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.
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