Slow down, please: The evolution of beak morphology in Tanagers

Which evolutionary model best explains the evolution of this bird group?

The early bird gets the worm. This saying not only applies to our everyday life, it can also be relevant for evolution. When a species colonizes a new area, its members might be confronted with numerous vacant ecological niches. Some individuals might adapt to feed on worms, while others prefer grains or fruits. This situation of ecological opportunity sets the stage for rapid diversification and the origin of new species. In other words, an adaptive radiation. From a theoretical point of view, you would expect an initial burst of species diversification followed by slowdown of evolutionary changes as the niches are being filled.

This scenario has been described for island populations, but does it also apply to species that spread across continental landmasses? A recent study in the Biological Journal of the Linnean Society tested this model with such a group of species: the tanagers. About 12 million years ago these birds colonized South America and diversified into more than 300 species with a wide range of beak morphologies. The ideal study system to explore the early burst scenario on a large spatial scale.

Three Models

Nicholas Vinciguerra and Kevin Burns collected data on the beak morphology for 333 out of 377 species of tanagers. Next, they summarized all morphological variation in a few metrics and compared three different evolutionary models to explain the observed variation:

  • Brownian Motion (random changes over time)
  • Ornstein–Uhlenbeck (evolution towards an optimal value)
  • Early Burst (the model described above)

The analyses revealed that the Early Burst model was the best-fitting model. The researchers noted “a rapid burst of bill shape evolution early in the evolutionary history of tanagers followed by a subsequent slowdown toward the present.” This finding supports the scenario that tanagers quickly filled the available morphospace in beak morphology when the ecological opportunities were present. Over time, the available niches filled up and the rate of evolutionary change dropped.

The phylogenetic analyses showed that an increase in new species (blue line) is accompanied by an early burst in beak morphology followed by a slowdown in evolutionary diversification (black lines). From: Vinciguerra & Burns (2021).

Subfamilies

More detailed analyses revealed that the Early Burst model also applied to specific subfamilies, namely the core tanagers (Thraupinae), the highland tanagers (Diglossinae), the warbler tanagers (Poospizinae), the saltators (Saltatorinae) and the honeycreepers and allies (Dacninae). Adaptive radiations nested within a larger adaptive radiation. Similar patterns have been found in Vangas after they colonized Madagascar, but the situation in the tanagers appears more extreme.

However, the accumulation of species and morphological disparity within vangas occurred 23 Mya within an insular system, whereas in tanagers this evolution has occurred on a continental scale in nearly half the amount of time.

Interestingly, the Darwin’s Finches – the textbook example of an adaptive radiation – did not follow the Early Burst model. The lack of this iconic subfamily in the list above can probably be explained by their recent evolutionary origin. The Darwin’s Finches are still in the early stages of an adaptive radiation. If we could wait a few thousands to millions of years, we might see a slowdown in evolutionary rate in these birds.

An overview of the different subfamilies within the tanagers. Five of these groups also showed an Early Burst pattern of diversification in beak morphology.

References

Vinciguerra, N. T., & Burns, K. J. (2021). Species diversification and ecomorphological evolution in the radiation of tanagers (Passeriformes: Thraupidae). Biological Journal of the Linnean Society133(3), 920-930.

Featured image: Purple honey creeper (Cyanerpes caeruleus) © Charles J. Sharp | Wikimedia Commons

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