The concept of biological species was advanced by Mayr (1957) and gained general acclaim through the advocacy of Stebbins, Clausen, Grant and others in botanical taxonomy. This concept underscores reproductive isolation which is, in turn, a by-product of genetic divergence.
The biological species, according to Grant (1957), is defined as “a community of cross fertilising individuals linked together by bonds of mating and isolated reproductively from other species.” In short, a biological species is formed by “groups of interbreeding natural populations that are reproductively isolated from such other groups.”
In this concept, two aspects are stressed:
(i) Interbreeding between members of the same species and
(ii) Reproductive isolation between the members of different species.
Since the bulk species of the angiosperms are outbreeding, this aspect allows a good quantity of gene exchange between populations and accounts for the variability within species; this aspect contributes little by way of isolation. Inbreeding, on the other hand, reduces variability by curtailing gene exchange and aids in spreading homozygous population under the condition that there are no harmful influences for the homozygosity.
Continued self fertilisation will create deleterious effects on the genetic makeup of populations. Under fluctuating or unreliable conditions, selfing has proved helpful over interbreeding by ensuring pollination which leads to rapid growth in the population.
This has been demonstrated in populations of Agrostis tenuis and Anthoxanthum odoratum which can tolerate heavy metals. In this case, autogamy is a contrivance for metal tolerance by insulating these populations against gene exchange from non-tolerant forms.
However, selfing can evolve for reasons other than isolation. In Armeria maritima, the high degree of self-fertility in metal tolerant populations compared to the normal ones is not subject to gene exchange from non-tolerant populations and has probably evolved in response to low density in colonising state.
Portulaca is another example where autogamy has led to the production of several pure line populations which are isolated to such an extent from one another that each one of them has been assigned a separate binomial.
Interspecific incompatibility, one of the corner stones of the biological species concept, is an isolating mechanism between species that has been facilitated by evading fertilisation in the prezygotic phase.
This is evident in Linum, Lycopersicon, Nicoticina, Petunia and Solatium. One of the main causes of this incompatibility of the related species in the lack of correlation between the length of the pollen tube and length of the style. Kostoff (1943) listed 68 interspecific crosses in Nicotiana which failed in view of the pollen tubes not being able to reach the ovules.
In such crosses, generally the maternal styles are very long for the pollen tube to travel the path to the ovules. However, such crosses with those species possessing shorter pollen tube as females are reportedly fruitful.
Still another type of infertility has been recorded in the genus Datura where the pollen tube grows faster in the style of the same species and frequently bursts in the styles of other species. In comparison with the rate of domestic pollen tube growth, that of the alien pollen differs from one recipient to another.
Discordant evolution of genotype and phenotype poses an entirely different problem. Genotype does not undergo divergence and phenotype does in polymorphic species, whereas genotype diverges and phenotype remains the same in cryptic species. Such cases frequently confuse many practicing plant taxonomists and correct decisions are difficult to arrive at. In evolution, the basic unit is a breeding population.
A fundamental tenet of the biological species concept is the criterion of reproductive isolation. In asexually reproducing organisms, where there are neither breeding populations nor reproductive isolation, interbreeding is not relevant. Among angiosperms, several genera exhibit the phenomenon of apomixis. As a consequence, the biological species concept cannot be applied to these cases.
The problem is so acute that “the most satisfactory solution in taxonomic practice seems to be a dualistic one …. It consists in defining the term species biologically in sexual organisms and morphologically in asexual ones”. While agreeing with this statement, Maheshwari (1967) proposed the terms ‘biospecies’ and ‘phenospecies’ respectively for them.
Although the value of the concept lies in population biology and involves fertility tests; such tests have been limited to a few selected individuals and the results then applied to the entire population. In allopatric populations, these tests can be performed only under experimental conditions and are of mere academic interest because gene exchange does not take place in natural populations.
Furthermore, local breeding populations differ considerably in their genetic properties and may have various kinds of kinship with such other populations. Among higher plants, there are many natural interspecific and inter-generic crosses which present formidable problems to the evolutionary taxonomists.
During the last three decades, the concept has stirred the imagination of biologists. In the domain of botany, the concept has remained largely conceptual rather than operational. Even Mayr (1957) admitted the hazards in applying the concept to animal organisms. Heywood (1974) observed that the “general acceptance of a concept is, of course, a different matter from acceptance in practice”.
Even the so called experimental studies which the concept itself encouraged lead to the interference that most species are not evolutionary units held together by gene flow and that efforts to accommodate all variations into units so defined are misguided. Simultaneously, the validity of the concept has been challenged on operational grounds by numerical taxonomists.
Sokal and Crovollo (1970) jumped to the conclusion that the biological species concept is “neither operational nor heuristic are of practical value”. They went to the extent of suggesting that this concept be abandoned in favour of the phenetic species concept which is the most suitable one to be associated with the taxonomic category ‘species’.