
(2) Thornhill R (1983) Cryptic female choice and its implications in the scorpionfly Harpobittacus nigriceps. Am Nat 122:765–788