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Experimental evolution of predation and sexual attractiveness

Fossils and DNA-sequence comparisons among species are like fingerprints and other clues found at crime scenes. We can often draw reliable conclusions about past events (evolutionary or criminal) from such physical evidence, but some people prefer eyewitness accounts. So evolutionary biologists are increasingly doing experiments that let us see evolution in action. Evolution is a change over generations, so the short generation times of microbes make them especially useful for experimental evolution.

Two recent examples, both published in Proceedings of the Royal Society, are "Experimental evolution of a microbial predator's ability to find prey", by Kristina Hillesland, Greg Velicer, and Richard Lenski, and "Experimental evolution of a sexually selected display in yeast", by David Rogers and Duncan Grieg.

Predation

Mxococcus xanthus bacteria hunt in packs, surrounding other bacteria and releasing enzymes that break them open, then taking up the nutrients released. Hillesand and colleagues hypothesized that if their prey were farther apart, M. xanthus populations might evolve traits that would let them cover more ground, looking for prey. To test this hypothesis, they grew M. xanthus for a year (many generations) on culture plates with either closely or widely spaced patches of prey bacteria. Sure enough, evolving in an environment with widely spaced prey bacteria led to greater improvement in the ability to find more prey patches per day.

"Optimal foraging" theory predicts tradeoffs between the ability to find and "process" prey. That didn't seem to be the case in these experiments, although they only measured how fast the evolved predators moved through prey patches, not (say) their rate of nutrient uptake there per hour. There did seem to be a tradeoff between hunting speed and spore production. Like hunting, spore production is a cooperative activity in M. xanthus. To confirm this tradeoff, they would need to let populations evolve in an environment that selects for greater spore production, to see whether this reduces hunting speed as a side-effect.

Sexual selection

Some individuals are more attractive to potential mates than others, so genes for attractiveness are assumed to spread by "sexual selection." The peacock's tail is the best-known example, but we don't know the genetic basis of traits that make peacocks attractive to peahens. Also, bird generations are long enough to make multigeneration evolution experiments difficult. Yeasts (single-cell fungi, thought to play a major role in sexual attraction among humans by producing ethanol) have much shorter generation times, but present their own challenges.

Sexual attraction among yeast sex cells -- there are two mating types, a and alpha, analogous to sperm or eggs, except that they are similar in size -- is based on chemical signals, similar to the role of odors in many animals. The authors reasoned that producing more signal should increase the chance of finding a mate, at least when competition for mates is strong. So they competed alphas differing only in how much signal they produce. (Weak signalers were genetically engineered to produce less signal.) They created strong or weak mate competition among alphas by adjusting the ratio of alphas to a's. It may help to think of the alphas as males and a's as females, and the two environments as colleges differing in male:female ratio.

When "males" outnumbered "females" (strong competition among males for mates) the frequency of males that were strong signalers increased over generations (to 85% from an initial value of 1%), because they were much more likely to attract one of the few "females." There was much less increase in the frequency of strong signalers when "females" outnumbered "males." Not a surprising result, perhaps, but this is apparently the first time sexual selection has been shown to increase the frequency of a particular gene responsible for attractiveness.

Earlier, I contrasted physical evidence with eyewitness accounts. Actually, eyewitnesses are often less reliable than fingerprints or DNA evidence in proving whether a particular person was at a crime scene. Experimental evolution of microbes is easy enough, however, that similar experiments will probably be repeated by others, giving us the equivalent of multiple independent eyewitness accounts documenting the same evolutionary process.

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