page_title: Exaptation
section_title: Examples
hierarchical_section_title: Exaptation / Examples
caption_attribution_description: English: Avian feather pigments. Left: turacin, purple Center: turacoverdin, green (with some blue iridescence on the lower end) Right: carotenoids, scarlet (with blackish melanin on the wings) Birds are ZFMK specimens of Tauraco bannermani (ZFMK 68.77, left) and Ramphocelus bresilius (ZFMK Müller56, right).
context_page_description: Exaptation and the related term co-option describe a shift in the function of a trait during evolution. For example, a trait can evolve because it served one particular function, but subsequently it may come to serve another. Exaptations are common in both anatomy and behaviour. Bird feathers are a classic example: initially they may have evolved for temperature regulation, but later were adapted for flight. Note here that when feathers were initially used to aid in flight they were doing so exaptively; however, since they have since been shaped by natural selection to improve flight, in their current state they are now best regarded as adaptations for flight. So it is with many structures that initially took on a function as exaptations, once molded for that new function they become adapted for that function. Interest in exaptation relates to both the process and products of evolution: the process that creates complex traits and the products that may be imperfectly developed. Exaptation was proposed by Stephen Jay Gould and Elisabeth Vrba as a replacement for what they considered to be a teleologically loaded term 'pre-adaptation'.
context_section_description: Exaptations include the co-option of feathers, which initially evolved for heat regulation, for display, and later for use in bird flight. Another example is the lungs of many basal fish, which evolved into the lungs of terrestrial vertebrates but also underwent exaptation to become the gas bladder, a buoyancy control organ, in derived fish. A third is the repurposing of two of the three bones in the reptilian jaw to become the malleus and incus of the mammalian ear, leaving the mammalian jaw with just one hinge.
  A behavioural example pertains to subdominant wolves licking the mouths of lead wolves as a sign of submissiveness. (Similarly, dogs, which are wolves who through a long process were domesticated, lick the faces of their human owners.) This trait can be explained as an exaptation of wolf pups licking the faces of adults to encourage them to regurgitate food.
  Arthropods provide the earliest identifiable fossils of land animals, from about 419 million years ago in the Late Silurian, and terrestrial tracks from about 450 million years ago appear to have been made by arthropods. Arthropods were well pre-adapted to colonize land, because their existing jointed exoskeletons provided support against gravity and mechanical components that could interact to provide levers, columns and other means of locomotion that did not depend on submergence in water.
  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. A recent example comes from Richard Lenski's E. coli long-term evolution experiment, in which aerobic growth on citrate arose in one of twelve populations after 31,000 generations of evolution. Genomic analysis by Blount and colleagues showed that this novel trait was due to a gene duplication that caused a citrate transporter that is normally expressed only under anoxic conditions to be expressed under oxic conditions, thus exapting it for aerobic use. Metabolic systems have the potential to innovate without adaptive origins.
  Gould and Brosius took the concept of exaptation to the genetic level. 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, a species 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 substantial increase in the level of oxygen in 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 DNA 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 new environments. Similarly, viruses and their components have been repeatedly exapted for host functions. The functions of exapted viruses typically involve either defense from other viruses or cellular competitors or transfer of nucleic acids between cells, or storage functions. Koonin and Krupovic suggested that virus exaptation can reach different depths, from recruitment of a fully functional virus to exploitation of defective, partially degraded viruses, to utilization of individual virus proteins.
caption: Bird feathers of various colors
exif: {}