Question

Difficulties in the application of biological species

Answer 1

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:

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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:

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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.....

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