Models of processes that promote genomic coherence. (a) The ecotype species concept and (b) the biological species concept both entail processes that lead to genomic coherence within populations and divergence (horizontal dimension) between populations. Black arrowheads indicate organisms or isolates. The crosses in (a) indicate the clones eliminated in the process, while the red arrows in (b) indicate recombination between genomes. Blue lines indicate speciation. (c) If only random lineage splitting and lineage extinction occurred, coherence would not be expected, and the designation of speciation events (dashed blue lines) would be arbitrary. In the ecotype (periodic selection) model in (a), which is applicable to organisms without significant genetic recombination, favorable mutations sweep to fixation, carrying the genome in which they first occurred along, so that diversity is reduced to zero at all loci. Accumulation of neutral mutations, prior to the next sweep, generates the sort of microdiversity illustrated in Figure 1. Gray bars are niche boundaries. In the biological species model, it is individual favorable mutations that are fixed, because recombination (indicated by red arrows) separates them from alleles at other loci in the genome in which they first occurred. Still, recombination at all loci will in time promote genomic coherence within populations and divergence between populations, because with time all alleles at all loci will be traceable to mutations that occurred within the population. The gray block indicates a barrier to recombination.