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How pre­dictable is evolution?

pub­lished 23 Feb­ru­ary 2013 | mod­i­fied 23 Feb­ru­ary 2013
Ecoli evolution plosUnder­stand­ing how and why diver­si­fi­ca­tion occurs is impor­tant for under­stand­ing why there are so many species on Earth. In a new study pub­lished on 19 Feb­ru­ary in the open access jour­nal PLOS Biol­ogy, researchers show that sim­i­lar — or even iden­ti­cal — muta­tions can occur dur­ing diver­si­fi­ca­tion in com­pletely sep­a­rate pop­u­la­tions of E. coli evolv­ing in dif­fer­ent envi­ron­ments over more than 1000 gen­er­a­tions. Evo­lu­tion, there­fore, can be sur­pris­ingly pre­dictable.

There are about 4.5 mil­lion nucleotides in the E. coli genome. Find­ing in four cases that the exact same change had hap­pened inde­pen­dently in dif­fer­ent pop­u­la­tions was intriguing.
Matthew Her­ron, co-​author, Uni­ver­sity of Mon­tana »

The exper­i­ment, con­ducted by Matthew Her­ron, research assis­tant pro­fes­sor at the Uni­ver­sity of Mon­tana, and Pro­fes­sor Michael Doe­beli of the Uni­ver­sity of British Colum­bia, involved 3 dif­fer­ent pop­u­la­tions of bac­te­ria. At the start of the exper­i­ment, each pop­u­la­tion con­sisted of gen­er­al­ists com­pet­ing for two dif­fer­ent sources of dietary car­bon (glu­cose and acetate), but after 1200 gen­er­a­tions they had evolved into two coex­ist­ing types each with a spe­cialised phys­i­ol­ogy adapted to one of the car­bon sources. Her­ron and Doe­beli were able to sequence the genomes of pop­u­la­tions of bac­te­ria frozen at 16 dif­fer­ent points dur­ing their evo­lu­tion, and dis­cov­ered a sur­pris­ing amount of sim­i­lar­ity in their evo­lu­tion.

“In all three pop­u­la­tions it seems to be more or less the same core set of genes that are caus­ing the two phe­no­types that we see,” Her­ron said. “In a few cases, it’s even the exact same genetic change.”

Recent advances in sequenc­ing tech­nol­ogy allowed Her­ron and Doe­beli to sequence large num­bers of whole bac­te­r­ial genomes and pro­vide evi­dence that there is pre­dictabil­ity in evo­lu­tion­ary diver­sity. Any evo­lu­tion­ary process is some com­bi­na­tion of pre­dictable and unpre­dictable processes with ran­dom muta­tions, but see­ing the same genetic changes in dif­fer­ent pop­u­la­tions showed that selec­tion can be deter­min­is­tic.

Her­ron and Doe­beli argue that a par­tic­u­lar form of selec­tion — neg­a­tive fre­quency depen­dence — plays an impor­tant role in dri­ving diver­si­fi­ca­tion. When bac­te­ria are either glu­cose spe­cial­ists or acetate spe­cial­ists, a higher den­sity of one type will mean fewer resources for that type, so bac­te­ria spe­cial­is­ing on the alter­na­tive resource will be at an advan­tage.

“We think it’s likely that some kind of neg­a­tive fre­quency depen­dence — some kind of rare type advan­tage — is impor­tant in many cases of diver­si­fi­ca­tion, espe­cially when there’s no geo­graphic iso­la­tion,” Her­ron said.

As tech­nol­ogy advances, Her­ron believes that sim­i­lar exper­i­ments in larger organ­isms will soon be pos­si­ble. Some exam­ples of diver­si­fi­ca­tion with­out geo­graphic iso­la­tion are known in plants and ani­mals, but it remains to be seen whether or not the under­ly­ing evo­lu­tion­ary processes are sim­i­lar to those in bac­te­ria.

The above news item is reprinted from mate­ri­als avail­able at PLoS Biol­ogy via Sci­enceDaily. Orig­i­nal text may be edited for con­tent and length.
(Source: PLOS Biol­ogy, 19.02.2013)
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