Abstract
BackgroundIt is widely agreed that species are fundamental units of biology, but there is little agreement on a definition of species or on an operational criterion for delimiting species that is applicable to all organisms.Methodology/Principal FindingsWe focus on asexual eukaryotes as the simplest case for investigating species and speciation. We describe a model of speciation in asexual organisms based on basic principles of population and evolutionary genetics. The resulting species are independently evolving populations as described by the evolutionary species concept or the general lineage species concept. Based on this model, we describe a procedure for using gene sequences from small samples of individuals to assign them to the same or different species. Using this method of species delimitation, we demonstrate the existence of species as independent evolutionary units in seven groups of invertebrates, fungi, and protists that reproduce asexually most or all of the time.Conclusions/SignificanceThis wide evolutionary sampling establishes the general existence of species and speciation in asexual organisms. The method is well suited for measuring species diversity when phenotypic data are insufficient to distinguish species, or are not available, as in DNA barcoding and environmental sequencing. We argue that it is also widely applicable to sexual organisms.
Highlights
In The Origin of Species, Charles Darwin showed how a single species can give rise to two different varieties and eventually to species by the accumulation of gradual changes
The same clades were strongly supported by bootstrapped phylogenetic analyses using parsimony, or neighbor-joining with sequence differences corrected for multiple hits with the parameter-rich maximum likelihood models selected by ModelTest [25], or corrected with the one-parameter Jukes-Cantor model
Our results show that a simple species criterion with a rigorous theoretical basis in population genetics and statistics, the 46 rule, can be used to detect species in even small samples of individuals from a variety of asexual organisms
Summary
In The Origin of Species, Charles Darwin showed how a single species can give rise to two different varieties and eventually to species by the accumulation of gradual changes. Darwin wrote: ‘‘varieties cannot be distinguished from species—except, first, by the discovery of intermediate linking forms; and, secondly, by a certain indefinite amount of difference between them; ... Sexual organisms exchange genes readily within clusters but not between them. These facts are widely accepted and biologists tend to treat species as fundamental units of biology; defining species continues to be one of the most difficult and contentious problem in biology [2,3,4,5]. It is widely agreed that species are fundamental units of biology, but there is little agreement on a definition of species or on an operational criterion for delimiting species that is applicable to all organisms
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