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Exaptation – Morphology and Physiology

1.	Okada N, Sasaki T, Shimogori T, Nishihara H. Emergence of mammals by : exaptation. Genes Cells. 2010;15(8):801-12.

This article looks at conserved non-coding elements (CNEs) as a possible source of biodiversity, in accordance with comparative genomics. It posits that CNEs may lead to the generation of new gene expression and that mammals may have received some of their specific characteristics by obtaining it through exaptation. By studying the human genome, researchers discuss specific morphological evolution of mammals and why exaptation was crucial.

2.	Linde-medina M. Adaptation or exaptation? The case of the human hand. J Biosci. 2011;36(4):575-85.

Researchers explore the human hand and argue that it came about because of exaptation as opposed to adaptation due to natural selection. Taking an in-depth look at the definition of adaptation, they argue that a genetic program was not responsible for skeletal pattern formation in the hand, meaning the matter of the evolutionary determination of the formation of the hand may not be as clear cut as most currently accept.

3.	Barve A, Wagner A. A latent capacity for evolutionary innovation through exaptation in metabolic systems. Nature. 2013;500(7461):203-6.

Using glucose as a viable source of energy also allows the usage of about 44 other carbon products that are sufficient for our metabolism. Stating this, it can be seen that metabolic systems have potential to innovate without adaptive origins – exaptation. With this being said, it may be difficult to distinguish between adaptations and exaptations because of the vast number of possibilities that are presented.

4.	Nickle DC, Goncharoff LM. Human fist evolution: a critique. J Exp Biol. 2013;216(Pt 12):2359-60.

The human fist may be an example of an exaptation as apposed to adaptation as thought by most because it came about as a tag-along trait that wasn’t by itself a target of natural selection.

5.	Pievani T, Serrelli E. Exaptation in human evolution: how to test adaptive vs exaptive evolutionary hypotheses. J Anthropol Sci. 2011;89:9-23.

Researchers describe testing adaptation and exaptation and list several human traits with tests and supports for their theories of why the stated trait is or isn’t an exaptation. A definition of exaptation given and it is explained why there is difficulty in distinguishing between it and adaptation.

Edited and Commented
https://en.wikipedia.org/wiki/Exaptation

Function may not always come before form: developed structures could change or alter the primary functions they were intended for due to some structural or historical cause.

Adding other examples
I have found a couple primary sources pointing to evidence of exaptations that haven't been listed in the article yet. These include the human hand/fist as it came about as a "tag-along" trait that wasn't a target of natural selection. Metabolic systems may be another example of an exaptation because of the many possibilities for innovation based off of their ability to use many carbon products as viable carbon derivatives sufficient for metabolism. I will add these examples and in-depth information in due time.

Additional information to "Preadaptation" section
I would like to add a few sentences to the "Preadaptation" section that will help better define exaptation and make it slightly more understandable. I feel it is a complex topic in evolutionary biology that isn't the easiest to understand for most people.

Wolf "behavioral" example
I feel as though some more evidence is required concerning the wolf behavior example - also, can behaviors be considered as exaptations? I somewhat understand the idea behind this example; however, I'm not totally convinced about it.

WIKIPEDIA ARTICLE EDITS START HERE:
https://en.wikipedia.org/wiki/Exaptation

Adaptation and Exaptation Cycle
It was speculated by Gould and Vrba in one of the first papers written about exaptation, that when an exaptation arises, it may not be perfectly suited for its new role and may therefore develop new adaptations to promote its use in a better manner. In other words, the beginning of developing a particular trait starts out with a primary adaptation toward a fit or specific role, followed by a primary exaptation (a new role is derived using the existing feature but may not be perfect for it), which in turn leads to the development of a secondary adaptation (the feature is improved by natural selection for better performance), promoting further development of an exaptation, and so forth.

Once again, feathers are an important example in that they may have first been adapted for thermoregulation and with time became able to catch insects and therefore serve as a new feature for another benefit. Large contour feathers with specific arrangements arose as an adaptation for catching insects more successfully, which eventually led to the flight seeing as the large feathers served better for that purpose.

Examples
Metabolism can be considered an important part of exaptation. As one of the oldest biological systems and being central to life on the Earth, studies have shown that metabolism may be able to use exaptation in order to be fit given some new set of conditions or environment. Studies have shown that up to 44 carbon sources are viable for metabolism to successfully take place and that any one adaptation in these specific metabolic systems is due to multiple exaptations. Taking this perspective, exaptations are important in the origination of adaptations in general. Metabolic systems have the potential to innovate without adaptive origins.

There may even be DNA evidence of exaptations occuring. It is possible to look at a retroposon, originally thought to be simply junk DNA, and deduce that it may have gotten a new function to be termed as an exaptation. Given an emergency situation in the past, an organism may have used junk DNA for a useful purpose in order to evolve and be able to survive. This may have occurred with mammalian ancestors when confronted with a large mass extinction about 250 million years ago and a substantial increase in the level of oxygen in the Earth’s atmosphere. More than 100 loci have been found to be conserved only among mammalian genomes and are thought to have essential roles in the generation of features such as the placenta, diaphragm, mammary glands, neocortex, and auditory ossicles. It is believed that as a result of exaptation, or making previously “useless” DNA into one that could be used in order to increase survival chance, mammals were able to generate new brain structures as well as behavior to better survive the mass extinction and adapt to a new life.

