Wednesday, July 17, 2013

Exaptations - Many Evolutionary Innovations May Have Non-Adaptive Origins.

An exaptation is a shift in the function of a trait during evolution. Traits can evolve because they serve one particular function, but subsequently they may come to serve another. Exaptations are common in both anatomy and behavior. Here are a few from Wikipedia:
A classic example is how feathers, which initially evolved for heat regulation, were co-opted for display, and later co-opted for use in bird flight. Another example is the lungs of many basal fish, which evolved into the lungs of terrestrial vertebrates but also underwent exaptation to become the gas bladder, a buoyancy control organ, in derived fish.[3]

A behavioural example pertains to subdominant wolves licking the mouths of alpha wolves as a sign of submissiveness. (Similarly, dogs, which are wolves who through a long process were domesticated, lick the faces of their human owners.) This trait can be explained as an exaptation of wolf pups licking the faces of adults to encourage them to regurgitate food.[4]

Another more recent and controversial example posits that the human ability to use logic and reason originally evolved to win arguments and convince others, as opposed to its putatively exapted purpose of seeking truth. This idea, called the argumentative theory of reasoning, attempts to explain why many cognitive heuristics which should help to solve problems instead bias decisions, instead of framing those biases as adaptations developed to help win arguments.[5]
In the report below, Santa Fe Institute External Professor Andreas Wagner and colleague Aditya Barve, both evolutionary biologists at the University of Zurich, studied E. coli bacteria in order to get a better understanding on how traits originate by studying all the chemical reactions taking place in the organism’s metabolism as conditions are changed.

Wagner says, "Our work shows that exaptations exceed adaptations several-fold."

There is an interview with Wagner from The Scientist below this article summary from the SFI.

I find this idea interesting in that it seems to be another way of understanding Gary Marcus's idea of the Kludge in brain development. A kludge (or kluge) is a workaround, "a quick-and-dirty solution, a clumsy, inelegant, difficult to extend, hard to maintain yet effective and quick solution to a problem, and a rough synonym to the terms 'jury rig' or 'Jugaad.' This term is used in diverse fields such as computer science, aerospace engineering, internet slang, and evolutionary neuroscience" (see Gary Marcus, Kluge: The Haphazard Evolution of the Human Mind, 2009).

Evolution has created some quick-and-dirty solutions to immediate problems, and some of them have downsides (for example, the same group of genes associated with creativity and genius are also linked to schizophrenia), but others, such as language, no doubt arose for the immediate needs of coordinating hunting or child-rearing, but have also created great poetry and literature.

Great exaptations: Most traits emerge for no crucial reason

July 15, 2013

Exactly how new traits emerge is a question that has long puzzled evolutionary biologists. While some adaptations develop to address a specific need, others (called “exaptations”) develop as a by-product of another feature with minor or no function, and may acquire more or greater uses later. Feathers, for example, did not originate for flight but may have helped insulate or waterproof dinosaurs before helping birds fly.

How common such pre-adaptive traits are in relation to adaptive traits is unclear. SFI External Professor Andreas Wagner and colleague Aditya Barve, both evolutionary biologists at the University of Zurich, decided to get a systematic handle on how traits originate by studying all the chemical reactions taking place in an organism’s metabolism.

Starting with the metabolism of an E. coli that can survive on glucose as its sole carbon source, they subjected the complex metabolic chemical process to a "random walk" through the set of all possible metabolisms, adding one reaction and deleting another from it with each step. They kept constant the total number of reactions and the bacterium’s ability to survive on glucose alone, but allowed everything else to change. Every few thousand steps they analyzed the altered metabolism’s reactions.

They found that most metabolisms were viable on about five other carbon sources – sugars, building blocks of DNA or RNA, or proteins – that are naturally common but chemically distinct compounds. To be certain that viability on these other carbon sources wasn’t a natural consequence of viability on glucose, they tested metabolisms starting with viability on 49 other carbon sources, and each time found that exaptations emerged allowing the metabolism to survive on any one of several other carbon sources alone.


Image caption: In this network diagram of E. coli metabolism combinations, the color of each node corresponds to the combination of carbon sources the network is viable on. There are 247 different phenotypes in this graph, that is, different combinations of carbon sources the networks are viable on. Networks viable on glucose alone are black. Two nodes are connected if they are both viable on the same carbon source, while the size of a node is representative of the number of other nodes it is connected to. The figure shows that the majority of networks (96 percent) are viable on many different carbon sources. The figures were generated using the Gephi software.


“We observed an incredible abundance of viability on carbon sources that these metabolisms were never even required to use,” Wagner says.

By varying the number of reactions in a metabolic system, the team also found a relationship between the system’s complexity (determined by number of reactions) and the extent of the exaptations, with larger networks having more of them.

