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4. GENETIC DIVERSITY...
individuals, populations or taxa is a quantitative estimate of how similar they are genetically.
The converse of genetic similarity is genetic distance (D) that measures the extent of gene
differences. When genetic distance is low, genetic similarity is high, and vice versa. The
expectation is that genetic identity gradually decreases with increasing time of divergence
because of different mutations accumulating in the two populations/species (Nei 1987).
In the past thirty years a wealth of data has accumulated concerning the genetic identity
(distance) between taxa of various ranks. From these studies, some general features have
emerged. The genetic identity between local races varies from 1.00 to 0.90 and is generally
much larger than that for interspecific comparisons. The genetic identity between welldefined
biological species is generally low and varies from 0.20 to 0.35 (Zielinski and Polok
2005). The process of divergence is continuous, proceeding both before and after speciation.
Thus, between the I value typical of conspecific populations and well-defined biological species
is a huge gap, informing about the stage of divergence. The classical example of the
gradual decrease of I value has arisen from Drosophila willistoni complex enzymatic studies.
Thus, genetic identity of 0.970 is typical of geographic populations that are fully compatible
reproducible. Semispecies or species at the early stage of divergence that overlap in
geographic distribution and show both postzygotic and prezygotic reproductive barrier have
I equal to 0.873, whereas subspecies, that are allopatric and exhibit incipient postzygotic
barrier, - 0.795. Fully isolated but nearly identical morphologically sibling and nonsibling species
that are phenotypicallydistinct differ more appreciably with I value 0.517 and 0.352,
respectively (Futuyma 1987). Subsequent analyses of more pairs of closely related species
demonstrated that even partial reproductive isolation is often associated with large genetic
distances (Avise 2004).
As judge by the value of genetic identity, L. multiflorum and L. perenne should be classified
as a single species, eventually as semispecies or species at the very early stage of divergence.
Even the subspecies status currently recommended by the Integrated Taxonomic
Information System of the USA (2007) seems to high. Although such suggestions are not
new in their original formulation (Naylor 1960; Bulińska-Radomska and Lester 1985; Zielinski
et al. 1997) the present data provide the strongest support for this hypothesis. There are dozens
of additional examples of species re-classifications following molecular analysis. Aconitum
noveboracense and A. columbianum are herbaceous perennials that occur in the USA.
The former is a rare species while the latter is a much more common species in particular in
mountainous areas. Likewise ryegrasses, these two species can not be reliable differentiated
based on morphological characters. Molecular analyses that revealed significant similarities
in allozyme and RAPDs (I ≥ 0.90) have championed their treatment as a single species
(Cole and Kuchenreuther 2001). Remarkable examples of high molecular similarities despite
morphological differences are provided by Pinus cembra and P. pumila, of which the latter
is supposed to be a dwarf mutant of the former (Polok et al., unpublished data). A peat-bog
pine, Pinus uliginosa has been thought to be a hybrid between P. sylvestris and P. mugo, but
a battery of DNA markers have proved instead that it is closely related (I=0.90) with P. mugo
with which it shares a gene pool (Polok et al. 2007).
Genetic similarity provides, therefore, a valuable service to ryegrass re-classification.
However, several cautionary points should be made about such mechanistic appraisal. Speciation
is highly variable process, differing greatly in mean tempo and mode in different kinds