Why Are There So Many Species?

The causes and ecological context of rapid morphological evolution in birds (2022) Crouch & Tobias, Ecology Letters, https://doi.org/10.1111/ele.13962

Image credit: Andrej Chudý , CC BY-NC-SA 2.0

The Crux

One of the biggest questions facing evolutionary ecologists is why some groups of organisms contain SO MANY species, while others are relatively sparse in comparison. We’ve discussed adaptive radiations on Ecology for the Masses before, which is when a burst of speciation occurs within a group, with new species adapting to fill new ecological niches. It could be that the reason for such uneven groups is that some clades, or related groups of organisms, are more prone to such adaptive radiations than others. If this is true, it would mean that such clades experience not only an increase in the number of lineages (species) that they contain, but also the number of traits they exhibit.

Increases in the speciation rate and trait evolution are the hallmarks of adaptive radiations, but they may not occur at the same time, which can lead to some different outcomes. Clades may diversify rapidly, without really evolving new traits, and this is known as a “non-adaptive radiation“. In contrast, a lineage may quickly evolve new traits without speciating, which is known as an “adaptive non-radiation“. To understand the causes and context of such evolutionary scenarios, today’s authors studied the history of bird evolution.

What They Did

Getting reliable results from an analysis of evolutionary patterns requires an enormous amount of work and data, and today’s study is no exception. They first used the phylogenetic trees from a previously published paper, identifying nodes in the tree (see Did You Know?) where the speciation rate shifted. They paired this analysis of speciation rate with an analysis of the evolution of morphological traits, or those relating to the bird’s body. In other words, they were interested in detecting where on the phylogeny there was an increase in the rate of the evolution of body-related traits.

Did You Know: Nodes

Nodes are the point on a phylogenetic tree where one lineage splits into two (or more). Think about us and how we are most closely-related to the great apes. Our most recent common ancestor is the node where the split to humans and the other great apes happened!

Using these data, the authors wanted to test whether the rate of morphological trait evolution was elevated before and after speciation rate shifts. They did this by comparing the rates of morphological trait evolution on the branches on either side of a node that was previously identified as a speciation rate-shift node (aka the mother and daughter branches). Doing so would give the the authors information on the relationship between morphological trait evolution and speciation.

What They Found

The authors identified 34 nodes within the tree as likely rate-shift nodes. Interestingly, there were no relationships between the locations of these speciation rate-shift nodes and the location of shifts in morphological trait evolution. For example, on average a shift in the rate of the evolution of morphological traits took place 7 million years before a shift in speciation rate.

What’s more, the rate of evolution of morphological traits before shifts in speciation rates was similar to the rate after the shift in speciation rates, meaning that the speciation rate-shift was decoupled from the shift in morphological traits. Because the changes in the rates of speciation and trait evolution were decoupled, these speciation events can be considered “non-adaptive radiations”.

Darwin’s finches (pictured) are a classic example of the adaptations that a group of closely-related organisms develop to utilized different resources. (Image credit: Kiwi Rex, CC BY-SA 4.0 via Wikimedia Commons)

Problems?

The phylogenetic tree used by the authors is approximately a decade old, and while it has been incredibly useful in a number of studies (including today’s paper), it contains a number of errors. Such errors are a normal part of science because taxonomy (the naming and grouping of organisms into groups) is a dynamic process, so the placement of species changes as scientists learn more about them. The authors employed a number of methods to help control for the many errors inherent to the tree that they used, but I would be interested in seeing their analyses applied to an updated tree.

So What?

Adaptive radiations are a classic hypothesis in evolutionary ecology, with such explosions in the number of new species normally tied to a spread in the number of niches that a group can exploit (think of Darwin’s finches). What today’s study showed is that, at least for these birds, explosions in speciation rate and increases in morphological evolution are decoupled from one another, with different factors driving each. I find it very interesting when classic ideas aren’t supported, and appreciate that studies like this one do the work to test classic and widely-accepted hypotheses.


Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions who has gone to the dark side of ecology by becoming a birder. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.

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