FINAL DRAFT STARTS HERE:
Exaptation: a Novel way of Thinking about Derivation of Function

Evolutionary biology generally suggests that traits are developed because of being fit for a particular function and as a result of natural selection; these traits can also be called as adaptations (Weible, 2012). However, a less spoken about area regarding developed features concerns those that are fit for their current role, but not necessarily designed for it. A specific trait may have evolved for a particular use, but was co-opted for a new use as time passed: traits that fall under this criterion are termed exaptations (Gould and Vrba, 1982). Basically, these are features that currently add to an organism’s fitness, but were not designed for the function it serves by natural selection. Simply because exaptations aren’t frequently spoken of in primarily adaptationist neo-Darwinian evolutionary theory doesn’t mean such features are rare or hidden (Gould, 1991). Although deviating from common evolutionary biology learning and vocabulary, exaptations are important when thinking about the derivation of function with respect to a feature an organism possesses and should be considered as an important part of development over time. The concept isn’t “anti-Darwinian” and even expands on Darwin’s ideas of “pre-adaptation”, providing a better explanation to what Darwin may have been trying to hint at (Pievani and Serrelli, 2011). Incorporation of exaptation as a crucial point of evolutionary biology would allow an explanation for innovations by organisms that aren’t direct results of adaptation but are the by-products of other adaptive traits; this would broaden the scope of evolutionary terminology and incorporate novel ideas regarding formation and function of traits. Gould and Vrba (1982) were the first to introduce the term exaptation for a feature that is coopted and increases fitness but not as a direct result of natural selection. They describe a series of events in organisms concerning exaptation due to the fact that when an exaptation arises, it may not be perfectly suited for its new role and may therefore develop new adaptations in order to promote its use in a better manner. In other words, the beginning of developing a trait is primary adaptation (toward a fit for specific role), followed by primary exaptation (a new usage is derived, but may not be perfected for that feature), resulting in secondary adaptation (the feature is improved by natural selection for better performance), and so forth (Weible, 2012). A given example of this is in the case of feather function in the evolution of birds, as the basic design of feathers was an adaptation for thermoregulation and with time an exaptation for catching insects. Large contour feathers with specific arrangements arose as adaptations for catching insects more successfully and as time passed, the feathers became exaptations for flight (Gould and Vrba, 1982). It can be deduced that although exaptation itself is separate from adaptation and natural selection, the two work together over long periods of time in order to evolve the best-suited traits in organisms.

Darwin himself was questioned about his theory of natural selection, particularly on the absence of transitional stages in the fossils that are found or even in currently living species. It would seem that if evolution occurred as a series of small changes over long periods of time, there would be more evidence of transitional stages; however, this is not the case (McLennan, 2008). One formulated answer was the changing or addition of a function of an already existing structure. This may have been due to more than one organ performing the same task in which case selection would increase how efficient one of them would be by modifying its role for another function (McLennan, 2008). The term preadaptation was first given for these seemingly unimportant traits that didn’t obviously relate to the main function of a feature. For example, when humans are born, a large fetal head must pass through a narrow canal and this would be impossible if it were not for the ability of the skull to change its shape due to delayed ossification (development) of the bones (Gould, 1991). Delayed ossification allows the large size of human heads, which is the key to human success. However, this phenomenon is not thought to be an adaptation but an exaptation because “lower” vertebrates and mammalian ancestors that must only break free from an egg actually share this feature of the skull with humans (Gould, 1991). It can be stated that delayed ossification of the human skull was in fact used to further evolve humans as mammals to a more advanced stage that permitted larger heads. As mentioned, this exaptation could be a key part of human evolution and a reason for the ability to have and use our large brains.

Exaptations are not only limited to looking at traits and trying to see if they arose from natural selection or if perhaps some function was later derived in order to facilitate fitness for an organism. It is possible to look at a retroposon, originally thought to be simply junk DNA, and deduce that it may have gotten a new function to be termed as an exaptation (Okada, Sasaki, Shimogori, and Nishihara, 2010). Given an emergency situation in the past, an organism may have even used junk DNA for a useful purpose in order to evolve and be able to survive. This may have occurred with mammalian ancestors when confronted with a large mass extinction about 250 million years ago and a substantial increase in the level of oxygen in the Earth’s atmosphere (Okada, Sasaki, Shimogori, and Nishihara, 2010). More than 100 loci have been found to be conserved only among mammalian genomes and are thought to have essential roles in the generation of features such as the placenta, diaphragm, mammary glands, neocortex, and auditory ossicles. It is believed that as a result of exaptation, or making previously “useless” DNA into one that could be used in order to increase survival chance, mammals were able to generate new brain structures as well as behavior to better survive the mass extinction and adapt to a new life (Okada, Sasaki, Shimogori, and Nishihara, 2010).

