I will review the diverse and very scattered literature on population differentiation in the absence of barriers to gene flow, and point out some of the largely unexplored, but important and fascinating, problems in genetic biogeography.

As far as animals are concerned, there can be no doubt that even in certain groups where premating isolating mechanisms . . . have played a major part in speciation, occasional hybridization occurs and may lead to a significant level of genic "introgression" from one species into another.

Hybridization, introgression, and speciation are examples of natural and dynamic evolutionary processes that exert great influence on how genetic diversity is organized.

More challenging is evidence that most archaeal and bacterial genomes (and the inferred ancestral eukaryotic nuclear genome) contain genes from multiple sources. If "chimerism" or "lateral gene transfer" cannot be dismissed as trivial in extent or limited to special categories of genes, then no hierarchical universal classification can be taken as natural.

(Doolittle 1999)

Bacteria reproduce asexually, yet they are also capable of obtaining genes from other organisms, even those of different kingdoms.

(Ochman et al. 2005)

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