How are new species formed? And when do we call a new subline a “new species”?

How are new species formed? And when do we call a newly diverged subline a “new species”? Clearly, the environment strongly influences selection and promotion of certain genes and allelic forms over others. Concepts and definitions of species have been debated for generations, yet today remain controversial. Conservatively assuming that there are about 10 million (~107) different species on Earth, excluding most eubacteria and archaebacteria, and a single origin of life about 4 billion (~4 x 109) years ago, … this would give us an average diversification rate of 0.0025, i.e. one new species appearing every 400 years. This estimate is VERY rough and does not take into account extinction events or “bursts” of speciation, and it is likely to be a severe underestimate because microbes are undercounted. More impressive than the number of species is the number of intermediate forms––Darwin’s “doubtful cases”––suggesting that speciation is a continuous process that is happening all the time.

 Microbes pose a particular challenge because of their genetic diversity, asexual reproduction, and frequent promiscuous horizontal gene transfer (HGT). However, microbes also present an opportunity to study and understand speciation, because of their rapid evolution––both in nature and in the laboratory––and small genomes that can easily be sequenced. In the  report below, authors review how microbial population genomics has enabled us to catch speciation “in the act”, and how the results have challenged and enriched our concepts of speciation, with implications for all domains of life on Earth.

 Authors describe how recombination (including HGT and introgression) has shaped the genomes of nascent microbial, animal, and plant species; the authors argue for a prominent role of natural selection in initiating and maintaining speciation. They ask how universal is the process of speciation across the tree of life, and what lessons can be drawn from microbes? Comparative genomics, showing the extent of HGT in natural populations, certainly jeopardizes the relevance of vertical descent (i.e. the species tree) in speciation. Nevertheless, authors conclude that species do indeed exist––as clusters of genetic and ecological similarity––and that speciation is driven primarily by natural selection, regardless of the balance between horizontal and vertical descent.

 PLoS Genet  2o16;  12: e1005860

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