One of the questions in modern biology is how some of these most dramatic examples of morphological variety can be explained. Two of these examples can be found in birds: one is Darwin’s finches which have famously very large variety of beak shapes which allows them to focus on different food types. Another similar example which is in many ways even more dramatic is birds called Hawaiian honeycreepers which inhabit Hawaii. On HI a single ancestor which came Asia (so they’re probably related to rosefinches which we discover presently in Japan and in Siberia) to the Hawaiian Islands about ten million years ago and produced enormous variation. Historically there were over forty species. They were some finches which fed on seeds; some of them became creepers, they’d elongated beaks to kind of capture insects below the bark; and some several lineages became what we call honeycreepers, that’s they evolved the super long beaks to feed on the nectar the flowers.
So this enormous variety that you discover on HI among the Hawaiian honeycreepers is one of the grand examples, one of the textbook examples of adaptive evolution in nature. How do you clarify this? So recently we decided to see at this a more structuralist perspective, that’s we generated a very full collection of high-definition 3D shapes using computed tomography scans, Court scans that are d in hospitals to produce stands of human organs by the surgeons, for example, by the physicians. We generated a collection of Court scans all the species of Hawaiian honeycreepers and their relatives. We also wanted to compare the variety of the skulls and beaks to that of Darwin’s finches, so we generated a similar collection of high-definition scans Darwin’s finches and their relatives. We also scanned some of the species which were ancestral to both Darwin’s finches and Hawaiian honeycreepers.
We wanted to compare the skull variety in these two groups (which are both very diverse) using the same landmarks within the same morphospace using a method which is called geometric morphometrics we apply the same landmarks onto the skulls, and we the computer to ask questions on how the variety compares and what happened during evolution of these two very diverse clades of organisms.
What we found was very surprising. We found that Darwin’s finches’ skull shapes were, as we expected, much more diverse than those of the ancestors, they occupied much more of the morphospace. The Hawaiian honeycreepers turn out to be even more diverse, they occupied the quantity of morphospace which was even greater than that of Darwin’s finches, including some of the truly unique skull shapes and beak shapes, particularly those which were adapted for feeding on the nectar. But the biggest astonishment was when we compared how the different parts of the skulls evolved and developed relative to each other. This is referring to what we call skull modularity.
The skull can be divided into several domains, several parts which have somewhat distinct functions. For example, a beak is one such module; the cranial vault which protects the brain, the upper portion of the skull is another module; the orbits which defend the eyes is another module. So when we applied landmarks, we applied them so that we can distinguish all these different modules in the skull and look how they changed during evolution relative to each other, that’s if all of them changed in the same way. For example, if, as a skull becomes bigger, all the modules become bigger, then it means that the skull, all these modules are highly integrated, they’re talking to each other very closely and they behave in very similar ways. If these modules are more independent, then we speak about increased modularity, that’s increased independence of those different parts and lower integration.
“So what we found is that in Darwin’s finches relative to the ancestral species there was increased modularity in the beak”
We already knew that, we already knew that the beak program is highly modular in Darwin’s finches, so that was a kind proof of our previous studies. We found that in ancestral birds the skull is highly integrated, that’s all the different parts modify in a very near correlation with each other. However, in Hawaiian honeycreepers we found that the integration was extremely low, most of the parts of the skulls, different modules of the skulls were incredibly flexibly connected to each other. It was truly love a Lego system, that’s different parts could be mixed and matched, they could modify with relative ease during evolution, and this is what we think was the foundation of the diversity: the fact that they were able to modify different parts of the skulls in different ways. This incredible flexibility, this modularity in their skull design is what was probably foundational to the variety and that’s what’s underlying their ecological success.
We don’t know yet how modularity is established, we don’t know yet how the different modules separate each other and how they speak to each other. We know they exist, and we know that this communication between different modules is critical to the development, but we don’t yet know how this modularity is established and we don’t know yet how this modularity can change, how evolution can modify the level of integration modularity. But we do look proof now, for example, when we’re looking at the skull modularity in the Hawaiian honeycreepers, that the flexibility which they evolved in the designs correlates really strongly with the quantity of variety that they were able to generate during the evolution.
Skull modularity is necessary for two reasons. One is an evolutionary reason, that’s we’d love to realize how variety can be generated for structures which are otherwise thought to be tough to change, such as skulls and animal heads you’ve multiple tissues, multiple organs such as the brain and the eyes, the musculature in the skull, and the skull has multiple different bones, they all have to speak to each other, and this integration is thought to be a limiting factor to their evolution beca all these different things have to modify together. The studies on Hawaiian honeycreepers, for example, propose this integration can change, that’s one can expand the flexibility of individual elements and create them modify relatively independently each other. This provides us with a very necessary clue how the skull as a whole can evolve as well.
Another reason why skull modularity is necessary is developmental. As a developmental biologist, I’d love to realize how modularity is established in the first place, how you, for example, set up different parts of the skull. Skull is made of multiple bone and cartilage elements. How those elements are established? How they’re individually regulated in terms of their size and shape? How they’re integrated into the overall whole, how do they speak to each other? Essentially it’s love a very complex puzzle with all the pieces coming together to form a functional system. So developmental mechanisms behind modularity integration are very poorly understood at the moment, and it’s necessary to realize these mechanisms before we can realize how they can be altered during evolution.
It’s necessary also to realize that these questions about integration are also necessary for human health. There are quite a few biomedical conditions some of these parts, for example, some of the bones are fd prematurely or they don’t f they should be, and this cas a no of different craniofacial abnormalities which are essentially problems with the integration of the different parts of the cranium. So cranial modularity is necessary to realize in the future all those perspectives: evolutionary perspective, how it can modify to alleviate morphological evolution; developmental perspective, how the modularity and integration are established at the genetic, molecular level, so what’re the developmental, for example, genes and pathways involved that allowed these different modules to interact with each other, how this interaction can change, how can it become stronger or weaker and how that can facilitate, again, the evolution of the all system. Finally, it’s necessary for biomedical reasons beca a lot of these interactions between different parts, between the brain and the skull and, again, between the skull and the musculature are critical for the proper development of the head. When something goes wrong, when some of these mechanisms which authorize for integration are affected, that can ca craniofacial abnormalities that have to be treated by surgical or other techniques.
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