The findings underscore the idea that traits we see now – even complex ones, like color vision – may have had neutral origins that sat latent for generations before spreading through populations, Wagner says.

"Our work shows that exaptations exceed adaptations several-fold," he says.

If exaptations are pervasive in evolution, he adds, it becomes difficult to distinguish adaptation from exaptation, and it could change the way evolutionary biologists think about selective advantage as the primary driver of natural selection.

Their work was published July 14 in the online edition of Nature.

Read their paper in Nature (June 14, 2013) [Behind a paywall]

Read a Q&A with Andreas Wagner in The Scientist (June 14, 2013) [BELOW]

* * * * * * *

Q & A: Evolution Makes Do

Evolutionary biologist Andreas Wagner argues that many evolutionary innovations may have non-adaptive origins

By Chris Palmer | July 14, 2013


Traits that initially confer a selective advantage often later become beneficial in some unrelated—and often surprising—way. A classic example of a so-called pre-adaptive trait, or exaptation, is the feather, which originated for a purpose other than flight. Although numerous anecdotal examples of exaptations litter the evolutionary biology literature, it is unclear how commonplace they are. Research published today (July 14) on simulated metabolic networks, however, suggests that exaptations may in fact outnumber adaptations several-fold.

“The paper is an important effort to characterize the frequency of exaptation as an evolutionary mechanism,” Richard Blob, an evolutionary biologist at Clemson University who was not involved in the study, said in an email. “The authors have identified an effective system to begin approaching this question in a systematic way.”

Leading the research was Andreas Wagner, an evolutionary biologist at the University of Zurich, who studies evolutionary innovation across many levels of biological organization, from genes to organisms to communities. Publishing in Nature with coauthor Aditya Barve, Wagner described the ubiquity of exaptations in metabolism, one of the most critical—and ancient—systems common to all multicellular organisms. To address this question, Wagner created sophisticated simulations of thousands of metabolic networks, each with a set of chemical reactions that allow for the synthesis of biomass from one of 50 different carbon sources—glucose, for example. Reflecting real-life metabolic networks, the simulated networks were also endowed with a modest number of random chemical reactions. Wagner found that all of the networks showed exaptations for the ability to utilize other carbon sources—as many as 20 different sources in some networks.

The Scientist chatted with Wagner about exaptations, their apparent ubiquity, and what they mean for the study of evolutionary biology.

The Scientist: Where did the notion of exaptations come from?

Andreas Wagner: Stephen Jay Gould first coined the term to describe traits that may be simple by-products of other traits. Also, Darwin said in his Origin of the Species back in 1859 that organs that serve a particular purpose may have originated for a completely different purpose. So even Darwin was aware that exaptations exist, although they didn’t have that word at the time.

TS: What are your favorite examples of exaptation?

AW: The origin of feathers, which help with stabilization during flight. But they probably originated for completely different reasons such as thermal insulation or waterproofing. There are also these fascinating proteins in our eye lens called crystallins, which originated from metabolic enzymes. At high concentrations they retain transparency, allowing nature to build lenses with high refractive indices, which are well suited to focus light.

TS: How did you become interested in the transition from pre-adaptive to adaptive traits?

AW: I consider this to be the last frontier in evolutionary biology. Natural selection we’ve known about for more than 150 years. So, we know a lot about how evolutionary innovations spread through populations, but we know very little about how they originate.

TS: Can all adaptive traits be traced to an exaptive origin?

AW: I would say the answer is yes. Gould argued very much in favor that [exaptive traits are] very frequent. And he was attacked from all sides because the common wisdom was that all traits originated for the same reason that they are still being kept around today. Nowadays, most people agree that exaptations occur and are fairly frequent, but we didn’t know whether they are more frequent than regular adaptations.

TS: What conclusions can you draw from your simulations of metabolic networks?

AW: Our work shows that exaptations exceed adaptations several-fold. Mere examples of exaptations [could not] address this question. You really need this kind of systematic approach where you study a sample of all possible metabolisms and what typical properties they have.

TS: So, evolution creates a pool of possible adaptations, only a few of which are expressed at a time?

AW: Exactly. Imagine we find a novel microbe that is able to survive on some carbon source, say citrate. Reflexively, a microbiologist would say, because citrate is an important carbon source in the microbe’s environment, that this ability is an adaptation. What our work tells us is this need not be the case. This could simply be the by-product of the ability to live on some other carbon source that we may not even have identified yet.

TS: What does this mean for the field of evolutionary biology?

AW: This opens a huge can of worms for evolutionary biologists because it becomes very hard to distinguish adaptation from exaptation.

TS: Why does it matter to have a clear delineation?

AW: If exaptations are pervasive, then natural selection—which few doubt is critical for the preservation and spreading of traits—may not be that important for the origin of innovations in life’s history.
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