By Alison Campbell 25/05/2020


The 2015 Schol Bio paper included a question about a group of birds known as honeycreepers, specifically, the 56 species endemic to the Hawaiian islands. (Or, were endemic: 18, or perhaps 19, are still living; the others are extinct.) Students who’ve already had a look at this paper as part of their preparation for the exam will know that it included 2 pages of ‘resource’ material (a combination of maps, images, and information) ahead of a question that asked them to

analyse the information provided in the resource material and integrate it with your biological knowledge to discuss the evolutionary processes that have resulted in the distribution and diversity of honeycreepers on the Hawaiian islands. 

These lovely little birds are indeed very diverse, particularly in terms of their beaks, which indicates that over time evolution has resulted in adaptations to a wide range of diets. (There’s a lot of variation in beaks and feet as far as birds are concerned – shape & to a large degree size are constrained by the requirements for flight, which set an upper weight limit. The really big birds – eg penguins, and the various ratites – are flightless.)

The Hawaiian honeycreepers have a history of rapid adaptive radiation, as the volcanic islands are very young in geological terms – you could describe their history as an example of punctuated equilibrium. The main islands range in age from Kaua’i, about 5 million years old, to Hawai’i, less than a million years old. As you can see from the map below, these are just the most recent islands in a chain that extends all the way back to Midway Atoll. The chain formed as the Pacific crustal plate moved over a stationary hotspot in the underlying magma.

The Northwestern Hawaiian Islands, protected as the Papahānaumokuākea Marine National Monument since 2006 (NOAA map).
Image source: Schultz, O’Malley, Kehn, Polovina et al., 2011

The common ancestors of the honeycreepers – & of their sister taxon, the rose finches – would have arrived on the main Hawaiian islands by island-hopping down the chain, touching down on Kaua’i some time around 5mya. In genetic terms you’re looking at an example of the founder effect: with a small population, their gene pool represented only a sample of their original population back in Asia – although it’s interesting that Lerner & her colleagues (2011) suggest that in fact the founding population could have been relatively large, perhaps hundreds or thousands of birds. And, in that founder population, it would be possible for genetic drift to result in fixation or loss of particular alleles.  As the colonising birds arrived on each new island in turn, they’d have encountered new niches and different selection pressures, resulting in adaptive radiation, with any new mutations or particular combinations of alleles being selected for if they provided an adaptive advantage.

It’s worth noting at this point that the ocean forms a geographic barrier so that birds on eg O’ahu are geographically isolated from Kaua’i and Moloka’i, but that speciation on any particular island would be sympatric. Students need to remember that it wouldn’t be sufficient to say just this; you’d also need to talk about the development of reproductive isolating mechanisms that prevent interbreeding and gene flow. For example, you could talk about the role of sexual selection, where one partner selects another on the basis of different colours or courtship behaviours. Or you could mention the possible role of niche differentiation, where competition for resources results in birds using different food sources, with those birds with beaks having features that make them better able to use those foods being at a selective advantage. If you have a look at the figure below, you’ll see that there’s considerable variation in beak size and form in the honeycreepers. One of them, the ‘akiapola’au, uses its bill in a similar way to woodpeckers (& our own extinct huia), feeding on insects that burrow into trees (convergence in behaviour & to some degree in beak shape).

Figure 2. Bayesian Divergence Date Estimates for Hawaiian Honeycreepers from Whole Mitochondrial Genomes Based on Three Island Age Calibration Points from Lerner, Meyer, James, Hofreiter & Fleischer, 2011.

The diagram shows that some honeycreepers – eg the i’iwi, apapane, and akepa – are found on multiple islands, while others (such as the Maui and Hawai’i creepers) are restricted to a single island. This may reflect the availability of particular types of food, though it could also be that a species migrated from one island to another and the first population subsequently became extinct. The Nihoa finch is a generalist, which would make it a good colonising species, while other species – eg the i’iwi – have specialised on particular food sources.

And it also shows that a great deal of the diversification in the Hawaiian honeycreepers happened around the time that the island of Oahu became available for colonisation – for whatever reason, the formation of this island had a big impact on the group’s evolutionary trajectory.

If you want to read even more about these birds, this excellent blog post by GrrlScientist gives a very thorough overview of the original paper (Lerner et al., 2011) on which the question is based.

 

Lerner, H.R.L., Meyer, M., James, H.F., Hofreiter, M., & Fleischer, R.C. (2011) Multilocus Resolution of Phylogeny and Timescale in the Extant Adaptive Radiation of Hawaiian Honeycreepers. Current Biology, 21 (21): 1838-1844. doi: 10.1016/j.cub.2011.09.039

Schultz, J., O’Malley, J., Kehn, E., Polovina, J., Parrish, F., & Kosaki, R., (2011) Tempering expectations of recovery for previously exploited populations in a fully protected marine reserve. Journal of Marine Biology, 2011 (5530). doi: 10.1155/2011/749131

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