Shedding light on the grey zone of speciation along a continuum of genetic divergence

“Species” used to be a clear-cut definition that meant one group of animals was no longer able to breed with another group of animals. For example, for a long time it had been believed that the Neaderthal man (Homo neanderthalensis) lived among, or nearby, the Modern Human (Homo sapiens), in Europe 28,000-35,000 years ago, but the two were “too different” to breed with one another. Then, “stretches of Neanderthal DNA” were found in the Homo sapiens genome. The same thing happened with the Denisovian Man (Homo denisoviensis), living in southwestern Siberia, near the borders of Mongolia and China; but then “stretches of Denisovian DNA” are also found in the Homo sapiens genome.

Additional studies in other Families comprising multiple Genuses and species. The principal rule is that “isolated populations accumulate genetic differences across their genomes as they diverge.” These “genetic differences” involve chromosomal insertions/deletions (indels) and rearrangements –– to the point where chromosomal pairs cannot line up during meisosis and, therefore, fertile zygotes cannot be formed. Thus, if a subpopulation lives isolated (inbreeding only) for some number of years (e.g. 2 million, 3 million) those subpopulations cannot breed with one another; hence, a new species is formed. This was first noted by Charles Darwin among finches in the Galapagos Islands.

Now, scientists are beginning to appreciate that gene flow between populations can “counteract this divergence” and tend to restore genetic homogeneity. Species barriers are expected to act locally within the genome, leading to predictions of a mosaic pattern of genetic differentiation between populations at intermediate levels of divergence ––– this is the “genic view of speciation”. At the same time, linked selection also contributes to speed up differentiation in low-recombining and gene-dense regions. Authors [see attached article] used a modeling approach that accounts for both sources of genomic heterogeneity and explored a wide continuum of genomic divergence –– made by 61 pairs of species/populations in animals.

Authors’ analysis provides a unifying picture of the relationship between molecular divergence and ability to exchange genes. They show that the “grey zone” of speciation –– the intermediate state in which species definition is controversial –– spans from 0.5% to 2% of molecular divergence, with these thresholds being independent of species life history traits and ecology. Semi-isolated species, between which alleles can be exchanged at some, but not all, loci are numerous, with the earliest species barriers being detected at divergences as low as 0.075%. Of course, these results have important implications regarding taxonomy, conservation biology, and the management of biodiversity!

PLoS Biol 2o16; 14: e2000234

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