Hardship of the biological species concept and ways of its overcoming (on example of birds). - V.S. Friedmann. - Berkut. 21 (1-2). 2012. - P. 127-182.
This paper seeks to rehabilitate the biological species concept viewed in a new perspective. It shows that the species may be more effectively defined as a population system, on the one hand maintaining isolation from population systems of related forms, and, on the other hand, preserving internal integrity. The former proves crucial in population systems coming into secondary contact involving hybridization, while the latter is of importance wherever environmental instability causes local population decline and range fragmentation. Importantly, the same communication mechanisms of conspecific recognition are involved in both maintaining the population system's isolation and in preserving its internal integrity. The idea of life's discrete structure was suggested by Nikolay Timofeeff-Ressovsky as the defining element of the biological view of the world. Biological forms do not grade into each other but are rather organized into units of which the species is the lowest one. The biological species concept assumes that membership in such units is important for the individuals that make up the unit. Since individuals cannot realize their reproductive potential on their own, but rather only as members of populations and communities, hence, assuming a sufficient level of divergence between the two population systems (higher-level populations of Short 1972), neither hybridization within the contact area nor even the fertility of the hybrids will be fatal to the isolation of the two biological forms. Though natural selection in most cases does not favour isolating mechanisms, conspecific recognition systems will (assuming they have sufficiently diverged while separated) keep the two forms separate during secondary contact and hybridization, even if mating is not assortative. Acceptance of conspecifics and rejection of heterospecifics plays out when hybrids and back-crosses attempt to integrate into population groups formed by phenotypically pure individuals inside the hybrid zone. Thus, the two forms remain distinct even when their genetic pools are almost united, because consistent differences in the organization of population systems and consistent differences in phenotypes (and morphology in general) are maintained beyond the hybrid zone. This happens despite a constant gene and phene flow across the hybrid zone which by themselves would be quite sufficient to form a hybrid phenotype within the larger part of the species range. Furthermore, the hybrid phenotype proves to be unstable in space and time even within the hybrid zone, where all individuals are of hybrid origin. Hybrids fail to form their own network of stable settlements even where hybrids outnumber non-hybrids. This paper shows ornithological evidences. 1. The unpopularity of the biological species concept at the end of the XX century owes to an incorrect definition of the traits responsible for the isolation of species-level forms. The authors and proponents of the biological species concept believed these to be traits that prevent mating between different forms and lower the survival rate among hybrids. In other words, conspecific recognition was believed to occur on the level of individuals forming mating couples and reproducing. Therefore, the biological species concept lost its appeal once it was shown through numerous detailed studies of hybrid zones in birds that recognition errors on this level occur very often, that hybrids and back-crosses can be fertile to such an extent that their presence would be expected to promote ever greater hybridization, and that the ecological and demographical situation at the species range boundary is particularly conducive to recognition errors that produce hybrids. 2. The biological species concept can be rehabilitated with new research evidence that shows that conspecific recognition plays out not on the level of individuals, for these commonly make recognition mistakes, but on the level of population systems, for the capability of individuals to integrate into such associations will depend on the individual’s phenotype. Pure phenotypes easily integrate into settlements of their own form, while hybrids and back-crosses integrate with all the more difficulty, the more intermediate and mixed their phenotype. In this case, separate forms are efficiently differentiated on the levels of ecology and morphology (and the achieved level of differentiation is maintained) despite a consistent and vast gene flow across the hybrid zone and even in cases of a practically common gene pool. 3. The historical dynamics of species ranges constantly creates natural experiments that can evaluate whether given forms have reached the species status. The first kind of experiment occurs in long-term disjunctions of species ranges, where the population system is tested for integrity. The second test occurs in zones of secondary contact accompanied by hybridization, where the level of isolation of the population system (in a situation of inflow of ‘foreign’ genes and phenes) is tested. This paper analyzes how studies of population-level isolation can be applied to both situations (which present difficulties for the biological species concept). It is shown that the same mechanisms are used to maintain the integrity of the population system both during disjunction inside the species range and during contact with a related form at the species range boundary. [Russian].
Key words: speciation, species limits, conspecific recognition, population, hybridization, hybrid zone, phylogeography.
Address: V.S. Friedmann, Lomonosov Moscow State University, Biological Faculty, Lab. of Ecology and Nature Conservation; Leninskiye Gory, 1, Moscow, 119992, Russia; e-mail: wolf17@list.ru.