Evolution loves history

In order to linkurl:evolve;http://www.the-scientist.com/news/display/23321/ novel traits, organisms may depend upon smaller, less dramatic mutations that they amass through their evolutionary history rather than suddenly acquiring a single mutation that gives them drastically different phenotypes, according to a study published online today (Jun 2) in __PNAS__.

By | June 2, 2008

In order to linkurl:evolve;http://www.the-scientist.com/news/display/23321/ novel traits, organisms may depend upon smaller, less dramatic mutations that they amass through their evolutionary history rather than suddenly acquiring a single mutation that gives them drastically different phenotypes, according to a study published online today (Jun 2) in __PNAS__. Whether an organism arrives at major evolutionary innovations through a single key mutation or a history of many accumulated mutations has been hotly debated by evolutionary biologists. The findings are "perhaps the most rigorous, clear-cut demonstration of the role of historical contingency," in evolution, said linkurl:Richard Lenski;https://www.msu.edu/user/lenski/ of Michigan State University, the study's main author. Examining linkurl:__E. coli__;http://www.the-scientist.com/2007/3/1/65/2/ cultures that his lab has maintained since 1988, Lenski found that one population of the bacterium had evolved the ability to metabolize citrate -- an unprecedented trait -- after more than 30,000 generations, or approximately 15 years. So Lenski and his colleagues consulted their "frozen fossil record," consisting of preserved individuals collected from the bacterial population at several points throughout its evolution. They revived bacteria from different stages of the population's 20-year evolution and "replayed" evolution in each. Clones sampled in the first 15,000 generations never evolved the ability to metabolize citrate (Cit+), while clones taken from later populations did evolve the characteristic. That means this big evolutionary jump relied on a chain of genetic events that took place over the course of the population's evolutionary history, Lenski said. "It's a very elegant demonstration that major changes may depend on accretion of minor changes before hand," said linkurl:Albert Bennett,;http://www.faculty.uci.edu/profile.cfm?faculty_id=4546&name=Albert%20F.%20Bennett%20http://www.youtube.com/watch?v=cE2wHXkVxXY a University of California, Irvine evolutionary physiologist who gave Lenski feedback on the study before it was published in __PNAS__. "What's really demonstrated here is that the way has to be paved before hand." Lenski and his collaborators suggested two possible mechanisms that could have lead to the Cit+ phenotype: the population either accumulated interacting mutations that added up to the functional ability to metabolize citrate, or random mutations, such as insertions, created altered DNA substrates that were then subjected to further mutations, yielding the unique facility. Both Lenski and Bennett said that they tend towards the former, functional, explanation for the evolutionary jump. "Presumably, this is how metabolic pathways got built up," said Bennett. Lenski cautions, however, that his findings do not fully resolve the question of whether evolution is a result of random or deterministic processes. "The real world is a mixture of these two processes," he said. "We're able to see the importance of both perspectives simultaneously." Still, the results suggest that major evolutionary advances are likely to depend on the past evolutionary history of an organism. "This suggests that key innovations may be things that are historically contingent," Lenski said. The researcher now plans to delve deeper into his evolutionary reconstruction experiments, elucidating exactly which mutations led directly to the Cit+ phenotype in his __E. coli__ population. "The key will be to find the one or more mutations that, at the time they occurred, produced the citrate mutation," he said. Lenski will also turn the dial further back to identify those mutations that preceded the emergence of the Cit+ trait. "It's going to be rather tricky to sort out which mutations it was," admitted the researcher.


Avatar of: Jeff Chamberlain

Jeff Chamberlain

Posts: 3

June 3, 2008

How did they implement the evolutionary pressure that caused this change? Obviously, a certain amount of the E. coli had to survive under the given conditions in order for the mutation for metabolizing citrate to manifest itself. Was there a survival advantage for the E. coli that evolved this trait?
Avatar of: kirby zeman

kirby zeman

Posts: 2

June 3, 2008

The mutation that caused the metabolism of citrate most likely is a small change in a similar path that was already in existence for another use. Each step in the cit+ metabolism may have had some utility for another reason along the way to developing the ability. It would seem improbable that a detailed multi-stepped path would evolve solely for citrate, with each step useless until the last step. If that were the case, the bacterium should be transiently filled with nearly an infinite number of multi-stepped pathways for many non-usable substrates. The genome is not large enough.
Avatar of: Keith Rasmussen

Keith Rasmussen

Posts: 1

June 3, 2008

When I read that up to generation 14,999 the little beasties couldn't metabolize citrate, but generation 15,000 could metabolize it, I expected to read this proved a sudden, random mutation caused the new ability. Instead, I read this proves incremental mutations capped by a final, critical mutation. I'm sure I'm missing something here, but what? Wouldn't the observed result happen in either case?
Avatar of: David Clark

David Clark

Posts: 4

June 6, 2008

E. coli mutants that use citrate have been selected before and published some years back. The problem is that E. coli cannot transport citrate and the alterations are in a transport system for citrate and isocitrate. Furthermore, wild type E. coli uses citrate when grown anaerobically.\n\nThe only amazing thing is that this trivial "discovery" got into PNAS. It's not what you discover but who you know.
Avatar of: John Collins

John Collins

Posts: 37

June 9, 2008

Numbering myself amongst those finding it incredible that this very incomplete investigative chronicling made its way into PNAS may I add a few perhaps more interesting oddities from the distant and not so distant past, purely for the delectation of this forum. \n\nIn the early 70s, the question was asked:is the presence of superfluous genetic information really a genetic load?: will it be lost if selective pressure for its maintenance is removed?; e.g. the presence of the proline operon in a culture kept for a thousand generations with proline (a very artificial setting). It was.\n\nIn a more real life situation, the recent work of Soren Molin's group (2007) has shown how a single mutation can lead to formation of a symbiosis between two micro-organisms when confronted by an adverse environment containing substrates that essentially demand a collaboration between the two organisms to efficiently use the new energy source.\n\nBye the way, E.coli also contains a weak beta-lactamase in the chromosome, and a gene which can easily mutate to active ß-galactosidase (it is called evolutionary beta galactosidase ebgA and its regulator ebgR). Our surprise at such serendipity (that such apparently useful phenomena are around when needed) are more a result of our teleological perspective than evidence that there is a huge dormant stock pile of redundant plasticity. For instance, although being a host to lambda bacteriophage is often a considerable burden it still hasn't lead to a loss of the MalB-gene product used as the virus receptor, which in a maltose rich, virus-free medium has a distinct advantage. Hosting virus as a vehicle for genetic exchange probably also has long term evolutionary advantages for organisms in a challenging, rapidly changing environment (not for those in a stable environment; see Pelagibacter ubique). Dobzhansky would have admonished us to search for a more global explanation to these situations within an evolutionary framework.

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