He fleas and she fleas

Originally published 26 February 2006

Has there ever been a more astute observ­er of the war between the sex­es than James Thurber?

With wry words and acer­bic pen, Thurber chron­i­cled the irrec­on­cil­able inter­ests of men and women in essays and car­toons that appeared in The New York­er dur­ing the 1930s, ’40s and ’50s.

There has been a sex­u­al rev­o­lu­tion between Thurber’s gen­er­a­tion and my own, but human nature does­n’t change, and there are enough innate dif­fer­ences between men and women to keep us laugh­ing at Thurber’s wit. Women may protest that he was a misog­y­nis­tic old fud­dy-dud­dy, but males of my gen­er­a­tion are pret­ty much con­vinced he got it right.

One famous sequence of Thurber draw­ings is called “The War Between Men and Women.” The con­flict starts with a man toss­ing a drink into the face of a woman (we do not know with what provo­ca­tion). High points of the sto­ry come with the “Cap­ture of Three Physics Pro­fes­sors” by a smirk­ing, gun-tot­ing lady, and the “Sur­ren­der of Three Blondes” to a trio of bewil­dered, bow-tied gentlemen.

Thurber’s females are gen­er­al­ly larg­er and more assertive than the males, who are often depict­ed as timid, mus­ta­chioed mil­que­toasts. At the end of Thurber’s war, how­ev­er, men come out on top. A scowl­ing female com­man­der hands over the sym­bol­ic base­ball bat to her bemused male coun­ter­part. La plus ça change, la plus c’est la même chose.

And then, of course, there’s that lit­tle clas­sic of a book he wrote with E. B. White called Is Sex Necessary?

It’s a peren­ni­al ques­tion, one that has befud­dled evo­lu­tion­ary biol­o­gists ever since Dar­win. To put a fin­er point on the ques­tion: Why are males necessary?

In many species, males do lit­tle except pro­vide sperm, then head off to the ball game or the club, so to speak. The female gets stuck with all the work of car­ing for the offspring.

If nat­ur­al selec­tion is all about insur­ing the sur­vival of an indi­vid­u­al’s genes, then the rate of genom­ic repli­ca­tion would be increased if all indi­vid­u­als of a species were born female and repro­duced asex­u­al­ly, with­out need of males. As Ras­mus Nielsen writes in the Feb­ru­ary 17 [2006] issue of Sci­ence: “[Asex­u­al­ly] repro­duc­ing females aris­ing in a sex­u­al pop­u­la­tion should have a twofold fit­ness advan­tage because they, on aver­age, leave twice as many gene copies in the next generation.”

Yet most com­plex organ­isms are sex­u­al. So how did sex­u­al repro­duc­tion arise? And why is it maintained?

It is com­mon­ly held that shuf­fling genes between two sets of chro­mo­somes helps counter the effects of harm­ful muta­tions. In the anal­o­gy of the geneti­cist John May­nard-Smith, it’s like owing two cars, both of which have a (dif­fer­ent) non­func­tion­ing part. By mov­ing parts about, you can often get at least one car going.

In the same issue of Sci­ence, Susanne Paland and Michael Lynch present a nifty exper­i­ment that shows males do have a role to play in keep­ing genes intact.

Paland and Lynch worked with pop­u­la­tions of the water flea, Daph­nia pulex, which con­ve­nient­ly comes in two vari­eties: sex­u­al and asex­u­al. They watched 14 asex­u­al pop­u­la­tions (all female) and 14 sex­u­al pop­u­la­tions (male and female). And lo and behold, as the the­o­rists guessed, the sex­u­al pop­u­la­tions had a low­er rate of dele­te­ri­ous muta­tions. Four times lower.

He fleas do appar­ent­ly help she fleas get rid of bro­ken genes. Hav­ing a male around is like keep­ing a junk car in the yard to use for parts.

But wait, that sure­ly can’t be all of it. What I want to know is: Among those pop­u­la­tions of all-female water fleas, who car­ries out the garbage? Who fix­es the toi­let when it won’t stop run­ning? Who does the barbecuing?

If males are only around to pro­vide back­up genes, where are they when you need them? Watch­ing a bowl game with their bud­dies at the local bar. Fish­ing. Asleep on the couch.

Does the adap­tive advan­tage for females of gene-mix­ing com­pen­sate for the cig­ar ash­es on the car­pet, the smelly socks under the bed, and the dirty dish­es piled in the kitchen sink?

Pop­u­la­tion geneti­cists are get­ting bet­ter all the time at observ­ing and track­ing muta­tions, but which of those colonies of water fleas are hap­pi­er, the sex­u­al or the asexual?

It may be that male water fleas exist to help female water fleas keep their genes in good repair, but when it comes to under­stand­ing the impor­tant human stuff, like who ends up with the base­ball bat, I’ll stick with Thurber.