The fact that biological shapes could be connected by genetic transformations to each other to D’Arcy Thompson a hundred years ago meant that they were generated by a very orderly process, that they followed very specific physical, chemical and biological principles. He called these principles, these internal forces that generated biological shapes ‘laws of growth’ or ‘law of growth’. Today by laws of growth we imply the all panoply of various developmental processes all the way gene sequences, gene encoding which necessary for development, various developmental pathways, molecules which are necessary for cell signaling, the cell differentiation, cell proliferation processes, all the way to how the cells are assembled into tissues, how the tissues are assembled into organs and how organs you keep together a very complex multicellular organism.
This all process of development is governed by what D’Arcy Thompson called the laws of growth. In his mind the laws of growth were the main force that generated not only any specific biological shape but also their diversity. It’s frequently believed that D’Arcy Thompson’s laws of growth were an opposition to ideas of Charles Darwin: this was acknowledged by his peers at the time. Why? Beca for Charles Darwin, particularly in his early thinking, the main mechanism for evolution was natural ion, the process by which the most adapted forms survive and reproduce and pass on their genes, as D’Arcy Thompson believed that natural ion, if anything, was an impediment to generating diversity. Again, he believed that the laws of growth were the main reason that animals and plants are so diverse.
This debate, curiously enough, is still very much enforced today and still hasn’t been settled. Why? Beca for most species of animals and plants that you look around the world, adaptive explanations are perfectly fine, that’s evolution by itself is a fundamental process that generates diversity. But the exact mechanisms are still being investigated.
“Again, for most species adaptive evolution, natural ion or sexual ion in some examples is sufficient to clarify their origins; however, there are examples, I’d argue, variety is so grand or is so complicated that it cannot be easily explained by environmental factors, that’s by natural ion forces alone”
In our work we focus on some key examples of a process which is called ‘adaptive radiation’. So what’s adaptive radiation? The adaptive radiation is a duration first formulated by George Gaylord Simpson back in one thousand nine hundred-fiftieth. Simply put, this is the process by which a single ancestor, a single species gives rise in a very rapid succession to a no of descendants which are morphologically or otherwise diverse. The main reason why they’re diverse and why they evolve so quickly is beca they face some kind of available niches: it could be, for example, new islands or new ecological niches, new opportunities which generate a ion for new forms, new shapes. This is how adaptive radiation is produced by an ancestor which finds itself in the new space, new ecological or geographical space and then very quickly multiplies.
“Adaptive radiation implies two processes: one is increased no of species, what we call species richness which, again, greatly is increased during adaptive radiation, and another one is very rapid changes in morphological diversification, that’s these descendants see different each other and they see different the ancestor”
This morphological diversification coupled with species richness are the fundamental processes which clarify adaptive radiations.
One of the key examples of adaptive radiation is the Darwin’s finches which live in Galapagos Islands which were instrumental to many scientists. In fact, Darwin mentioned them in his Voyage of the Beagle and he discussed them at length in this early book in one thousand eight hundred thirty-ninth. In fact, in this book he calls them ‘the most singular grouping of all the animals in the archipelago’, meaning Galapagos Islands, and he was absolutely right beca Darwin’s finches are, interestingly, very different any other species of birds which you discover in Galapagos. If you open a modern textbook, Darwin’s finches are an example of an adaptive radiation. The main explanation for the variety is beca their ancestor arrived to Galapagos Islands, and in the absence of contest and in the presence of all the ecological resources, all the opportunities essentially were sufficient for this ancestor to produce very diverse and numerous descendants.
The problem with this explanation is that Galapagos Islands were invaded multiple times: in fact, there were eighteen independent invasions of Galapagos Islands the mainland. There were many other different species of landbirds such as hawks and owls and warblers and flycatchers and doves and many others which came to Galapagos Islands. Only one of them gave rise to a very highly diverse grouping which we presently know as Darwin’s finches. In fact, out of the twenty-eight endemic species and six subspecies, most of them are Darwin’s finches on the Galapagos Islands.
