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Dynamics of competing species in a model of adaptive radiation and macroevolution

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We present a simple model of adaptive radiations in evolution based on species competition. Competition is found to promote species divergence and branching, and to dampen the net species production. In the model simulations, high taxonomic
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   1 Dynamics of competing species in a model of adaptive radiations and macroevolution Birgitte Freiesleben De Blasio 1 , Fabio Vittorio De Blasio 2 ,  1  Department of Statistics, Institute of Basic Medical Sciences, University of Oslo, Norway, 2 Department of Geosciences, University of Oslo, Norway. Corresponding author: Dr. Fabio Vittorio De Blasio c/o International centre for Geohazards P.O. Box 3930 Ullevaal Stadion  N-0806 Oslo  Norway Phone: +47 22 02 31 22 E-mail: fvb@ngi.no    2    ABSTRACT We present a simple model of adaptive radiations in evolution based on species competition. Competition is found to promote species divergence and branching, and to dampen the net species production. In the model simulations, high taxonomic diversification and branching take place during the beginning of the radiation. The results show striking similarities with empirical data and highlight the mechanism of competition as an important driving factor for accelerated evolutionary transformation.   3   INTRODUCTION The process of adaptive radiation (AR) is believed to play a major role in the evolution of diverse life forms on Earth. In the fossil record, large-scale AR’s are seen as ‘explosions’ of new taxonomical groups during  particularly active periods in time. The greatest AR of all times was the Cambrian explosion that gave rise to most of all known animal phyla. Small-scale AR’s usually occur in limited and isolated environments. A  prime example is Darwin’s finches on the Galapagos Island that have adapted from a single immigrating species to occupy different ecological niches [1]. Despite great variability in size, AR’s share common characteristics. (1) They are initiated when new resources becomes available to a founder species, e.g. because it develops a key character, or in the wake of a mass extinction. (2) The availability of resources triggers rapid evolution of morphologically distinct groups to fill ecological niches not yet occupied. (3) The creative phase is followed by a longer phase of species multiplication with limited creation of novelty [2]. A characteristic pattern of AR’s is that high taxonomic groups are established early and anticipate the creation of low level groups (Fig. 1). Why does the branching of major groups take place at the srcin of the radiation and becomes rare afterwards? The question is part of a fundamental controversy that spans the last century of evolutionary thinking, namely if the srcin and proliferation of novelties has been gradual—or—if history of life has moved in leaps. In other words, are the forces that shape long-term evolution (macroevolution) identical to the ones that operate at the level of individuals (microevolution)?   4 According to the widely accepted Neo-Darwinian Theory [2,3], adaptive evolution is a gradual process that is well explained by the conjunction of mutation, natural selection and genetic drift. Still, other evolutionists argue that macro-evolutionary patterns are more than simply the accumulation of micro-evolutionary processes over long periods of time. For instance, in the macro-mutation hypothesis [4] it is claimed that major taxonomic groups have formed as a result of infrequent, but large ‘jumps’ in genotypic appearance. More recently, it is stated in the Theory of Punctuated Equilibrium [5] that evolution proceeds through long  periods of stasis ‘punctuated’ by rapid bursts of speciation. The fast  proliferation, in turn, leads to selection on higher taxonomic levels and introduces a hierarchical and directed structure to macroevolution [6].  Neither theory is able to account for the uneven appearance of high-level taxa in the fossil record. For example, if large morphological jumps may occur at any time, why is the creation of high taxonomical categories only  present in the beginning of a radiation? And, which forces prevent the radiation from progressing forever? It has been argued that genetic stability has increased during evolution, thereby favouring early development of higher taxa [7]; the hypothesis of genetic robustness has also been studied theoretically [8]. An alternative hypothesis that has gained increasing acceptance is that major morphological innovation decreases during the radiation as a consequence of ecological saturation following niche occupation [9]. Valentine et al. [10,11] have used computer models to study the influence of eco-space colonisation on AR’s. In the models, an ecological niche capable of supporting a species is represented by a tessera in a two-dimensional ‘adaptive space’. Thus, each tessera can be occupied by at most one species, and species may colonise new tesserae by sending a daughter species. Local colonisation of a neighbouring cell (microevolution) is assumed to be more frequent than large jumps spanning the distance of more tessera (macroevolution). Large jumps are mostly successful in the   5  beginning of the radiation when space is relatively free, and hence, the model rightfully explains the uneven appearance of major groups. However, the concept of macro-mutation is controversial, and the use of tessera makes the model stiff. Competitive interactions play a key role in ecological dynamics, and competition also appears to be essential for species proliferation.  Noticeably, an AR often produces morphologically different species where seemingly no adaptive reason for the diversification exists; e.g. the finches on the Galapagos Islands are similar in key characteristics when occurring on different islands, while they differ markedly when present on the same island. The non-adaptive diversification shows that competition between species exploiting the same resources is important for creation of diversity [1] Recent laboratory bacterial experiments have significantly increased our understanding of evolutionary processes and suggest that species branching is promoted by competition [12,13]. The test-tube bacterial experiments show spatial patterns, which resemble grand-scale AR’s observed in fossil lineages. This apparent scale-independence may reflect that small- and large-scale AR’s are driven by the same forces of mutation and natural selection [14]. We present a simple model for the dynamics of AR’s based on inter-species competition. In the model we use a morphospace representation of species, which measures the disparity of organisms in shape, form and structure. Each axis quantifies a single phenotypic character, and each species is represented by a single point in the space when measured for the various characters. Phenotypic similarity between species implies that they are likely to exploit the same resources [15]. The observation is used to ‘translate’ competition among species for ecological niches and resources into strong competition among neighbouring species in morphospace. However, even species that are far morphologically may compete for common resources, like the competition for light among
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