Other half of Darwin's theory passes test

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" Long before it's in the papers " October 13, 2008

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" Other half " of Darwin's theory passes test

Oct. 13, 2008World Science staff

Some flir­ta­tious yeast cells have con­firmed a part of Charles Dar­win's the­o­ry of ev­o­lu­tion that was nev­er tested as suc­cess­fully as the rest of the the­o­ry, bi­ol­o­gists say.

This some­what spe­cial part of the the­o­ry is the con­cept of ev­o­lu­tion through " sex­ual se­lec­tion. "

 

 

 

 

 

 

 

 

 

 

One com­mon ex­ample of a re­sult of evo­lu­tion through sex­ual se­lec­t­ion is the pea­cock's tail. (Im­age cour­tesy G. Ri­ta­ma)

 

 

 

 

In gen­er­al, ev­o­lu­tion­ary the­o­ry holds that spe­cies grad­u­ally change be­cause of cer­tain mu­ta­t­ions that spread through their po­p­u­la­t­ions. These mu­ta­t­ions spread if, and only if, they're ben­e­fi­cial—so that in­di­vid­u­als pos­sess­ing them sur­vive long­er, re­pro­duce more or both. Thus the mu­tat­ed trait ap­pears in­creas­ingly of­ten in suc­ceed­ing genera­t­ions.

Ev­o­lu­tion has been ob­served in ac­tion nu­mer­ous times, be­cause in short-lived spe­cies, many forms of ev­o­lu­tion oc­cur fast enough for hu­mans to watch the changes oc­cur.But one form of ev­o­lu­tion has not been di­rectly seen: ev­o­lu­tion through sex­u­al se­lec­tion, notes a pa­per in the Oct. 7 on­line is­sue of the re­search jour­nal Pro­ceed­ings of the Roy­al So­ci­e­ty B.

This va­ri­e­ty of ev­o­lu­tion is what bi­ol­o­gists be­lieve ac­counts for the ap­pear­ance of sex­u­al-advertising traits such as a pea­cock's bright tail, or per­haps mu­si­cal abil­ity.Such traits are be­lieved to evolve for much the same rea­son as oth­ers: those who have a cer­tain char­ac­ter­is­tic mate more, and thus spread the genes for that fea­ture. The chief dif­fer­ence be­tween this form of ev­o­lu­tion and oth­ers is that with sex­u­al se­lec­tion, the driv­ing fac­tor in the pro­cess is sex­u­al com­pe­ti­tion, rath­er than oth­er ex­i­gen­cies of sur­viv­al more gen­er­ally.

Sex­u­al se­lec­tion is an in­tri­guing as­pect of ev­o­lu­tion be­cause it drives the ev­o­lu­tion of traits that on their face, seem less than clearly ben­e­fi­cial, said Dun­can Greig of Uni­ver­s­ity Col­lege in Lon­don, one of the pa­per's au­thors.

" For ex­am­ple a pea­cock's tail might be con­spic­u­ous to preda­tors, " he not­ed in an e­mail. Or for a hu­man equiv­a­lent: " Fer­rari drivers might be more likely to end up splat­ted against a tree than Buick drivers. " For both ex­am­ples, " the sim­ple ex­plana­t­ion is that the cost is more than bal­anced by the ben­e­fit of ex­tra mat­ing. "

In the new pa­per, Greig, along with Da­vid W. Rog­ers of Im­pe­ri­al Col­lege in Lon­don, claim to have ob­served ev­o­lu­tion through sex­u­al se­lec­tion for the first time. " Our yeast sys­tem is a pow­er­ful tool for in­ves­ti­gat­ing the ge­net­ics of sex­u­al se­lec­tion, " they wrote.

Yeast cells oc­cur in two dif­fer­ent mat­ing types, some­what akin to male and fema­le. Each type sig­nals to po­ten­tial part­ners of the oth­er type by pro­duc­ing an at­trac­tive chem­i­cal, called a pher­o­mone. But cells vary widely in how strongly they can sig­nal; the dif­fer­ences are ge­net­ic.

Rog­ers and Greig en­gi­neered one of the " sex­es " of yeast cells, called MAT-alpha, to have ei­ther very high or very low sig­naling strength. They then mixed both types of cells with those of the op­po­site " sex " group, called MATa. This mix­ing was done in two dif­fer­ent ways: in one, the MAT-alpha cells were few, and so faced lit­tle com­pe­ti­tion among each oth­er; in the oth­er, they were many, so that they faced tough com­pe­ti­tion for mat­ing op­por­tun­i­ties.

Only un­der the high-com­pe­ti­tion situa­t­ion, the strong-sig­nalling gene var­i­ant spread quickly through the popula­t­ion at the ex­pense of the weak-sig­nalling var­i­ant, Rog­ers and Greig found. This matched the pre­dic­tions of sex­u­al se­lec­tion the­o­ry, they added.

" We have tested the sim­plest pos­si­ble sex­u­al se­lec­tion sce­na­rio, " they wrote. " Ob­serv­ing the real time ev­o­lu­tion of nov­el sex­u­ally se­lected traits, and pref­er­ences for them, is the ul­ti­mate test for sex­u­al se­lec­tion the­o­ry. "

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