At first, the maxim ‘reduce, reuse, recycle’ may seem unrelated or tenuous to apply to the evolution of the subphyla known as the vertebrates, or Vertebrata. This slogan is typically used in relation to environmental issues, as the 3R’s of sustainable development. However, natural selection and evolution can be viewed as a form of sustainable development. As organisms progress and speciate, the selection pressures reduce/remove traits that are no longer advantageous, re-use those which are still beneficial, and recycle some organs or body parts into new characteristics, which are of higher value to the individual’s survival or reproduction success.
In this essay, these processes and their outcomes will be explored and discussed using illustrative examples.
The process of reduction or elimination of morphological and physiological traits across evolutionary time has been widely documented, examples and evidence will be discussed in this paragraph.
The most widely recognised example of the reduction process in vertebrate evolution is that of the limbs, perhaps because the comparisons are easily demonstrated, using extinct and transitional forms. Sometimes described as ‘regressive evolution’ (Cajeb, 2019), the “loss of structures is an adaptive evolutionary response to changed conditions of living, which make an organ or part functionally irrelevant or disadvantageous” (Cajeb, 2019), and therefore the selection pressures result in an increasingly rudimentary trait. It was described by Darwin that ‘disuse’ is the main driving force in the reduction of organs, using examples such as the reduction of mammal eyes in cave dwelling or burrowing species, and the loss of effective wings in birds who inhabit isolated oceanic islands, who have little need to fly (Darwin, 1859); he hypothesised that adaptive changes in behaviours precede changes in morphology and physiology.
The principal example of this of this process would be the loss or vestigialization of limbs in Squamates (snakes and legless lizards). Due to changes in habitat or behaviour, ancestral tetrapod reptiles began to fill new ecological niches, where efficient movement through dense grassland and burrows was favoured. In these habitats, lateral undulation (the type of movement where wave-like motions propel the animal forward) is a more energy-efficient means of motion than quadrupedal locomotion (Wiens, 2001). Evidence of this regressive evolution can be found in both extant and fossil organisms, where intermediate degrees of limb loss and body elongation are extremely common in some groups of lizards, such as scincidae (skinks) (Lande, 1978). Studies have concluded the molecular basis of loss of limbs in snakes, as they evolved from their limbed ancestor, may be determined by the loss of function in a single enhancer, and these changes could involve regulation of Hox genes that act prior to Shh (‘Sonic Hedgehog’, an important signalling molecule), or other genes that are critical for initiation of limb development (Kvon, 2016).
The ‘reuse’ of organs and characteristics can be likened to the saying ‘if it isn’t broken, don’t fix it’, as it applies to traits that have remained at a relatively similar level of benefit to the organism over evolutionary time. Many instances of homologous morphological and physiological characteristics can be seen to persist in the vertebrates over evolutionary history, suggesting that adaptive evolution pressures were never strongly applied. An example of this form of evolution is a physiological state that is shared by a huge proportion of extant vertebrates: Ectothermy. This process can be defined as the direct correlation between external heat sources, and internal body temperature, and the need to pursue these heat sources, in order to self-regulate. Virtually all aspects of the behaviour and physiology of ectotherms are sensitive to temperature, including foraging ability, courtship, and rates of feeding and growth (Angilletta, 2002). Despite the requirement of a stable ambient temperature, which could be a constraint, this characteristic has prevailed through time from the earliest vertebrates, to many successful modern lineages, such as the reptiles, amphibians, and fish.
The final type of evolution being explored in this essay as an example of sustainable development is ‘Recycling’. This process occurs when an organ or body part is transformed to create a novel one, which is more advantageous or efficient than the previous form. This progression could be described as the main method of species development and is documented widely in the vertebrates. Extensive examples of this from this phyla could be examined: fins into limbs, dorsal nervous system/ neural crest into central nervous system, exoskeleton of bony fish into internal skeletons, endoderm into gill structures (Gillis, 2017), and the notochord into the spine. The instance that I will explore is the development of advanced vertebrate middle ear structures, from components of the jaw (Manley, 2010).
The middle ear conveys airborne sound to the liquid-filled inner ear using 3 distinct structures, which are the result of a long evolutionary process; the transformation has been reconstructed from fossil vertebrates by analysis of homologous structures (Sienknecht, 2013). Multiple instances of the convergent evolution of this organ (such as in mammals, lizards, and potentially marsupials), demonstrate the extensive benefits of the ear structure. The evolutionary advantages brought about by the utilisation of the ear to detect sounds led to less beneficial structures within the jaw to be recycled and repurposed. The quadrate bone in the upper jaw of early vertebrates evolved into the incus, the articular bone in the bottom jaw evolved into the malleus, and the stapes arose from the hyomandibular, which became free and purposeless once distinct neck regions developed, and distanced the head from the main trunk of the body (Manley, 2010).
The evolution of the vertebrate middle ear is an excellent example of how evolution recycles structures which have become inoperative or disadvantageous, into new beneficial morphology, under adaptive selection pressures.
In summary, the principles of ‘Reduce, Re-use, Recycle’ provide a very useful framework for considering the development of new traits in vertebrates, and the course of speciation. It is engaging to see evolution as a form of sustainable development and link the process to the 3R’s. Evolution tends to favour developments which increase an organism’s efficiency, success, and longevity, and this has parallels with humanity’s objectives for sustainable development.
Angilletta, M., 2002. The evolution of thermal physiology in ectotherms. Journal of Thermal Biology , 27(4), pp. 249-268.
Cajeb, N., 2019. Evolution by Loss. In: Epigenetic Principles of Evolution. Albania: Academic Press, pp. 493-534.
Darwin, C., 1859. The Origin of Species by Means of Natural Selection or the Preservation of Favored Races in the Struggle for Life. 1st ed. London: John Murray.
Gillis, J., 2017. The Origin of Vertebrate Gills. Current Biology , 27(5), pp. 729-732.
Kvon, E., 2016. Progressive Loss of Function in a Limb Enhancer during Snake Evolution. Cell, 167(3rd), pp. 633-642.
Lande, R., 1978. Evolutionary mechanisms of limb loss in tetrapods. Evolution, Volume 32, pp. 73-92.
Manley, G., 2010. An evolutionary perspective on middle ears. Hearing Research , 263(2), pp. 3-8.
Sienknecht, U., 2013. Developmental origin and fate of middle ear structures. Hearing Research, 301(1), pp. 19-26.
Wiens, J., 2001. How Lizards turn into Snakes: A Phylogenetic analysis of Body-Form evolution in Anguid Lizards. Evolution, 55(11), pp. 2303-2318.