One of my pet peeves is the common description in the media of bacteria "learning" to "outsmart" antibiotics. As anyone with a basic comprehension of evolution knows, learning has nothing to do with it. Learning is what happens during the lifetime of an individual, and it occurs in direct response to some information that the individual encounters. When bacteria become resistant to antibiotics, it is not by learning. The individual bacteria do not sense the antibiotic and change to become resistant. Rather, individual bacteria in a population that happen to be resistant because of some genetic difference (or in whom a mutation conferring resistance arises by chance or through gene transfer from another population) will survive and reproduce more effectively than individuals lacking the genetic characteristic that confers resistance. Over many generations of this process, the gene providing resistance to the antibiotic will be found in the majority of bacteria -- not because it "spreads" and not because individual bacteria develop resistance, but because the bacteria that are the most abundant in the population after many generations are obviously the descendants of the ancestors that left the most offspring, namely those who survived the antibiotics.
Unfortunately, the desire to consider evolution "learning" when discussing bacteria is not limited to non-specialists, and some scientists seem only too eager to reinforce this misconception. As a prime example, consider this story posted on PhysOrg:Thinking ahead: Bacteria anticipate coming changes in their environment. In this report, Saeed Tavazoie of Princeton claims "What we have found is the first evidence that bacteria can use sensed cues from their environment to infer future events".
What they found is that bacteria (E. coli) that move from the outside world to the human gut undergo a switch to anaerobic metabolism before they actually enter the low oxygen surroundings of the digestive tract. They do this by instigating the change in response to a rise in temperature, which they encounter when they enter the mouth. In other words, a rise in temperature is predictably followed by a drop in oxygen in the world of these bacteria, and they respond to the former by making changes appropriate to the latter. Is this "predictive behaviour" that the bacteria "learned"? The authors seem to believe so. In fact, they argue, the bacteria can "learn" to do the opposite, if you establish an experimental environment in which a change in temperature is not followed by a reduction in oxygen availability. As the story says,
Remarkably, within a few hundred generations the bugs partially adapted to this new regime, and no longer turned off the genes for aerobic respiration when the temperature rose. "This reprogramming clearly indicates that shutting down aerobic respiration following a temperature increase is not essential to E. coli's survival," says Tavazoie. "On the contrary, it appears that the bacterium has "learned" this response by associating specific temperatures with specific oxygen levels over the course of its evolution."
Lacking a brain or even a primitive nervous system, how is a single-celled bacterium able to pull off this feat? Whereas higher animals can learn new behavior within a single lifetime, bacterial learning takes place over many generations and on an evolutionary time scale, Tavazoie explains.
No. Individuals that did not exhibit the usual response of turning off aerobic metabolism when they experience a temperature increase -- who under normal conditions would not have done well, but in this experimental situation did better than alternatives -- left more offspring. Over many generations, this became common in the population because these individuals and their progeny consistently reproduced more successfully than alternatives within the population. This is simply microevolution and it has nothing to do with "learning" or "associating" or "inferring" or anything else of the kind.
Another author interviewed in the report does not do much better with these issues when talking about their simulation of evolution.
"To predict mealtimes accurately, the microbes would have to solve logic problems," says Tagkopoulos, a fifth-year graduate student in electrical engineering and the principal architect of the Evolution in Variable Environment framework.
And sure enough, after a few thousand generations, an ecologically fit strain of microbe emerged which did exactly that. This happened for every pattern of cues that the researchers tried. The feeding response of these gastronomically savvy bugs peaked just when food was offered, says Tagkopoulos.
The bacteria are not solving logic problems. The distribution of gene combinations is changing over time because individuals in the variable population that happen to have certain traits that confer advantages relative to alternatives in a particular environment leave more offspring in every generation. The sole problem of logic is equating evolution over generations with learning in a single lifespan.
From the actual article:
Within this in silico ecology, evolving organisms with random initial networks compete against each other in structured environments where signals and resources fluctuate with a distinct temporal correlation structure. In a typical experiment, the combinatorial states of multiple signals convey information about the availability of extractable energy resources in the near future. Cells that can efficiently "learn" such correlations are able to express the energy-extracting metabolic pathway at the appropriate time, giving them sizable fitness advantage over their competitors.
Replace "Cells that can effectively 'learn' such correlations" with "Genes or combinations thereof that happen to produce cells that respond in a specific way to stimuli correlated with relevant forthcoming environmenal change" and this would be ok. Sure, it's more complicated, but given the major difference in accuracy, I think the page space would have been well invested.
Bacteria may be the dominant life forms on the planet, but their success is a result of evolution, not learning.