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Learning vs. lifespan?

For my first 100 posts, I’ve ignored journals with “evolution? in their names, to make the point that evolution is at the heart of biology, rather than an “appendix.? Much of our DNA can be deleted without obvious ill effect, but taking evolution out of biology would kill it as an explanatory, hypothesis-driven science. Point made? This week I untie my hands and discuss a paper from one of the 30+ scientific journals that focus on evolutionary biology.

“Learning ability and longevity: a symmetrical evolutionary tradeoff in Drosophila? by Joep Burger, Munjong Koss, and others, will appear soon in the journal, Evolution.

The ability to learn is useful under a wide range of conditions, but is it always beneficial? If so, why do most species have limited learning ability? Is there some evolutionary constraint, such as head size, that prevents evolving greater learning ability? Apparently not. Artificial selection for learning ability has been successful in several species. When artificial selection imposed by humans achieves something in months that natural selection has failed to do in millions of years, that suggests that the “improvement? has some cost that exceeds its benefits, at least in nature. But could the ability to learn really have a cost that exceeds its benefits?

To find out, the authors of this week’s paper first looked at fruit-fly populations that had evolved previously in another laboratory, under selection for increased learning ability. Fruit flies don’t like the bitter taste of quinine, so they won’t lay eggs in a material that contains it. Earlier researchers, Mercy and Kawcki, put quinine in either orange- or pineapple-flavored substrate. After some of the flies learned to associate one of the fruit odors with quinine, they removed the quinine and let the flies lay their eggs. The next generation of flies was raised only from eggs laid in the substrate that had not had quinine during the learning period. Some of these eggs could have been laid at random, of course, but the assumption was that flies with more ability to learn and remember associations would be over-represented. This turned out to be true. After 30 generations of selection, the flies were better at learning. The authors confirmed that this was true for learning how to avoid shocks as well as quinine. In humans, increased learning ability could come from cultural evolution, but there was no evidence that the flies were setting up universities or apprenticeship programs, so this was strictly a genetic change.

The question was, did this increased learning ability have any cost? The authors measured various traits, including time to maturity, egg laying, and lifespan. Lifespan was 10% lower for males and 15% lower for females, compared to the less-learned control population. This could be because selection was based on egg-laying preferences, as described above.

They also did the reverse comparison, looking at fly populations selected (by Arking and colleagues) for longer lifespan – actually, for ability to reproduce later in life -- to see whether they had lost some of their learning ability in the process. Sure enough, the ability of long-lived flies to remember which substrate had the quinine was 39% lower than in control fly populations.

The authors suggest that there may be one or more genes that affect both learning and lifespan, but in opposite directions. One allele (version of the gene) increases learning at the expense of lifespan, whereas another allele does the opposite. They mention two particular genes that could have such an effect, but they haven’t yet tested to see whether there were differences in allele frequencies between control and selected populations. They also discuss alternative explanations, such as genetic linkage -- an allele for learning ability just happens to be physically linked with an allele that limits lifespan – or negative effects of inbreeding in small laboratory populations. They argue, plausibly, that such explanations are unlikely, but it would be nice to know for sure which genes are involved. Interestingly, the effect on learning was age-specific, with longer-lived flies learning less well when young, but better when old, relative to controls.

Tradeoffs are very common in evolution. Dolphins swim better than ducks, but they can’t fly. Changes in human learning ability and lifespan today are largely due to culture (education, pensions, etc.), but there must still be some genetic changes over generations as well. Should we expect any genetic increase in human longevity to be associated with decreased learning ability, or vice versa? Humans aren’t fruit flies, but at the cellular level we are surprisingly similar. On the other hand, even if some genes exist only in a low-learning/high-longevity version and a high-learning/low-longevity version, this tradeoff could be reduced by subsequent evolution of other genes. When microbes evolve resistance to antibiotics, for example, there is often an initial cost, such as lower growth rate without antibiotics, relative to control populations. Further evolution, involving different genes, often reduces those costs. Maybe, in the long-run, we can live long and learn.

Other recent papers:
Residual reproductive value and male mating success: older males do better

MHC-mediated mate choice increases parasite resistance in salmon


The first hominin of Europe


Density-Dependent Cladogenesis in Birds


Loss of Egg Yolk Genes in Mammals and the Origin of Lactation and Placentation
Discussed by PZ Meyers

Phenotypic Mismatches Reveal Escape from Arms-Race Coevolution
Discussed by Ed Yong


Cooperative problem solving in rooks (Corvus frugilegus)

Comments

My first thought was the old saw, "a little learning is a dangerous thing". How could a fruit fly ever learn more than a little, and how could it tell if it learned the wrong thing? We have enough trouble with that ourselves.

So, I was surprised that you can get definitive results even when everything learned is valid. Even without the positive results here, the danger of too readily learning the wrong thing makes learning ability not an obvious win.

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