Here’s another good reason for choosing a research project that I forgot to mention in those old posts.
Study the balance between two opposing forces or effects that ordinarily or even necessarily co-occur because they arise from the same source.
I think of this as a “double edged sword” question. For instance, consider the effects of predators on the evolutionary diversification of their prey (Meyer & Kassen 2007). On the one hand, predation should promote diversification of prey by generating novel ecological opportunities and selection pressures. On the other hand, predation reduces prey population sizes, which should inhibit diversification by lowering prey densities. Reduced prey densities weaken resource competition among prey and any associated selection for diversification.
As another example, consider evolutionary rescue, whereby a population that’s so poorly adapted to its environment as to be doomed to extinction adapts to its environment to the point where it’s able to persist (Bell 2013). On the one hand, a “harsher” environment might promote evolutionary rescue, by imposing stronger selection for fitness improving variants. On the other hand, a harsher environment should inhibit evolutionary rescue, by reducing population size. That reduces the supply rate of beneficial mutations, increases the probability that beneficial mutations will be lost to drift, and gives evolution less time to work before the population goes extinct.
As a third example, dispersal famously is a double-edged sword for metapopulation persistence. Dispersal allows rescue effects and recolonization of extinct populations (good), but also synchronizes population dynamics and so increases the risk that all populations will crash at once, leaving no source for recolonization (bad) (Yaari et al. 2012).
As a fourth example, consider the effects of migration on adaptation. On the one hand, migration introduces new variation into the population, which provides fuel for evolution by natural selection. On the other hand, introducing maladapted variants into a population faster than selection can purge them can keep the population maladapted.
“Double edged sword” questions are interesting because the opposing forces arise from the same source and so ordinarily (even necessarily) co-occur. That is, they’re really questions about the effect of one variable or factor X on some other variable Y. But it’s not obvious how X will affect Y, because the net result of its different effects of X on Y could go either way (and might depend on other factors, of course). That to my mind is more interesting than a situation in which opposing forces arising from different sources just so happen to affect some variable of interest. In general, things that just so happen to be the case are less interesting than things that have good reason to be the case, or that necessarily are the case.
Note that the more opposing forces there are, the less interesting a double edged sword question becomes. If an increase in X could lead to an increase in Y for 57 different reasons, and lead to a decrease in Y for 72 other reasons, you’re not going to be able to sort them all out. That’s not so much a double edged sword question as a, um…[does addition in head]…129-edged sword question. And any answer you do manage to get is likely to be some variation on “it depends; it’s really complicated”. Well, unless you can reframe the question using some sort of unifying theoretical framework so as to cut the sword down to two edges (a la #4 on this list).
A double edged sword question usually stops being a good question when the connections between cause and effect are very indirect. In general, indirect effects are idiosyncratic. Fundamental questions with idiosyncratic answers often aren’t good questions.**
Finally, double edged sword questions aren’t interesting if it’s obvious which of the two opposing forces or effects will win. For instance, when you stir your tea, on the one hand you accelerate convective cooling. On the other hand, you add kinetic energy, which heats the tea. But it’s blindingly obvious that the former effect completely dominates the latter.
*Not that competition for resources always selects for diversification, of course (Fox & Vasseur 2008 Am Nat, Vasseur & Fox 2011 Am Nat). Cases in which competition selects for character convergence are very understudied empirically. Setting up some chemostats with two stably coexisting algae each limited by a different nutritionally-essential resource and then running them long enough to test whether the algae evolve character convergence would be a difficult and somewhat high risk but very high reward experiment, I think.