Bench Press

What would you do you if you were curious about the relative importance of selective pressures on a population of lizards on Caribbean islands? Since you’re reading Bench Press you might be inclined to turn to the power of computer modeling which can provide numerous advantages particularly when traditional experiments can’t be conducted. We’ve seen examples of computer models analyzing near earth asteroids, potential epidemics, and classic math and physics problems. However we’ve also seen that at times purely mathematical approaches can result in errors as well. Our inability to accurately describe problems with numbers 100% of the time makes it imperative that we continue to think creatively about ways to design experiments to test hypotheses.

That’s why I was particularly impressed by a paper in Nature by Ryan Calsbeek and Robert M. Cox, who wanted to explore the importance of selective pressures on anole lizards in the Caribbean. Field experiments to measure the effect of selective pressures are rare for a variety of reasons. A major one being the difficulty of finding animals and environments which can be manipulated in a controlled manner. Drs. Calsbeek and Cox didn’t let this stop them as they utilized a group of small islands in the Caribbean, each small enough to throw a ball end to end, as their test beds. There they removed the resident brown anole lizards and replaced them with experimental animals which had been carefully measured, tested for stamina, and tagged to identify at the end of the experiment.

Now that they had their experimental populations they needed to set up islands that tested the hypothesis that competition played a larger selective role than predation in island anole lizard populations. They established islands that had low and high density populations, and for each density type they setup islands inhabited by lizard-eating birds alone, lizard-eating birds and snakes, as well as islands free of predators (accomplished with a generous covering of netting as seen below). An unmodified control island was also monitored as a natural reference population.

They distributed the lizards in May and four months later at the end of the breeding season, September, they came back to capture the survivors and census the population. While this was difficult work they were able to collect a large amount of data which confirmed the hypothesis that competition is a more powerful selective force in these populations. They saw no real phenotypic differences in the lizards on islands experiencing differing predation, but saw that lizards surviving on crowded islands were significantly bigger and had greater stamina than those on less crowded islands (seen below). This indicated competition between lizards pushed the population while predation did not.  

While their clever experiment does a great job explaining the relative importance of selective pressures on this particular species of lizards on islands in the Caribbean it may not say anything about natural selection in other species. Despite that this paper remains awesome because as much as I like to see technology change the way we do science, I still appreciate a well constructed experiment to answer tough questions.

(Nature: Experimentally assessing the relative importance of predation and competition as agents of selection)