I know you’ve done it. You’ve gone fishing in two different streams, or maybe even just looked at photos of brook trout from different areas, and thought “why are these fish so different from one another?” And, I’m not just talking about length. You may have noticed that brook trout can show a very large range in color, patterning, body depth, fin size, etc. Even fish within the same stream can sometimes look completely different. What gives?
It’s a great question. And the answers potentially have big implications for our understanding of how trout respond to fragmentation. I’ve already told you that fragmentation results in population isolation, which can lead to loss of genetic diversity and eventually lead to local extirpation. But, there are other changes that can occur before an isolated population collapses- the population size might decline, the ratio of males: females might become skewed, fish may start behaving differently, and, that’s right, individuals may start taking on different physical appearances.
The question is whether fragmentation leads to predictable changes in morphology across populations. If so, it indicates that fragmentation could be a factor that influences the evolutionary trajectory of populations in a predictable way. For example, if fragmented populations show a tendency to have reduced body coloration, then we would know that fragmentation somehow operates as a selection pressure, and that bright coloration is somehow not advantageous in fragmented populations (this scenario is purely hypothetical, by the way).
So, scientists being scientists, someone went out and tried to determine if fragmentation is an evolutionary selection pressure that acts on brook trout morphology. In a recent paper, researchers from Canada sampled individuals from 14 brook trout populations in Newfoundland, Canada. These populations are all genetically distinct, and upstream of barrier waterfalls (begs the question as to how the brook trout got there. Let’s leave that story for another day, but it was all natural, promise). At each site, the team took measures of body size, weight, and sex. They also took very detailed pictures so that they could later digitally measure things like body color, number of spots, body depth, fin length, hump size, jaw length, etc.
Is fragmentation a significant selection pressure on brook trout morphology? Yes and no. The researchers definitely found that populations had very different morphologies. Isolation has prevented gene flow, which has put each population on it’s own evolutionary trajectory. But, population size and standing genetic diversity, which act as a proxy for the strength of the effect of fragmentation, didn’t predict morphology that well. This indicates that fragmentation, itself, isn’t a factor that influences morphological change. Rather, current habitat conditions seemed to have a stronger influence of morphology. Trout in warm, slow streams tended to grow larger and develop a larger hump on their backs (reminder: warm is relative, these populations are in Canada), fish from faster streams tended to have longer pectoral and pelvic fins, and sites with more acidic water had fish with redder color tones.
Interesting, the environmental associations tended to be stronger in females than in males, and females also had more morphological traits that were correlated to habitat. For example, females developed redder tones in deep, fast, warm water, but the association was weaker for males. Why this is the case isn’t entirely clear. But, it likely has something to do with the fact that sexual selection acts much more strongly on males. Sexual selection is a form of natural selection that is specific to traits that increase reproductive success. Think about brook trout spawning behavior, and how sexual selection may act differently on each sex. Females build redds and then wait for eligible bachelors to arrive. Males have to compete for access to females, and subordinate males aren’t going to produce many offspring. This means that natural selection (via sexual selection) is going to strongly favor males that have traits associated with fighting ability during spawning, even if perhaps they are a little less adapted for the environment outside of spawning. Sexual selection acts less strongly on females, and so natural selection is going to favor females that are generally adapted to their local environment. As a result, female morphology is more strongly correlated to habitat than male morphology.
So, why do we care about these results as fish managers and conservationists? While many organizations are making strides to increase movement corridors and reconnect populations, streams are still becoming fragmented by loss of thermal habitat, road crossing, dams, etc. The results of this study suggest that fragmentation, itself, doesn’t seem to pose a strong selection pressure. But, the habitat that the fish become isolated in does. By building a road crossing, we could be effectively deciding the morphological fate for brook trout populations. How this could influence population survival remains unclear, but changes in morphology don’t seem, at least right now, to result in rapid loss of population survivability.
The more you know…
*Note: Content in this post is my own and may not reflect the opinion of the manuscripts' authors or the agencies they represent. I encourage you to read the manuscript, found here, so you can contribute to the discussion.