The Evolution of Different Species with Similar Structures Or Functions In
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FEATURE ARTICLE How Does Evolution Explain Blindness in Cavefish? • MIKE U. SMITH ABSTRACT difficult to imagine that eyes, though useless, could in any way be Commonly used evolution assessments often ask about the evolution of injurious to animals living in darkness, I attribute their loss solely blindness in cavefish or salamanders, running speed in cheetahs, and/or the to disuse” (p. 110). long necks of giraffes. Explaining the loss of function in cave animals, The evolution of different species with similar structures or however, is more difficult than explaining evolution involving gains of functions in spite of their evolutionary ancestors being very dissim- function resulting from natural selection. In fact, the evolution of cavefish ilar or unrelated is called “convergent evolution” (Biology Online, blindness is not yet well understood by scientists. This article presents the 2016). The main question that has confounded biologists for years three current hypotheses for explaining the evolution of blindness in has been: How do so many different species that inhabit caves end Astyanax mexicanus Mexican tetras ( ), related to the Next Generation — Science Standards and the Advanced Placement curriculum. up with very similar phenotypes how do the phenotypes con- verge? In particular, how does blindness evolve in cavefish and in essentially every other species of animal whose life is spent in the Key Words: blindness; evolution; inheritance. dark (called “troglodytes” or “troglobionts”)? The surprising answer is that we don’t know the full story yet, but the mechanisms under- lying the evolution of blindness in the Mexican cavefish have begun to be elucidated. Introduction In the classroom, understanding how evolu- The evolution of tionary biology could explain loss of function in Measures of understanding of evolution the case of troglodytes provides an excellent oppor- often ask students to explain the evolution different species with tunity not only for identifying student misconcep- of blindness in cavefish or salamanders, tions, but also for understanding central concepts. running speed in cheetahs, and/or the long similar structures or These include: Disciplinary Core Ideas such as nat- necks of giraffes (Bishop & Anderson, functions in spite of ural selection, adaptation, and the interplay of 1990; Demastes et al., 1995; Project genetics and the environment (“evo-devo”)pro- 2061, n.d.; Settlage, Jr., 1994). These items their evolutionary moted by the Next Generation Science Standards are primarily used to identify student mis- (NGSS Lead States, 2013; Lerner, 2000); as well conceptions, such as inheritance of ancestors being very as more advanced AP “Big Idea 1” (evolution), acquired characteristics and evolution dissimilar or Science Practices 1 (using models), 3 (scientific driven by need or “pressure” (Nehm et al., questioning), 5 (using scientific explanations and 2010). To the surprise of many, students unrelated is called theories), 7 (relating knowledge across scales, con- typically have much more difficulty with “ cepts and representations); and more specific con- the cavefish question than with cheetahs convergent cepts such as genetic drift, fitness, and homeotic (Nehm & Ha, 2011), but students are not evolution”. genes (Biology Online, 2008, 2009, 2012). Troglo- the only ones who have difficulty explaining dytes are an excellent case study for students to the evolution of a loss of function (e.g., blindness) vs. a gain of func- learn to explain the basic evolutionary question of how species change tion (increased running speed). Even Darwin (1872) got the explana- over time (adapt to shifts in their environment) (Table 1). The exis- tion of blindness in cavefish wrong, attributing it to disuse: “As it is tence of vestigial structures such as the nonfunctional eyes of cavefish The American Biology Teacher, Vol. 79, No 2, pages. 95–101, ISSN 0002-7685, electronic ISSN 1938-4211. © 2017 National Association of Biology Teachers. All rights reserved. Please direct all requests for permission to photocopy or reproduce article content through the University of California Press’s Reprints and Permissions web page, www.ucpress.edu/journals.php?p=reprints. DOI: https://doi.org/10.1525/abt.2017.79.2.95. THE AMERICAN BIOLOGY TEACHER BLINDNESS IN CAVEFISH 95 Table 1. Common troglodyte characteristics (Gross, 2012; McGaugh et al., 2014; Protas et al., 2007; Retaux & Caslane, 2013). Characteristics Blindness Loss of pigmentation Enhanced tactile & chemical senses (including taste) Increased food finding ability Increased starvation resistance Reduced energy needs Enhanced lipid storage Reduced fecundity (larger, fewer eggs, etc.) Slower, more