system, along with a profusion of other complex organ systems under extreme environmental conditions. It is incredulous to think that a random, unguided mutation-selection model of evolution could cobble together all functional components with such precision to establish this irreducible suite of adaptive traits, let alone create all of their precursor lineages, and more than ~30,000 teleost species found worldwide. What’s more, almost all of the adaptations that enable these cavefish to thrive and reproduce within limestone caves are, and would have to be, promoted as evidence of convergent evolution. 3) Convergent evolution or engineered adaptability? There is a growing trend in evolutionary biology to infer ‘convergence’ whenever similar character states (traits) arise independently between lineages and/or species. Such inferences most often derive from genetic, phylogenetic, geographic and biological analyses that incorporate estimates of time. Most often, phyogeographic or phylogenetic character mapping strategies are involved (Avise 2000). One of the world’s leading college textbooks of biology defines convergent evolution as: “The evolution of similar features in independent evolutionary lineages”(Urry et al. 2020). Twenty-five years earlier, that textbook defined convergent evolution as: “The independent development of similarity between species as a result of their having similar ecological roles and selection pressures”(Campbell 1996). One scientific dictionary defines ‘convergence’ as: “The evolution of unrelated species occupying similar adaptive zones, resulting in structures bearing a superficial resemblance” (King and Stansfield 2002). A second scientific dictionary defined convergence as: “The independent evolution of structural or functional similarity in two or more unrelated or distantly related lineages or forms that is not based on genotypic similarity and common ancestry” (Lincoln et al. 1998). Only one of these definitions avoids the term ‘evolution’; one avoids the term ‘independent’; all of them include the terms ‘similar’ or ‘similarity’ and either ‘species’ or ‘lineages’. Furthermore, we can safely imply that each definition is not limited in scope, but is applicable across all multicellular taxa (fungi, plants, animals). In the Astyanax cavefish model, rapid convergent evolution is the most prevalent explanatory hypothesis for their common array of traits (see previous discussion). However, they are presumed to have had a ‘head start’ as they could not depend upon slow and gradual production of numerous new mutations. Accumulation of standing genetic variation in ancestral surface fish populations is thought to explain not only their rapid response times, but also widespread convergence of the specialized adaptations they exhibit today. As outlined in the Introduction, cave populations of blind A. mexicanus are estimated to have diverged from surface populations sometime between 8.1 million and ~ 20,000 years ago, with persuasive support for the lower reference date, and perhaps even younger (Fumey et al. 2018). At least five independent cave invasions (Gross 2012) have led to 29 distinct populations of cavefish in the El Abra region of Mexico (Fig. 8). At least ten or more of these populations are “significantly distant from one another” (Gross 2012). Thus, multiple cave colonization events over an evolutionarily short timeframe have resulted in “convergence on cave-derived morphological and behavioral traits” across multiple, geographically separated cavefish populations (Bradic et al. 2012; Coghill et al. 2014; Herman et al. 2018). And, Herman et al. (2018) have confirmed that cave populations are polyphyletic, and therefore derived “from more than one common evolutionary ancestor or ancestral group”. This pattern of cave colonization and convergence on highly-similar integrated adaptations is not limited to Mexico. At present, over 200 such cavefish species have been Río Sabinas Río Frio Río Mante Río Mesillas Río Puerco Río Valles Río Choy Río Naranjo Río Coy Río Tampaon Arroyo Lagarto N Sierra de Guatemala Sierra de el Abra Sierra de Colmena Sierra de Nicolás Pérez La Región de la Sierra de el Abra Figure 8. Location and distribution of known populations of Astyanax mexicanus in Mexico. There are 29 known populations of A. mexicanus cavefish within karst cave habitats across the Región de la Sierra de el Abra. Almost all of Astyanax surface fish and cavefish models utilized for research today were collected from the rivers and cave systems in this region. Solid black circles: cavefish populations. (Map and population distributions modified from Figure 2 in Gross, 2012; Figure 1 in Fumey et al. 2018). described, and all of them have evolved independently from surface ancestors. Thus, each cavefish species is a replicate of the same natural experiment, testing the evolutionary response of a sighted surface fish to the absence of light and the limitations on food in a subterranean environment. The evolutionary responses converge on loss of eyes and pigmentation and the augmentation of other senses, such as taste, smell or mechanosensation, as well as a more efficient metabolism, changes in feeding behavior, altered activity BOYLE, ARLEDGE, THOMAS, TOMKINS, AND GULIUZZA Testing the cavefish model 2023 ICC 135
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