Body size evolution during a metapopulation expansion

Animals and plants come in a wondrous variation of size. This variation is obvious among species, but it can also be tremendous within species. Fish are a shiny example, like in salmonids, where for a given age, some fish can be twice as long as others, and much heavier. Growth is very plastic in fish, and it does explain a large part of this variation in correlation with trophic resource and local density, that drive competition for resource. Such process can be particularly well observed in recent metapopulations: as time passes, core populations tend to become more populated, increasing local competition over resource. As a consequence, body size at age should decrease over time (red arrows in the figure below).

But there are other mechanisms that may drive the evolution of growth, and therefore body size at age. As the metapopulation itself expands, new boundaries populations are created by dispersing individuals, and these individuals may not be a random sample of their source population. If they are presenting higher body size at age than the population mean for example, then we could observe a gradient along the expansion front where body size at age would continuously evolve toward higher values (green arrows).

This spatial sorting is expected because body size at age is often partly heritable, so these dispersers present genetic characteristics that will drive the foundation of the new population, provided the subsequent gene flow with core populations is not too strong.

We turned to the invasion of Kerguelen islands by introduced brown trout to investigate theses hypotheses. We managed to squeeze our database to obtain more than 21000 captures of one year old trout, along with their body size and day of capture, distributed over 42 populations spanning 50 years of monitoring. And we looked at how body size changed in each of these populations over time, depending on their foundation dates.

What we found was both reassuring and surprising. In fact, in naturally founded populations, body size evolved the way we expected: it increased along the expanding front (black curve, left panel on the figure above), yet at a reduced pace. In brown trout, migrating (and therefore potentially dispersing) individuals are usually the ones growing faster, so increased body size in newly founded populations makes sense. When populations got more crowded however, body size decreased quickly probably under the effect of competition for resource (greyish curves, left panel).

When we looked at populations introduced by human (right panel), the story was way different. First, body size on average was much smaller. Second, it was also a bit higher in recent populations compared to ancient ones, but this could not be due to spatial sorting (since no dispersers founded these populations). Finally, we did not find evidence for decreased body size in old populations where density should be higher. There are a number of possibilities to explain all these differences, but in a nutshell: even in remote areas such as subantarctic Kerguelen Islands, the footprints of human presence on evolution is staggering.

You may find more details in our recent publication on the matter, part of Lucie Aulus’s PhD:

https://doi.org/10.1098/rsbl.2021.0366

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