Marine microbial ecosystems underpin global biogeochemical cycles and
play a central role in the regulation of Earth's climate. These
communities are extremely diverse, and their taxonomic composition
varies considerably across ocean basins. It has however been difficult
to establish links between taxonomic diversity and ecosystem function,
and the ecological and evolutionary mechanisms underpinning taxonomic
variation are not well understood. Here we use an individual-based
eco-evolutionary model in which taxonomic diversity emerges as a
consequence of evolutionary history. Using this model we are able to
show that virtually unlimited genetic divergence can be supported in
highly abundant and rapidly evolving assemblages, even in the absence of
niche separation. With a steady stream of genetic, epigenetic and
plastic heritable changes to phenotype, competitive exclusion may be
weakened, allowing sustained coexistence of nearly neutral phenotypes
with highly divergent lineages. This response may help to explain
observed patterns of taxonomic diversity and functional redundancy -
without recourse to hidden dimensions of niche partitioning. In light of
these results we suggest that individual-level variability is a key
driver of species coexistence and the maintenance of microbial
biodiversity.