Concept of Species

Early man used plants for many purposes. The man divided the plants to edible, medicinal and fuel for his convenience. In the plant kingdom species is the basic unit. The species may be defined as the largest group of organisms in which two individuals can produce fertile offspring, typically by sexual reproduction.

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History of Species

The history starts with the great philosopher Plato. He proposed that the concept of edios or species and believed that all objects are shadows of the ‘eidos’. In the year 1686 English scholar John Ray stated that “No matter what variations occur in the individuals or the species if they spring from the seed of one and the same plant, they are the accidental variations and not such as distinguish a species permanently and further he states that one species never springs from the seed of another nor vice versa”. Therefore, Ray defined species as groups of plant truly breed within their limits of variation in his book titled “Historia Plantarum”. In the year 1753 Linnaeus gave a new definition for species. In his work Species Plantarum work, he used mainly the floral structure and sexual characters, In his work he described systematically around 5900 species of plants. Therefore, he used a sexual system “natural system” for defining species. His concept was accepted widely.

       The word taxonomy was introduced by De Candolle (17778-1841) in his book Theorie Elementaire de la Botani-que (1813). He defined the term species as “a species is a collection of all the individuals which resemble each other more than they resemble anything else. He divided plants into two major groups, nonvascular and vascular plants. In De Candolle book titled Prodromus systematis Naturalis Regni Vegetabilis (1823-1873) described 161 families and 58,000 species of cotyledon plants. Wallace (theory of evolution) and Darwin stated that species as the fundamental units of evolution. In the year 1920 new branch genetics born based on genetics many modern species concepts were developed.

Modern species concept

In modern species concept, the relationship between individuals was considered. These relations could be phonetic or phylogenetic. Phenetic relies on similarities between present properties of organisms with no consideration or references in how they possess them. The source data for phenetic are morphology, cytology, phytochemistry, anatomy, embryology and even some generic features. Phylogenetic describes the pathways of ancestry and it is called Cladistic which is same as phylogenetics.

Concept of Biological species

In the 19th century (1942) Mayr the Zoologist used the term biological species. The term defined as “groups of actually or potentially interbreeding natural population which is reproductively isolated from other such groups”. Thus groups of related plants which are distinct at the level of biological species concept (BCS). It is a non-phylogenetic species concept. Because a species is a group of reproducing natural population incapable to effectively mate or breed with other such groups, and which inhabits a particular niche in nature. The disadvantages of this concept are given below

1. It is inapplicable for sexual organisms.
2. It is impractical in instances of geographically isolated (allopatric populations).

The isolating species concept and recognition species concept are part of the biological concept or genetic concept, because of both the concept as part of gene recombination. The disadvantage of the biological concept is inapplicable on sexual organisms and impractical on allopatric.

Morphological Species Concept

In the year 1978 Cronquist defined species as the smallest groups that are constantly and determinately distinctive discontinuity in the series of biotype.In few cases, the species are morphologically indistinguishable (sympatric) and are clearly different lineages.

Ecological Species Concept (ESC)

Ecological competition is the key factor for ecological species concept. In the year 1976 Van Valen stated that “A species is a lineage (or a closely related set of lineages) occupying an adaptive zone minimally different from that of another lineage in its range and which evolves separately from all lineages outside its range”. In the year 1986 Colinvaux stated that “A species is a number of related populations the members of which compete more with their own kind than with members of other species”. The disadvantage of this concept is it is not significant to determine the degree to which two or more entities are competing ecologically.

Evolutionary Species Concept

This is a single lineage of ancestor-descendant populations of organisms which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate”. The disadvantage (problem) of this concept is gaps in the fossil record levy prejudice limits between species, especially those which experiencing regular size/ shape evolution.

Cohesion species concept

This is an evolutionary lineage that serves as the arena of action of basic microevolutionary forces, such as gene flow, genetic drift, and natural selection”. This was proposed by Templeton (1994).

Phenetic Species Concept

In the year 1993 Riddley defined the species as “ A species is a set of organisms that look similar to each other and distinct from other sets”. For better classification, the following principles to be followed

1. The information and characters in the taxa should be attained.
2. Each and every trait has an equal weight when forming natural taxa.
3. The similarity between two taxa (units) is a result of their individual resemblance among the many traits used to compare them.
4. Taxa can be identified and recognized
5. This is viewed and practiced as an empirical science.

Phylogenetic Species Concept

This concept was proposed by stace, 1989, Agapow et al., 2004. After the rediscovery of G.MenDel’s  laws of inheritance in 1900 and the development of the modern theory of chromosome, all these led to the Cladistic speciation. It defines the species as a group of organisms that share an ancestor.

Practical problems

1. i) It is rarely possible to reconstruct with certainty the past evolutionary pathway:

This lead to difficult to devise a satisfactory method of designation a branching pattern by means of a single linear sequence which is so important in flora and systematic treatment. However, attempts have been made to construct a sequence starting with the most advanced: this could ensure that each taxon recognized is monophyletic or polyphyletic.

Pluralistic Species Concept

Campbell and Reece, 2002 stated that the need to use more than one species concepts in order to be applicable arose the idea of a pluralistic species concept. It is recognizing that “the factors that are most important for a cohesion of individuals as a species vary.

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