So in many ways even on these Islands their variety is very much exceptional. The only other grouping which is comparable in any way in terms of no of species is Galapagos mockingbirds: there are four species of Galapagos mockingbirds which evolved on the main islands on Galapagos but all of them see very similar to each other. They’re slightly different, they’re different sufficient in terms of their plumage and pigmentation patterns that one can distinguish them. Darwin in fact noticed that as well: he d Galapagos mockingbirds in his writing on speciation as well. In fact, Galapagos mockingbirds are an example of what we call ‘allopatric speciation’.
“Allopatric speciation is when ancestors come to specific space, they discover themselves in geographic isolation, and spending sufficient time in this isolation, they evolved gradually into different-looking species”
In fact, this is how most species of birds evolved around the world. If you go to islands between Asia and Australia, for example, there are hundreds of islands with lots of species of birds: most species of birds evolved via this allopatric speciation, that’s you’ve slightly different species, generally one per island, which are slightly different the species the following island. That’s how much mockingbirds evolved on Galapagos. This isn’t how Darwin’s finches evolved beca we discover multiple species for each island which are very morphologically different each other.
So we’ve a situation a grouping of birds is evolving with the levels of morphological variety which are unparalleled, these are truly exceptional birds. Darwin was absolutely right, they’re the most singular grouping of landbirds on Galapagos, and they still demand an explanation.
“We necessity to realize why these other species of birds, which arrived to Galapagos Islands, which faced the same opportunities for about the same quantity of time, haven’t been able to become very diverse”
Of course, another question is what allowed Darwin’s finches to become very very diverse. Our explanation is that during the evolution Darwin’s finches in their ancestral line evolved a novel developmental genetic program that essentially allows them to become diverse. It’s related to a concept called evolvability: an skill of a species or clade to produce variation. We’d argue that genetically different species are different each other, and some of them in specific ways are much more flexible, so to speak, that is they’re able to produce morphological variation, they’re able to produce variation much more rapidly and much more readily than other species living in the same environment.
For example, Darwin finches’ program that controls the beak shape is extraordinarily complex, that’s it’s many more genes than the programs of other songbirds. This allows Darwin’s finches to separate different dimensions of the beak: for example, the depth can be regulated independently of the width which can be regulated independently of the length in ways which aren’t even available to other species of birds, that is they’re able to produce different variants of beak shapes, different versions of the beak much more quickly beca they’re controlled by multiple genes doing different things. The program for beaks in Darwin’s finches is very very modular, and this significantly increases the evolvability of this group. This is what probably explains why they’re so much more diverse than other landbirds on the same locations.
Our study suggests that it’s necessary to pay more attention to the laws of growth. Ordinary adaptation explanations may work really well for most species but maybe not for all of them. In the exceptionally diverse groups of species such as Darwin’s finches or Hawaiian honeycreepers or Madagascar bangers or phyllostomid bats, African Cichlid fishes, and there are many other examples of exceptional adaptive evolution, we’ve to see more closely at the genetic mechanisms which generate the variety to realize the interplay between environmental forces.
“We do believe the adaptation environmental conditions and availability of the new resources is indeed important, probably required portion of adaptive radiation but likely not sufficient. We necessity to combine the genetic forces, the laws of growth, the innovations at the development genetic level with natural ion to clarify adaptive radiation that we observe in nature”
So we’ve to see at these stories this perspective, both genetic and environmental perspectives.
It’s clear that understanding genetic principles of development is quite necessary for understanding the origins of variety in the living world. It’s clear that in order to produce a new shape, a new biological shape, you’ve to modify something at the genetic level, you’ve to modify something about the developmental program. But are we prepared to think, are we prepared to accept that at minimum in some cases the innovation at the genetic level is the driving force for producing dramatic biological variation in examples such as Darwin’s finches and Hawaiian honeycreepers?
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