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On Biodiversity...


Director of the Instituto de Biología Evolutiva (CSIC-UPF)

Conservation biology viewed from the perspective of evolutionary biology

The author suggests that it is essential to determine what evolutionary patterns have led to today’s biological diversity in order to understand the key factors producing biodiversity, and so have a historical framework allowing today’s rich biodiversity to be put in context.

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 One of the big challenges facing scientific research in the wake of the sequencing of the human genome –and that of many other species– is to describe and understand biological diversity, whether within each species (the study of poly­morphism or variation) or between species (analysis of divergence). Evolutionary biol­ogy provides the fundamental tools and concepts with which to describe HIGHLIGHTSProfile: Xavier Bellés
and understand biological diversity. And this knowledge leads us to no less than a direct understanding of the keys to how life, and its essential mechanisms, work.

It is in this context that the Instituto de Biología Evolutiva [Institute of Evolutionary Biology] (IBE), set up in 2008 as a joint centre by the Spanish National Research Council (CSIC) and the Pompeu Fabra University, operates. Here we will discuss some examples of research lines relating to biodiversity and its conservation.

It is currently estimated that there are between 1.5 (Robert May, 2000) and 1.9 (Arthur Chapman, 2009) million described species. This wide variation is basically a result of problems of synonymy in the taxonomic catalogues. It is noteworthy that over 60% of described species are arthropods, with 1.15 million species, and the immense majority of them (1 million) are insects. All other known species are divided among the plants (over 300,000 species, or approximately 16% of the total), fungi (almost 100,000 species, approximately 5% of the total), molluscs (around 85,000 species, approximately 4.5% of the total) and vertebrates (over 63,500 species, approximately 3.4% of the total). We therefore have to admit that we do not know exactly how many valid species have been described. And our uncertainty increases further when we try to evaluate how many species there are in total on our planet. The most conservative estimates envisage a range of values of between 3.5 and 111.5 million species. The most recent evaluation was published in 2011 by Camilo Mora and colleagues, and predicts that there are 8.7 million eukary­otic species on the earth, 2.2 million of which are marine creatures. This work also estimates that 86% of land species and 91% of marine species have yet to be described.

In any event, the number of species currently inhabiting the earth is very large, indeed probably larger than it ever has been in the past. This is a result of life’s evolution over more than 3.5 billion years, i.e. more than three quarters of the time during which life on our planet will be possible. From a common ancestor, life has diversified into millions of different lineages through a contingent succession of processes of speciation and extinction. Thus, a number of authors, such as Jack Sepkoski, Robert May and Michael Benton, estimate that the species alive today are just 2-4% of all the species that have ever existed. Nevertheless, a large number of combinations of rates of speciation and extinction are possible, and this can lead to an end state in which there is wide diversity, such as that we see today. It is also essential to determine what evolutionary patterns have led to today’s biological diversity in order to understand the key factors driving biodiversity, and so have a historical framework allowing us to put today’s rich biodiversity in context.

It is clear that in order to investigate the patterns of diversification over time, the first source of information to consider is the fossil record. From a catalogue of the different forms of life in the past we can deduce the rate at which groups of organisms have appeared and disappeared at different times in the earth’s geological history, and ultim­ately, we can identify gen­eral patterns of biological diversification. However, the fossil record is far from being an ideal source of information, as we know it is incomplete and biased. Soft-bodied organisms rarely fossilise, making it impossible to quantify the biodiversity further back than the last hundred million years before the Cambrian. What is more, there is a marked tendency towards the formation of fossils in particular environments, as shown by the fact that 95% of described fossils are marine creatures, while 85% of today’s known biodiversity is terrestrial.

