The following points highlight the three most serious difficulties faced in the application of biological species concept. The difficulties are: 1. Insufficient Information 2. Uniparental Reproduction 3. Evolutionary Intermediacy.
Difficult # 1. Insufficient Information:
Sexual dimorphism, age differences, polymorphism and other such types of variations often give rise to doubts as to whether a certain morphotype is a separate species or only a phenon within a variable population.
Proper studies of life-history, population analysis etc. can unmask such doubts. However, such difficulties are also faced by the neontologists who normally work with preserved material and by the paleontologists who also must assign phena to species.
Difficult # 2. Uniparental Reproduction:
Self-fertilization, parthenogenesis, pseudogamy, vegetative reproduction, are some forms of uniparental reproduction that do not fulfill the criteria of interbreeding. As per definition, a population is an interbreeding group, and, therefore, the term population in “an asexual biological population” is a contradiction.
The biological species concept based on the presence or absence of interbreeding between populations is, therefore, inappropriate for uniparental reproducing organisms.
How to solve this dilemma has been discussed by Simpson (1961) and by Mayr (1963). Fortunately, there are usually well-defined morphological discontinuities among kinds of uni-parentally reproducing organisms. These discontinuities are apparently produced by natural selection among the various mutants, which occur in the asexual clones.
It is customary to utilise the existence of such discontinuities, and the amount of morphological difference among them, to delimit species among uni-parentally reproducing types. Species recognition among asexual organisms is based not merely on analogy but also on the fact that each of the morphological entities, separated by a gap from other similar entities, seems to occupy an ecological niche of its own—it plays its own evolutionary role.
Difficult # 3. Evolutionary Intermediacy:
It is important for a taxonomist to have a thorough knowledge of all stages of differentiation between the individual variant and the well-characterised distinct biological species. Many species pass through intermediate stages like biotypes, races, subspecies, ecotypes or semi-species.
In such incipient speciation, populations will be found which are in the process of becoming separate species and have acquired some but not yet all of the attributes of distinct species. The taxonomist, thus, may encounter various difficulties which may result from such evolutionary intermediacy.
(a) Acquiring of reproductive isolation without sufficient morphological changes:
Sibling species pose greatest problems to a taxonomist as these species — although having acquired reproductive isolation — are feebly or not at all separated morphologically.
(b) Acquiring strong morphological differences without reproductive isolation:
It is undoubtedly inappropriate to call all morphologically distinct population a species. On the other hand, the isolating mechanisms between any two species may occasionally break down. No generalized solution is possible when morphological divergence and acquisition of reproductive isolation do not coincide. In these cases, the specialist should delimit his species in such a way that they form biologically meaningful, natural entities.
(c) Hybrid complexes:
The occasional breakdown of reproductive isolation may occur even between good species. This, most frequently lead only to the production of hybrids. Such hybrids are either sterile or of lowered viability and, therefore, this may not cause any taxonomic difficulties.
Sometimes, hybrid individuals are described as species before their hybrid nature is discovered. Such names lose their validity as soon as the hybridism is established. Only populations are recognised as taxa and hybrid individuals are not populations.
Two types of hybridization generally take place:
(i) Sympatric Hybridization:
When two parental species maintain their genetic integrity over a more or less wide area in which they occur together, it is advisable to uphold their biological species status even though in a portion of their ranges there is a breakdown of the isolation. No taxonomic recognition is given to hybrid populations that result from such a local breakdown of reproductive isolation.
Hybridism in plants may lead to the instantaneous production of an allopolyploid — an individual that combines the chromosome sets in two parental species. Such hybrids may give rise by uniparental reproduction to a new population that is reproductively isolated from both parents, and behaves like a new species if it is able to reproduce and, by occupying a new ecological niche, is able to compete with other species (including the parents).
Geographical isolates occasionally have intermediates between subspecies and species, often referred to as semi-species. On the basis of certain criteria they would be considered species; on the basis of others, they would not. It is, therefore, convenient for taxonomists to attach such doubtful populations (as subspecies) to the species with which they are most nearly allied.
In some cases, the isolates have probably reached the species level but have remained members of a super-species. Such populations are often referred to as allospecies. Other instances of evolutionary intermediacy are the circular overlaps and other borderline cases.