Experimental evolution: play dead or fly away?
Last week's paper discussed trade-offs between seed size and seed number. Many such trade-offs (growth vs. reproduction, more seeds vs. taller stem, etc.) follow directly from conservation of matter or energy, but what about other sorts of trade-offs? It has been suggested, for example, that there is a trade-off between competitiveness and dispersal ability. Why should this be? For seeds, at least, a larger seed gives the seedling a head-start against competitors, but smaller seeds travel farther on the wind.
This week's paper proposes another trade-off, for which the mechanism is less obvious. "Drop or fly? Negative genetic correlation between death-feigning ability and flying ability as alternative anti-predator strategies", was written by Tatunori Ohno and Takahisa Miyatake and published in Proceedings of the Royal Society B (vol. 274, p. 555-560).
Prey that are unable to fight back against predators (using horns or nasty chemicals, for example) can flee or hide. Insects can fly away or drop to the ground and "play dead." It's not that predators necessarily refuse to eat a dead insect, but that insects lying on the ground and not moving are hard to see.
The authors of this paper measured flying ability in adzuki bean beetles and compared that to the length of time they would play dead (or "feign death"). Comparing 21 populations of these beetles from around Japan, they found a strong negative correlation. Good fliers weren't good at playing dead, and vice versa.
What would cause such a negative correlation? One possibility is that good fliers escape from predators, so there is no selection for playing dead. Within a population of slow-flying beetles, on the other hand, those that drop to the ground and feign death long enough for the predator to leave are more likely to survive and reproduce. Under this hypothesis, there is no intrinsic trade-off between flying ability and playing dead. Instead, the apparent trade-off would come from different death-feigning selection regimes for good vs. bad fliers.
This hypothesis is wrong in this case, however. We know because Ohno and Miyatake selectively bred beetles for good vs. bad flying ability and separately for short vs. long immobility when playing dead. They found the same negative relationship seen in the field, even though their laboratory experiments didn't include predators. In only eight generations, they got 40-fold differences in duration of death-feigning. (Who said evolution is always slow?) They concluded that there must be some kind of physiological or behavioral trade-off between flying and feigning death.
They did mention a third possibility, namely genetic linkage. Maybe, in their experimental population, there was a gene for flying well that just happened to be close to a gene for not playing dead. I agree with the authors that this is unlikely. They give various examples showing that more active insects are more likely to flee and less active ones are more likely to hide. They suggest that some chemical signal in the beetles' brains stimulates flying and simultaneously reduces the ability to remain still and feign death. I wonder whether prolonged selection for both flying speed (when fleeing) and length of immobility (when feigning death) could break this link.
Overall conclusions? First, evolutionary biology is increasingly becoming an experimental science. With strong selection, a population can behave very differently in only a few generations. Regarding trade-offs, I would not conclude that every claim of a trade-off between X and Y should be accepted uncritically. Some evolutionary innovations that improve competitiveness might simultaneously improve dispersal, for example. On the other hand, this paper shows that we can identify and quantify trade-offs even when their mechanistic basis has yet to be established.