Fortunately, biological diversity can also be studied using the information in molecular phylogenies, as shown by the global analyses of diversification in mammals based on molecu­lar phylogenies, carried out at the IBE as part of Víctor Soria-Carrasco’s doctoral thesis work, under the supervision of José Castresana. From phylogenetic trees it is possible to analyse diversification by studying the distances between cladogenetic events. This information can be shown on graphs of lineage accumulation over time (Lineages Through Time, LTT), which offer a simple way of exploring the pattern of diversification within the phylogen­etic tree (Figure 1A). From there it is possible to use various types of models describing diversification processes. The simplest is the model of pure speciation (pure birth), in which lineages emerge at a constant rate, producing a straight line on the semi-logarithmic LTT graphs (Figure 1B). The pure birth model is a specific case within the speciation and extinction (birth-death) models, in which extinctions are not allowed. The next simplest is a birth-death model, in which the rates of speciation and extinction are constant (Figure 1C). These models can be made considerably more complicated if we allow ad hoc changes in the diversification rate. This leads to more complex patterns that allow for variations in the speciation rate in a pure birth model or changes in the extinction rate, or in both the speciation and extinctions rates, in a birth-death model.

Other possible models depend on density, so that the diversification rate varies gradually according to the number of lineages. Density-dependent models are very interesting, as by linking the rate of diversification to the number of lineages present at a given time, they allow a relationship between diversification and the ecology to be assumed. Thus, models of a gradual decrease in the diversification rate would reflect the idea that there is a limit on the number of available niches, and therefore, the rate at which new lineages appear should decrease as the ecological niches are filled (Figure 1D). Moreover, gradual growth models make it possible to explore the idea that the increase in the number of lineages favours an acceleration in speciation processes. This is the interesting idea, which as yet lacks widespread support among specialists, that biodiversity catalyses further biodiversity. Evolutionary biology therefore gives us valu­able tools with which to estimate biological speciation and extinction rates over time. And understanding how these rates have looked in the past will give us a fuller appreciation of how dramatic today’s rate of biodiversity loss is. As is well known, this process is largely anthropogenic, and understanding it will help focus species conservation criteria in a more global and holistic way.   

Evolutionary biology also gives us the tools with which to establish the genealogy of species, the famous tree of life mentioned by Charles Darwin in On the Origin of Species. The tree of life is also useful when defining the criteria for species conservation. From the strictly evolutionary point of view, it would be of the utmost importance to protect those species that represent particular branches of the tree, i.e. species that are the sole representatives large evolutionary groups, as we would lose a whole exclusive organism type if they were to disappear. For example, the Filasterea class of protists comprises just two species: Capsaspora owczarzaki (Figure 2A) and Ministeria vibrans (Figure 2B), which are being studied by Iñaki Ruiz-Trillo’s group at the IBE, who are researching the origins of multicellularity. Capsaspora lives in symbiosis in the hemolymph of the tropical fresh-water mollusc Biomphalaria glabrata, while Ministeria lives free and feeds on bacteria. Neither them appears to face any particular threat or to be at risk of extinction, but if they were, protecting them would be a priority as they contain the information for an entire class of organisms. Corallochytrium Iimacisporum is a similar case, as it is the only species representing the Corallochytrea class of protists. We could even find examples of species that represent a whole phylum, such as Trichoplaxadherens, which is the only representative of the phylum of placozoa, or Symbion pandora and Symbion americanus, the only two species in the Cycliophora phylum, or XenoturbeIla bocki and Xenoturbella westbladi, the only known representatives of the phylum Xenoturbellida. Again, there are no reasons to fear for these species’ survival, but if they were endangered, they would need to be put at the top of the conservation agenda on account of the biological and evolutionary information they contain.

Figure 3. Image of the Montseny newt, Calotriton arnoldi, in the typical copulating position, in which the male (underneath and facing forward in the photo) holds the female (on top and facing away) with its tail. / Photo courtesy of Salvador Carranza, Instituto de Biología Evolutiva (IBE).