Evidence of exaptation isn’t limited to looking at genome sequencing, it can also be seen in phylogenetic trees as in the example crystallins or soluble proteins existing in the lens of all vertebrates and some invertebrates. These are able to refract light in order to better focus images on the retina as well as remain stable for a long period of time, a necessary quality for the eyes where proteins cannot be repaired or replaced (Weible, 2012). Phylogenetic and functional analyses show that these alpha crystallins originated after two duplication events: the first when a copy of the heat shock protein gained the function of protection against damage throughout the body while the second copy acquired a change in function, namely the ability to focus light. The protein that was now stable and had the ability to focus light changed its location to a restricted area, the eye (McLennan, 2008). This example shows that exaptation may involve a change in function as well as location. Furthermore, the transition of aquatic vertebrates to land dwelling ones requires the changing of fins to limbs and therefore an exaptation of using limbs that were present for support and movement on land – something not originally intended for the fins (McLennan, 2008). Organisms used traits they already possessed in order to survive in a new environment even if they weren’t perfectly suited for it. A final phylogenetic example involves skrraa call in certain birds that was first used in male-male combat but eventually changed to be involved in male-female courtship in a specific ancestor. These calls sound similar and had a change of function over time (McLennan, 2008). Overall, looking at phylogenetic trees and seeing the development of new functions over time is an important measure of exaptation and aids in its understanding.

The human hand and its function is an exaptation in the way that the thumb is long and robust along with the joint surfaces and muscles are larger, probably as a result of using and making tools (Linde-Medina, 2011). The hands didn’t necessarily evolve to do this, especially in cases such as using them for writing or playing musical instruments. The hand obviously has a functional role for humans but its overall shape should be considered an exaptation due to the fact that the features it possesses cannot be explained in terms of function (Linde-Medina, 2011). Furthermore, it was studied that the skeletal pattern of the hand wasn’t the result of a genetic program and therefore further supports the theory that the formation of the human hand isn’t as clear-cut as many people seem to think.

Some exaptations may not be obvious to the naked eye: such as the fact that metabolism can be considered an important part of exaptation. As one of the oldest biological systems and being central to life on the Earth, studies have shown that metabolism may be able to use exaptation in order to be fit given some new set of conditions or environment (Barve and Wagner, 2013). Studies have shown that up to 44 carbon sources are viable for metabolism to successfully take place and that any one adaptation in these specific metabolic systems is due to multiple exaptations. Taking this perspective, exaptations are important in the origination of adaptations in general (Barve and Wagner, 2013). Metabolic systems have the potential to innovate without adaptive origins but may in fact be due to exaptations. Given this, it may be difficult to pinpoint the exact reason due to the many possibilities that are presented in metabolism. In fact, defining exaptations isn’t an easy task given its definition and many variables that may play a role in accuracy.

In conclusion, although there are disagreements by some scientists concerning exaptations, one should consider the unpredictable changes during the course of human history and exaptation as playing a role by giving rise to new co-opted functions that diverged from the adaptations by natural selection (Gould, 1991). An actual consequence of exaptations is the possibility of accelerated evolutionary change due to the appearance of some complex traits as a result of exaptation, instead of gradual adaptation (Pievani and Serrelli, 2011). As a term that is used fairly sparingly, the concept of exaptation is needed because adaptation is not enough to explain the traits that weren’t derived as a result of natural selection (Gould and Vrba, 1982). Although somewhat recognized as pre-adaptation, exaptation focuses on the shift of a role using features that were adapted for another use. There are multiple facets to the development of function using features that organisms possess, and it should be known as such. Making exaptation a part of the evolutionary biology common lexicon would allow better understanding of such concepts as well as possibly opening doors to other novel ideas that may help gain understanding of the development of lineages on earth.

CITATIONS:
Barve A, Wagner A. 2013. A latent capacity for evolutionary innovation through exaptation in metabolic systems. Nature, 500(7461): 203-6.

Gould S. 1991. Exaptation: A crucial tool for an evolutionary psychology. Journal of Social Issues, 47(3): 43-65.

Gould S.J. and Vrba E.S. 1982. Exaptation – A Missing Term in the Science of Form. Paleobiology, 8(1): 4-15.

Linde-Medina M. 2011. Adaptation or exaptation? The case of the human hand. Journal of Bioscience, 36(4): 575-85.

McLennan, D. 2008. The Concept of Co-option: Why Evolution Often Looks Miraculous. Evolution: Education & Outreach, 1(3): 247-258.

Okada N, Sasaki T, Shimogori T, Nishihara H. 2010. Emergence of mammals by emergency: exaptation. Genes Cells, 15(8): 801-12.

Pievani T, Serrelli E. 2011. Exaptation in human evolution: how to test adaptive vs exaptive evolutionary hypotheses. Journal of Anthropology Science, 89: 9-23.

Weible, D. 2012. Ritualization and Exaptation: Towards a Theory of Hierarchical Contextuality? Biosemiotics, 5(2):211-226.