On getting empirical with the obvious

Sewall Wright knew how to pose for cool science portraits

Sewall Wright, titan of genetic drift and master of posing for cool science portraits

I’ve been thinking lately about the value in doing “simple” things. As a PhD student, my time is constantly incentivised by productivity (what am I doing right now that is working towards a publication?). But that doesn’t jive well with most general types of curiosity. Not the “big idea” types of questions, the little stuff: I wonder what happens if I change this parameter? Can I do this in R? Does that pattern happen in plants…birds…crocodiles….?
Paradoxical to these daily feelings, simple questions often beget great scientific impact.
A nice example is providing empirical evaluation of established theory. I, like many students, am guilty of lapping up most textbook theory as if it was handed to me by God on a stone tablet. That’s why I’m thankful for investigators like Jennifer Lohr and Christoph Haag, who recently provide a comprehensive experiment published in Evolution that explains how a fundamental aspect of molecular ecology (population size) relates to three big evolutionary co-factors (genetic load, inbreeding depression, hybrid vigor).
If you’ve taken an popgen course, a good evolution class, or read some primary literature on genetic drift and population size, here are the predictions you’d likely make:
Population size goes down, genetic drift increases, so:

  1. genetic load increases
  2. hybrid vigor increases
  3. inbreeding depression decreases

These factors have some pretty clear implications for the evolution of things like dispersal and the investment in local adaptation, and the associated theoretical literature that these predictions stem from is dense. But empirical evaluations are rare and have been limited by different effects between traits, which may relate to local adaption. Lohr and Haag use groups of Daphnia that have varying levels of genetic diversity to conduct outbreeding/inbreeding trials, establish clones, and measure life history traits.
The result? Well, yeah, it is what you’d expect.
As genetic diversity decreases, hybrid vigor increases, genetic load increases, and inbreeding depression decreases. While not surprising, these authors went to great lengths providing evidence for theory central to the way we think about how population size influences evolutionary trajectory.
Even if you already have a good feel for these ideas, the introduction of this paper is worth reading for clarity alone. Maybe you could pass it along to your own students or maybe even suggest they find another theory to test themselves. Something “simple” might be sitting right under their nose.
Lohr, J. N., & Haag, C. R. (2015). Genetic load, inbreeding depression and hybrid vigor covary with population size: an empirical evaluation of theoretical predictions. Evolution. DOI: 10.1111/evo.12802

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