The Fascinating Story of Speciation in Bulimulid Land Snails
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Where did they come from and where did they Galapa-go? The fascinating story of speciation in Bulimulid land snails Figure 1. A Bulimulid snail crawls across some lichen covering a lava rock in the humid highlands of San Cristobal. (Author) Audrey Bennett Bill Durham – Sophomore College: Evolution and Conservation in Galapagos Stanford University 14 October 2018 Abstract Hiding under leaf piles and in small pools of moisture in the porous rocks, the small, herbivorous bulimulid snails exist almost inconspicuously from the average tourist’s eye. However, a closer inspection into their natural history reveals an amazing story of diversification, adaptation, and evolution. This genera of snail, upon arrival to the Galapagos Islands about 2.77 million years ago (Parent and Crespi, 2006), has since radiated into 71 different species, making it the largest adaptive radiation on the islands (Parent and Crespi, 2006). The extent of this radiation in such a small area is rivaled only by the radiation of a similar bulimulid group in Baja California (Chambers, 1991, pp. 307-325). This raises some central questions: What factors contribute to successful radiation of the the bulimulid snails? Has reproductive isolation within an island or between islands been more important for speciation? Several hypotheses are proposed: Hypothesis 1 is that colonization event order will follow the Progression Rule, moving from eastern to western islands. Hypothesis 2 is that island isolation and species richness are negatively correlated. Hypotheses 3, 4, and 5 are that plant diversity, an island’s elevation, and the island area are positively correlated with the land snail species richness on that island. Data from case studies of these snails appear to support hypotheses 1, 3, 4, and 5. Lastly, this report investigates the threats that land snails face and how their adaptations to specialized environments has potentially lead to their vulnerability. https://galapagosconservation.org.uk/wildlife/galapagos-land-snail/ Figure 2-4. Bulimulid snails occupy a variety of habitats and have a variety of shell morphologies. (Parent) INTRODUCTION Natural history The group of Galapagos Bulimulidae includes about 71 species of land snails, all of which are endemic and distributed on most of the islands of the archipelago. (Coppois and Wells, 1987) They are also found in five of the six vegetation zones on the islands (Parent and Crespi, 2006). Adults range from 6-25 mm in length, and display a wide range of morphological traits. This is perhaps due to the variety of microclimates they inhabit. Adults range in color from white to dull brown to dark black, often with stripes or spiral bands. The ground living species tend to be duller in color than the arboreal ones, which have paler, polished shells. There is also a large diversity in shell shape, from short and globose to long and slender (Coppois and Wells, 1987) Potential as a study system In recent years, scientists have gained interest in these previously overlooked snails due to their amazing radiation and evolutionary history. By their very nature, islands are ideal study sites to investigate evolution, colonization, and radiation due to their isolation and simplified ecology. These snails are a particularly interesting study system since their evolutionary history is rich in speciation but very poorly understood. When lineages have diversified within islands, the clearest cases of diversification occur in taxa with low vagility on large islands with diverse habitats. This system fits that description quite well, with the islands’ distinct moisture zones providing the diversity in habitat. This combination of factors often leads to great diversification because with their limited mobility, populations are readily genetically isolated, and the diversity in habitat creates plenty of niches for which populations can specialize. Therefore, these Galapagos land snails are an ideal study system for radiation and speciation. These snails have also exhibited a remarkable adaptability to a variety of habitats. A single species has radiated into species that can now survive in dry, dusty habitats as well as moist, vegetation-rich regions. They can also survive long periods of drought, which contributes to their persistence and rapid speciation. The nature of the habitats on the Galapagos contribute to the amazing radiation as well; conditions of life such as climate, vegetation, and substrate, change within very short distances, allowing for distinct species to exist in close proximity but utilize different resources (Coppois 1984). Scientists around the world have begun to recognize the potential in this system. Christine Parent, one of the most prominent researchers on these snails, focuses on several evolutionary questions, such as: What is the sequence of species formation? Does it match the geological sequence of island formation? Are species often formed within in an island, or do they need to colonize a new island to split definitively from a common ancestor? (Parent and Coppois, 2016). Additionally, so little is understood about this genera (their small size and elusiveness contributes to this) that scientists are still trying to answer fundamental questions about their natural history, which then inform conservation efforts. Figure 5. Phylogenetic tree of the Bulimulid snails of the Galapagos based on combined mtDNA COI and nDNA ITS1 sequence data. The snail outlines are roughly proportional to actual size. Species on older islands connect at deeper nodes. Numbers above branches are Bayesian posterior probabilities. (Parent, Caccone, and Petren, 2008) Adaptive radiation Before delving into hypotheses, a strong understanding of adaptive radiation will be useful in understanding the mystery of the high species richness. Adaptive radiation is defined as the differentiation of a single ancestor into an array of species that inhabit a variety of environments and differ in morphological and physiological traits to exploit those environments (Schluter). The mechanism is as follows: more ecological niches, whether that be habitat type or food source, allows for more specialization, which given enough time can lead to reproductive isolation and isolation. Thus, phenotypic differences between populations originate from differences in environments they inhabit and resources they consume. These differences subject the species to unique selection pressures, which drives unique adaptations to that ecological niche. In addition, competition can drive co-occurring populations to exploit new resources and environments (Schluter). For example, competition for habitat space could drive a subset of a snail population in a region to move to a more arboreal habitat. This also suggests that rates of such divergence are dependent on the number of niches available, which could be a partial explanation to the snails’ incredible species richness. Given the high barrier to entry, islands as isolated as the Galapagos have a wealth of different resource types underutilized by different taxa, providing a great opportunity for divergence (Schluter). Therefore, ecological opportunity is largely dependent on timing; a species colonizing a remote archipelago or surviving a mass extinction is in a powerful position to speciate, given the number of available ecological niches. This paper will investigate the mechanisms of this impressive example of radiation to better understand the factors that influence species richness on islands. Island biogeographical theory says that species richness is dependent on three main rates: colonization, speciation, and extinction (Parent and Crespi, 2006). My hypotheses will focus on the dynamics of these three rates, which will in turn contribute to the overall understanding of how such a high number of species has remained distinct in a relatively small land area (7880 km2) (worldatlas.com). Figure 6. Species richness on an island is affected by colonization, speciation, and extinction rates. The three main influences of these factors are listed above them. This paper will investigate colonization, speciation, and extinction of the snails and how it affects species richness. COLONIZATION The initial colonization event, their “lucky break” that allowed such speciation, is thus a key part of the snails’ evolutionary history. However, mechanisms of dispersal and the exact common ancestor is still surprisingly unknown. Parent, Caccone, and Petren (2006) found that there was one single colonization event from the mainland, while other genera of snails on the Galapagos like Bapstinus have about three. They hypothesize that the Galapagos bulimulids are closely related to the continental South American bulimulids due to morphological affinities. However, this has not been genetically confirmed, and extinctions and range shifts often makes identifying the initial colonizer difficult to confirm. The time of the initial divergence from their closest relatives is also unknown, since there is not a reliable molecular clock for this taxonomic group. Dispersal mechanisms The bulimulid snails have certain adaptations that could have aided in their survival across the 1000+ km of ocean. They can close their operculum to prevent moisture loss, and can go into a state of aestivation, which is a state of dormancy and lowered metabolic rate to cope with arid conditions. One hypothesized mechanism has been travel via birds. Rafting on vegetation, though originally proposed, seems less likely since on certain islands, there