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“Other half” of Darwin’s theory passes test

Oct. 13, 2008
World 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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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.” 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, would seem less than 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.”