efficient metabolism Attraction to vibration Loss of schooling Loss of aggression Dramatic sleep reduction Shorter lifespans Reduced genetic diversity Smaller population sizes is also an important component of the evidentiary support for the the- ory of evolution (Senter et al., 2015; Anonymous, 2011). Also, study- ing weird creatures that live in a dark world far beneath the surface can be fun because students often find these troglodytes interesting and motivating. To explain troglodyte evolution, first we need some background. The Lives Of Troglodytes Figure 1. Astyanax mexicanus surface fish and cavefish. (Jeffery, 2009, Fig. 2. Used by permission.) The underground world of caves is largely unknown even though most of the world’s unfrozen water (94%) is stored, not in the oceans, but underground (Culver & Pipan, 2009). There are nearly respectively) populations were then separated when the water 50,000 caves in the U.S. alone (Retaux & Casane, 2013). So, what receded and connections between the two dried up. This example is life like in a cave, and what do we know about life there that of the isolation of subpopulations has many parallels with the might help explain the evolution of cave blindness? Of course, it appearance of Darwin’s finches (and other animals and plants) on is dark, and the lack of sunlight means the absence of photosynthe- the Galapagos Islands. Just as with Galapagos organisms, the first sis. Therefore, there are no primary producers to convert light members of a species to arrive in a cave were likely few and varied energy into biomass for consumption up the food chain. So how little from their relatives left behind, left behind, but over time do organisms survive? What do they eat? How do they find mates? cavefish and surface fish subpopulations evolved differently What kinds of animals are found there? Why are they all blind? (diverged) so that some sequences in the genomes of cavefish of And how did they get into the caves in the first place? the same genus are found to be different from their surface relatives There is a surprising diversity of life in caves. Almost all the (O’Quin et al., 2015). Of course, new spring rains sometimes bring major phyla are represented, including fish, beetles, salamanders, new immigrants into a cave such that the population immediately shrimp, and spiders, among many others (Protas et al., 2011). after a rain actually consists of (sighted and blind) subpopulations. There are 86 cavefish species alone (Jeffrey, 2009). The ancestors Probably the most studied of the troglodytes is Astyanax mexicanus of most of these species that Darwin called “wrecks of ancient (Figure 1) found in certain caves in northern Mexico (Figure 2), a tetra life” (Darwin, 1872, p. 112) were likely washed into the caves by related to the common aquarium fish that students may be familiar spring flooding. The cave and surface (hypogean and epigean, with. In the absence of photosynthesis, they mostly survive on the film 96 THE AMERICAN BIOLOGY TEACHER VOLUME. 79, NO. 2, FEBRUARY 2017 increases the number of taste buds on the ventral surface of the head, which helps cavefish find food more effectively (Gross, 2012). Natural selection for this increase in taste buds would, therefore, also promote blindness. The third hypothesis is based on neutral mutation and genetic drift. All too often textbooks use the terms “evolution” and “natural selection” interchangeably, ignoring the importance of genetic drift. Genetic drift is “the process of change in the genetic composition of a population due to chance or random events rather than to nat- ural selection, resulting in changes in allele frequencies over time” (Biology Online, 2008). Genetic drift differs from natural selection because observed changes in allele frequency are completely at ran- dom, not the result of natural selection for a trait. Genetic drift can have a relatively larger impact on smaller populations such as a typi- Figure 2. Distribution map for Astyanax mexicanus. cal population of cavefish. According to the neutral mutation and genetic drift hypothesis, therefore, normal mutation processes in a small population of cavefish sometimes produce neutral mutations of bacteria that break down bat and cricket guano (feces), although (mutations that lead to phenotypic changes that natural selection sometimes flooding brings in additional biomass food (Leighton, does not act on), and in the absence of natural selection, totally ran- 2015; Gross, 2012). Darkness also means that finding mates is more dom events can sometimes result in the increased frequency of such difficult. Mating and sexual selection in most animal species is often mutations over time. Such changes could include eye degeneration. based on coloration, but troglodyte species typically have no colora- So, what’s the right