Abstract
The rapid advance in genetic sequencing technologies has provided an unprecedented amount of data on the biodiversity of meiofauna. It was hoped that these data would allow the identification and counting of species, distinguished as tight clusters of similar genomes. Surprisingly, this appears not to be the case. Here, we begin a theoretical discussion of this phenomenon, drawing on an individual-based ecological model to inform our arguments. The determining factor in the emergence (or not) of distinguishable genetic clusters in the model is the product of population size with mutation rate—a measure of the adaptability of the population as a whole. This result suggests that indeed one should not expect to observe clearly distinguishable species groupings in data gathered from ultrasequencing of meiofauna.
Highlights
The nature of the process of species formation has been the subject of debate since before the time of Darwin
The mechanism that leads to the clustering that we describe in §3 is very robust and independent of the precise model specification, and we have limited our study to the two variants mentioned, with specific choices for the competition function and the nature of the offspring produced
Under fitness neutrality, any increase in the value of carrying capacity K is effectively equivalent to an increase in m, the model suggests that for organisms occurring in large abundances the scenario without ecological species can arise even when, at individual level, mutations are small compared with niche width
Summary
The theory of adaptive dynamics has unveiled robust mechanisms by which evolving populations of sexually or asexually reproducing organisms can self-organize to form well-defined ecospecies [5,6] These ecospecies have distinct phenotypes and are thought to be reproductively isolated, and so should be recognizable as genetic species in the sequence data. The output is a database of thousands of homologous gene sequences of organisms selected at random from the environmental sample Using these data, biodiversity is quantified by counting clusters of sequences (operational taxonomic units, OTUs) with mutual genetic distance smaller than a prescribed threshold. In order to define the processes of competition and reproduction, we need a criterion which specifies how close two individuals are to each other This ‘distance’ between individuals i and j will be taken to be the distance between their ecotypes, Dij 1⁄4 jxi 2 xjj in variant I. This is illustrated using numerical simulations, but analytic results are available which give criteria for cluster formation [8,9]
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