Recent reading: 4 March 2022

A view along a packed bookcase
“Library” (Flickr: Stewart Butterfield)

It’s now two weeks since I resumed in-person teaching, and so far, so good. It’s shockingly refreshing to actually interact with students directly, even with everyone masked, and to be able to just improvise with a specimen picked up on a walk around the campus. And field trips are back! There’s war in Europe (and the U.S. is at sort-of-but-not-war with Russia?) and the IPCC has dropped a huge list of things we need to do to cope with coming climate change but on Saturday I guided a bunch of students through keying out Ceanothus spinosus at the side of a trail in the Santa Monica Mountains. We might not be out of the woods yet, but we’re in different woods.

Here’s what I’ve been reading, recently:

Wahl LM and MM Tanaka 2022. Hazardous loss of genetic diversity through selective sweeps in asexual populations. American Naturalist doi: 10.1086/717813

A new-ish spin on the disadvantage of asexuality: asexual populations are poorly able to recover variation following a hard selective sweep.

  • Very nice rundown of the processes that make recombination advantageous in the Introduction of this paper: negative epistasis, Muller’s ratchet and mutational meltdown, extinction vortices.
  • Models clonal populations in a specific region of parameter space, where variation is mutation-limited and population size is large enough to evade Muller’s ratchet, but small enough that a selective sweep can reduce variation to a hazardous degree.
  • Authors note this effect may introduce a limitation on the efficacy of evolutionary rescue, in which adaptation to a hostile environment reduces effective population size enough to kick a population towards mutational meltdown.

Ghosh et al. 2022. Preferential allocation of benefits and resource competition among recipients allows coexistence of symbionts within hosts. American Naturalist 10.1086/718643

Population dynamics models of a host and two symbionts, one mutualistic and one nonmutualistic, which show that if the host can preferentially allocate rewards to the mutualist and the two symbionts compete within the host, both symbionts can be maintained in the association. This makes coexistence of the mutualist and nonmutualist possible under a wider range of conditions than previous theory identified.

Castillo AI et al. 2022. Genie: an interactive real-time simulation for teaching genetic drift. Evolution: Education and Outreach doi: 10.1186/s12052-022-00161-7

Presents a very (conceptually) simple web-based simulator of genetic drift in a spatially distributed population with an interface allowing manipulation of mutation rate and spatial population structure, and introduction of new mutations on command.

  • Briefly outlines class activities studying simulating drift and gene flow, spatial corridors, and drift/mutation balance.
  • Tracking student comprehension of a standardized set of genetic drift concepts before and after instruction with and without the new simulator doesn’t show a greater benefit to its use, however.
  • May be a nice option for a very simple interactive, and potentially more intuitive than other options I’ve used in class.

Boyle JH et al. 2022. Temporal matches and mismatches between monarch butterfly and milkweed population changes over the past 12,000 years. bioRxiv doi: 10.1101/2022.02.25.481796

Presents a new chromosome-level genome assembly for Asclepias syriaca and uses it in concert with range-wide data for this species and monarch butterflies to reconstruct the demographic histories of both back to the last glacial maximum.

  • Apparently A. syriaca is panmictic across eastern North America! The authors present STRUCTURE results for K = 2 and K = 11 showing sample-bars divided in horizontal stripes — they didn’t run K = 1? Lots of folks don’t; the authors say, on page 5, that you can’t, but that is actually not correct. (Though reading that statement made me seriously doubt myself for the time it took to dig up the paper presenting the STRUCTURE method.)
  • The headline results are more characterized by key events that aren’t found in the population genomic data: the authors find a population expansion for milkweed coinciding with deforestation of eastern North America by European settlers, but no matching expansion for monarchs; and no evidence of monarch or milkweed effective population size losses to match the census declines documented in the last 40 years.

About Jeremy Yoder

Jeremy B. Yoder is an Associate Professor of Biology at California State University Northridge, studying the evolution and coevolution of interacting species, especially mutualists. He is a collaborator with the Joshua Tree Genome Project and the Queer in STEM study of LGBTQ experiences in scientific careers. He has written for the website of Scientific American, the LA Review of Books, the Chronicle of Higher Education, The Awl, and Slate.
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