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Rock-paper-scissors for high stakes

Chapter 3 of The Origin of Species is titled "Struggle for Existence", which Darwin uses "in a large and metaphorical sense including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny." Differences among plants and animals in their success in leaving progeny depends on their adaptation to the physical environment, but also their interactions with each other. For example, "A plant which annually produces a thousand seeds, of which only one of an average comes to maturity [this must be true, if population size is constant], may be more truly said to struggle with the plants of the same and other kinds which already clothe the ground."

If the traits that maximized survival and reproduction were always the same, those with those best traits would quickly displace those with alternative traits. But changes in the physical and biological environment mean that no one genotype is consistently best. This week's paper is about frequency-dependent selection, where the fitness of each genotype depends on how common it is. If less-common genotypes tend to increase in frequency, no single genotype will take over.

The paper is "An experimental test of frequency-dependent selection on male mating strategy in the field" by C. Bleay, T. Comendant, and B. Sinervo, of the Universities of Bristol and California (Santa Cruz), published on-line in Proceedings of the Royal Society.

Male side-blotched lizards have orange, blue, or yellow throats. Orange males bully blue males, taking over their territories. Yellow males resemble females enough that they can sneak into orange territories and mate with females. But blue males guard their females so closely that they aren't fooled, so they outcompete yellow males, but they can't defend their territory against orange males. (There may be some lessons here for border security of large vs. small countries, but never mind.) Field observations show successive replacements consistent with these observations: blue => orange => yellow => blue and so on. It's like the rock-paper-scissors game, except that each individual male is stuck with his throat color and therefore (in practice) with the corresponding strategy.

Field observations were consistent with frequency-dependent selection. For example, yellow males can often fool orange males (who are trying to maintain large territories) but not blue males (guarding one mate), so yellow males should leave more progeny when orange males are more common. This seems to be the case, but could this correlation be due to some other factor, such as interactions between weather and temperature tolerance differences among the different types of males?

To find out, Bleay and colleagues experimentally manipulated the frequency of the different genotypes, seeding lab-raised lizards at field sites on the California coast. Some sites got mostly orange males, some mostly blue, etc. Then, as females at each site became pregnant, they were brought into the lab to lay eggs, so that DNA analyses could be used to identify who had fathered each egg. After any resulting lawsuits were settled, mothers and babies were returned to the field.

As predicted, orange males sired more babies when their competitors were mostly blue (because they could expand into their territories) than when they had to worry about sneaky yellow males, or compete with other orange males. Yellow males did best against orange and blue best against yellow, also as predicted, but these differences were much less clear. Also, up to 93% of variation in male reproductive success was unexplained, with differences in quality of territories thought to be important.

I've mentioned before that evolutionary biology is increasingly an experimental field. Most of the experiments I've discussed have been done under laboratory conditions, where we can hold everything (almost) constant except for the experimental variables being manipulated. This paper is a good example of experimental evolution under field conditions.

Figure 2 from the paper, copyright by the Royal Society and reproduced under US "Fair Use" guidelines.

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