Not-so-parallel evolution in stickleback
Stickleback populations in lakes diverge morphologically from populations in adjoining streams. We find that the direction and magnitude of that divergence is not always the same when we compare multiple lake-stream pairs. Variation among pairs in the amount of lake-stream gene flow is one cause: more gene flow means less divergence. However, environmental differences among lakes and among streams also generate deviation from parallel; the more environmentally different two pairs are, the less parallel their lake-stream divergence. Thus, both adaptive and non-adaptive processes contribute to a continuum of (non)parallel lake-stream evolution. (photo credit: Thor Veen)
How repeatable is evolution? That is, given similar starting conditions, and a similar amount of time, will multiple populations evolve towards the same outcome ('parallel evolution') or not? And, if they deviate from a shared evolutionary outcome, is that because of historical accidents unique to each population, or a more deterministic, adaptive process? We set out to answer these questions by studying patterns of morphological, genetic, and environmental divergence among pairs of lake and stream populations of threespine stickleback fish (Image 1), on Vancouver Island.
Vancouver Island was covered by a massive glacier until about 12,000 years ago. When the glacier thawed at the end of the last ice age, it left behind hundreds of freshwater lakes and streams (Image 2). Shortly thereafter, an ocean-dwelling fish, the threespine stickleback, swam upriver to found permanent lake and stream populations. Crucially, each watershed was colonized (roughly) independently by (roughly) the same oceanic genotypes. As a result, Vancouver Island hosts a ‘natural experiment’: each watershed is an independent instance of evolution by stickleback from similar starting conditions. So, did they evolve to the same end-point or not (Image 3)?
Specifically, we asked (1) whether stickleback from adjacent lake and stream populations are morphologically and genetically divergent; (2) whether replicate lake-stream pairs (independently evolved in different watersheds) have diverged to the same magnitude, in the same directions; and (3) what might explain lake-stream evolution that is not strictly parallel.
To the field! We planned to collect extensive morphological, genetic, and environmental data for each stickleback lake-stream pair, while maximizing the number of pairs visited. Targeting eighty fish and environmental data from each of sixteen lake and stream pairs seemed ambitious but possible with a large enough work force. In May 2013, we convened that work force—a gigantic field team, comprising two sub teams totaling four professors, two postdocs, two graduate students, five undergrads, and three middle school science teachers (Image 4).
A typical day consisted of bushwhacking in the rain, setting 50 minnow traps in the rain, collecting fish in the rain, sampling for DNA and preserving fish under a tarp to stay out of the rain, and spending several waterlogged hours in lake or stream collecting habitat data, in the rain (Image 5). At our fastest, we sampled one population in one day. At our slowest, when fish were not abundant or avoided our traps, we slogged through one population in four days. By the end, in mid-July, we had amassed 2,500 fish from 16 pairs (32 populations! plus 3 marine populations), pickled in formalin and ready for processing. It was an epic field season. It was only the beginning.
The real grind began back in the lab (where it was dry, at least). There, two postdocs and two undergraduates worked for two years to photograph and dissect fish to measure nearly 100 morphological traits (Image 6), digitize field data sheets, extract and sequence DNA, process the DNA data through bioinformatics pipelines, scan for parasites, and identify food items in stomachs. Even just managing and curating the resultant data into analyzable form required the recruitment of another postdoc for several months. By early 2016, we had tens of gigabytes of genomic data, thousands of fish’s worth of morphological measurements, and environmental, parasite, and stomach content data for all 32 sites. It was a monumental dataset, intimidating and exciting at the same time. We required a year, and the help of yet another postdoc, to fully analyze the data, write the manuscript, and advance the manuscript through review. Science is slow!
What did we find? First, genetic exchange between adjoining populations constrains divergence, as expected. The higher the gene flow, the smaller the lake-stream divergence. But which traits were diverging varied: divergence was sometimes very parallel, sometimes not at all, depending on which watersheds we were comparing. This heterogeneity was explained by the environmental data. Not all lakes are the same, and not all streams are the same. Variation within each habitat 'category' (lake or stream) predicted which lake-stream pairs would be highly parallel, or deviate from parallel. Thus, morphological evolution is non-parallel in part because of adaptation to non-parallel selection. Evolution is partly repeatable, but even some of the non-repeatable variation can be attributed to adaptation.
Like most science, this project was not fast. It was not easy. It has been a dedicated collaboration spanning almost a decade, with multiple researchers who complemented each other’s skills and expertise, shared their ideas, buoyed morale through difficult times, and shared in the beauty of remote field locations (Images 2, 5, 7), successful data collection and analysis, and scientific story telling. We feel that this work will advance the field by underscoring that parallelism is a continuum, and that novel insights into the evolutionary process arise from moving beyond “is evolution repeatable?” to “how repeatable is evolution, and why?”.
The paper in Nature ecology & evolution.
Read Dan Bolnick, Katie Peichel and Andrew Hendry's recountings of the history of this study here.
Find many more images and videos of field work on Vancouver Island at the following links.