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What's driving morphological trait evolution?

My personal motto has pretty much always been to do the thing I’m most afraid of, within reason of course. Following this guideline has opened me up to a world of opportunities and personal growth. So, I decided for this blog post I’d take the same approach and dive into an area that I’m not very comfortable with. For me this area of discomfort is the intersection between large diverse communities and evolution. 

Trophic interactions define our understanding of ecosystem resource cycling and diversity. Understanding the spatial and evolutionary scales at which trophic ecology influences biodiversity of central import to understanding long term macro-evolutionary dynamics.  A 2019 paper, “Reef fish functional traits evolve fastest at trophic extremes”, dives into the impact trophic position exerts on trait evolution in coral reef environments. Coral reefs sit on the upper limit of ecosystems teeming with  biodiversity. That said, they are an excellent setting for trophic studies, such as this.   

Researchers examined three hypotheses regarding the drivers of evolution: the height constraint hypothesis (HCH), the niche variation hypothesis (NVH), and the trophic extremes hypothesis (TEH). Under the HCH, high trophic levels would evolve more slowly than lower trophic levels because of the constraints imposed by high trophic levels feeding on evasive lower tere prey. Alternatively, NVH suggests the existence of a positive relationship between niche breadth and phenotypic variation. If NVH were the true evolutionary driver we would see intermediate trophic levels experiencing the highest rates of trait evolution. TEH, on the other hand, that organisms on the extreme ends of the trophic level, either highly or lowly situated, would experience the highest rates of trait evolution because these groups would have the most difficulty acquiring resources, creating a strong motivator for trait evolution. 

To actually put these hypotheses to the test, the team used previously published trophic level data to create a phylogenetic reconstruction of over 1,500 coral reef species. To measure the rate of trait evolution, researchers used an R package to map the multivariate rate of morphological evolution across trophic levels. Ultimately, this yielded a number of interesting results. Primarily, they observed that top-level predators experienced the fastest rates of most morphological evolution. This automatically rules out the HCH. They also observed that mid-level predators had the slowest rates of morphological evolution, scoring points for the TEH. Herbivores had the quickest rates of ME, while omnivores had the slowest, suggesting against the NVH. 

Given these findings, the TEH best supported the patterns seen, meaning that organisms at trophic extremes, those having the most difficultly with resource acquisition, experienced the highest levels of morphological trait evolution. I feel after reading this article that, like me, species that face the greatest challenges also experience the most growth. 


Citations: 

Borstein, Samuel R.; Fordyce, James A.; O'Meara, Brian C. (2019). Reef fish functional traits evolve fastest at trophic extremes. NATURE ECOLOGY & EVOLUTION 3, 2. 

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