In conservation biology this criterion, which we could call “evolutionary representativeness,” can also take us to the opposite extreme, i.e. to those species that are endangered, but nevertheless have related species that are not. These species, which form short branches on the phylogenetic tree, would have much lower priority for conservation, if we follow evolutionary criteria alone. There are numerous examples of this situation, but to illustrate it we could take the case of the Montseny newt, Calotriton arnoldi (Figure 3), described and studied extensively by Salvador Carranza, at the IBE. Calotriton arnoldi is a species of newt discovered in 2005 on the rocky outcrop of Montseny, approximately 50 km from Barcelona. The Montseny newt presents a series of morphological, osteological and genetic characteristics that make it a unique species, entirely distinguishable from its sister species, the Pyrenean newt (Calotriton asper). The genetic data allow us to estimate that the two species parted ways about a million and a half years ago. It is the only vertebrate endemic to Catalonia, the only salamander endemic to Spain, and one of the amphibians with the most limited distributions in Europe. Exhaustive exploration since its discovery show that the Montseny newt lives in just seven streams, with a total distribution of approximately 40 km2.

Its sister species, the Pyrenean newt, is found on both sides of the Pyrenees, at altitudes of 500 to 2,500m. It particularly favours clean, slow moving, relatively shallow water, but is also able to adapt to ponds, streams, rivers and even cattle troughs. In 2008 the IUCN gave the Pyrenean newt “near threatened” status as it occupies just over 20,000 km2 and its habitat is in decline. Why not concentrate conservation efforts on the Pyrenean newt and forget about the Montseny newt, if the latter’s evolutionary information appears to be redundant. For various reasons. Even from the evolutionary perspective, the loss of the Montseny newt would represent a loss of direct information about the different evolutionary processes the species has undergone, giving it its particular morphological, ecological and behavioural characteristics. In this creature’s case we do not have such strong ecological arguments for its conservation as for an emblematic species such as the Iberian lynx, with its role as an umbrella species and regulator of abundant prey such as rabbits, or the mesopredators, such as the fox or mongoose, or the universally popular case of lovable African elephants, whose disappearance could have dramatic ecological impacts on the African jungle and savannah.

Lastly, we should not forget the arguments in terms of their heritage value (or sentimental value, if you prefer). The Montseny newt, the Iberian lynx, or the African elephant, although on different scales of locality or universality, are part of the world’s heritage. Conserving the Montseny newt would have a similar significance to preserving the Romanesque churches of Taüll, although there are many other examples of Romanesque churches, or the Roda Bible, even though there are many other medieval bibles, or one of Joan Miró’s “Constellations”, although there are 22 more of them. In the early 1970s, when there were just a few hundred specimens of blue whale (Balaenoptera musculus), a well-known economist, Colin W. Clark, asked whether it would be better business to stop hunting blue whales and develop a programme for their sustainable exploitation, or catch all the remaining specimens and invest the profits in growth stocks. His answer was that it would be better business to kill them and invest the money. As Edward Wilson replied at the time, aggressively ‘economistic’ arguments contain the fundamental error of ignoring the real value offered by biodiversity to the health of our planet’s ecological systems, and ultimately, to ours. Although less tangible, Clark’s reasoning also produced a visceral rejection among nature lovers, who wish to conserve nature for sentimental reasons or for its heritage value, as discussed above. And this value is not insignificant.

Profile: Xavier Bellés

Xavier Bellés is a CSIC research professor and director of the Instituto de Biología Evolutiva [Institute of Evolutionary Biology] (IBE), CSIC-UPF. He was the coordinator of the CSIC’s Natural Resources Area from 2004 to 2008. His research interest is in insects, particularly metamorphosis. He has published around twenty books and over a hundred papers in Science Citation Index (SCI) journals, and is on the editorial board of five SCI journals. He also devotes some of his free time to science popularisation. He is a member of the Royal Academy of Exact, Physical and Natural Sciences in Madrid, the Royal Academy of Sciences and Arts in Barcelona, the Institut d’Estudis Catalans and the Institut Menorquí d’Estudis. He has been awarded the Prix Maurice et Therése Pic by the Société Entomologique de France (Paris), the Premio de Literatura Científica by the Fundació Catalana pera la Recerca (Barcelona) and the Prisma de Bronce from the Casa de las Ciencias (A Coruña).

Published in No. 09

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  • Lychnos. ISSN: 2171-6463 (Spanish print edition),
    2172-0207 (English print edition), 2174-5102 (online edition)
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