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Then species was defined on a priori basis and regarded as unchanging and fixed. After the knowledge of a number of organisms, people started facing difficulty as there are species which belong to different genera. Various attempts have been made to define a species. Broadly ; 5 different concepts are given:. Linneaus species concept.
Evolution Library: Topic Page
It was presented by Dobzansky in It was suggested by L. Van Valen in The species concept kept on changing from time to time. Ernst Mayr stressed that the non-breeding of natural populations rather than the sterility of individuals be taken as the decisive species criterion. After the knowledge of some monotypic taxa; philosophical problems, biological and evolutionary problems like gene flow etc. Morphologic species or Morphospecies concept is also called as classical phenetic species concept or Linnaean or classical species concept.
Morphological or classical species concept of readily recognized and morphologically defined species is practical and efficient system for information retrieval in most of the flowering plants.
This concept is useful and meaningful even for those plants where hybridization is common e. Utility of this concept is that it deals with reproductive isolations. Krukeberg stressed that the bio-species also differ in their ecological contexts. Grant used the biological species concept and created a new diploid species of Gilia Polemoniaceae experimentally through artificial relation over then generations in 16 years. Mayr modified the definition of biological species concept to stress ecological aspects along with reproductive isolation and told that species is a reproductive community of populations reproductively isolated from others that occupies a specific niche in nature.
However, Hengeveld criticized the use of word niche in definition as it is difficult to define it accurately. Hull critically evaluated the biological species concept and found certain difficulties in using it. He felt that the numerical phenetic species concept has more problems, particularly on deciding which phenetic unit is the one to be called species. The genetic species concept assumes that the biological factors of gene flow and reproductive isolation are operative but the way to define species is by a measure of the genetic differences or distance among population.
It is the numerical phenetic species concept using a quantitative measure or genetic rather the morphological or other distance. The problem comes in knowing the genetic difference between populations. Paleontologists work upon fossil species generally paleospecies or chronospecies in which arbitrary time limits are used to delimit paleontological species. Vaughan termed a collection of paleospecies in a monophyletic succession as genes. Paleospecies are usually time-oriented morphospecies.
Evolution in genetical species keeps them sharply separate. This concept avoids the difficulties with determining actual or potential levels of interbreeding as gene flow and allows some degree of inter-specific hybridization. Simpson also suggested that niche included multidimensional relationship of a taxon to its environment and not just its microgeographic situation.
It is a shift from ecological view point to a historical context resulting from apomorphic changes within single branch of a cladogram. Donoghue and Mishler called it as phylogenetic species concept. Willis stated that each species is an internally similar part of a phylogenetic tree. It includes experimental taxonomic studies also. The variation taking place in reciprocal transplant and hybridization etc. Most common examples are Ecotypes, Ecospecies, Coenospecies etc.
Closely, related but ecologically distinct population which are largely interfertile are called ecotypes. Those complex of species which cannot be sorted out taxonomically from a population and in which there is hope of eventual resolution are called species aggregate or Microspecies. Species group or Superspecies. Apomictic groups are included in apogameon and agameon. Danser used the term comparium and commiscuum for coenospecies but was on the ability to hybridize and on geographic factors. Grant defined syngamone as the sum total of species of semispecies linked by frequent or occasional hybridization in nature, a hybridizing group of species, the most inclusive interbreeding population.
It was given by Van Valen Compilospecies is one which is morphologically distinct Taxonomic species concept but not reproductively isolated single species according to biological concept , e. The two species which are morphologically not isolated but reproductively distinct are called sibling species, e. According to Grant Microspecies is the population of predominantly uniparental plant groups which are themselves uniform or slightly uniform and are differentiated morphologically from one another.
These species are restricted to a limited geographical area. The term microspecies was first introduced by Jord and often known as Jardanons in comparison to Linnaeons which are the normal species as suggested by Linneaus. The species concept that is most acceptable for more sexually reproducing flowering plants is the biological concept. The reproductive barriers that keep species apart are most important for limiting gene exchanging and for maintaining the integrity of each unit.
The flow-chart determining population and species as suggested by Dayer and SlobodchiKoff is given below:. Unlike animals many plants hybridize freely in nature and are thus not reproductively isolated. In Epilobium species interspecific hybridization is very common. According to biological species concept the whole group is placed under complex species.
Earlier the taxonomists were not knowing that the individual species possess certain degree of capacity for variation. They believed that taxonomy means that the description of a single plant is the description of the species to which it belonged. But now the scenario is quite different. Now variation in the same species also has to be studied.
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So population study is a must. Mayr , 47, 52 proposed the rule that we should name populations and not species, though an individual can be chosen as the nomenclatural type. Population is the group of plants that grow in a particular habitat at a particular time. Several populations constitute the species.
In such cases there are bound to be discontinuities between the different parts of the population. The more in dispersed such population systems are, the more will be difference between them. A local breeding population growing together with inbreeding capacity to form a common gene pool is recognized by the biosystematists as the unit of evolutionary change. Populations are therefore dynamic and show variations with in the population and variation between the populations of the same species.
Variation is a deviation in structural, functional or developmental characters of an organism from its parents, from others in the same population.
Species Concept: History, Types and Categories | Taxonomy
Genotypic variation is the difference in genotype within population or species as a result of mutation, recombination or gene interaction with some evolutionary significance. Phenotypic variation is variation in structural and functional characters resulting from environmental factors on one or more genotypes; this may not be of evolutionary significance.
Variety is the term applicable to diversity, variant population, variability, character variation etc. Table 1. Variant is an individual or a group of individuals within a population that is definiable and recognizable. It is a neutral term used without taxonomic significance. Mostly the term is applied to an individual or group of individuals with one or only a few distinctive characteristics. It may or may not be equivalent of the taxonomic category or variety. Diversity refers to a number of types of organisms or taxa in the plant kingdom.
Total diversity of plant kingdom is approximately 2,50, vascular plants. Among which 2, 35, are angiosperms, gymnopserms and 10, pteridophytes. Diversity can be seen in various types also, e. It is of three fundamental types, i. Developmental variability can be seen in Oak Quercus. Other example is of Bean plant where the first leaves are opposite and simple which later on become alternate and pinnately compound.
Genetic variation in the population is the result of mutation and recombination. Mutation may be as small as substitution of single nucleotide pair in the DNA molecule or as big as chromosomal aberration structure or number. Recombination is a reassortment of chromosomes, crossover segments etc. It generally takes place in meiosis and fertilization. It results in hybrid formation, pollination, dispersal, population size etc. Recombination also results from hybridization and introgression. Introgression is process of successive hybridization causing the migration of genetic material from one species infraspecific into another.
Speciation is the process by which new species are formed from other ancestral ones. The most widely accepted hypothesis explaining the process of speciation is the geographical theory of speciation or allopatric theory of speciation. According to it the first step in speciation is reproductive isolation brought about by physical separation or geographical separation of population allopatric population. Separation results commonly from shrinkage of the original range of a species so that the populations become physically isolated.
Separation may also results by large range or dispersal when new populations become established beyond the range of pollination of original gene pool. The second step is the independent differentiation and evolution of these reproductively isolated populations. Eventually, if the ranges of the populations merge again and the reproductive isolation persists, then the speciation process is considered complete. Due to first step in differentiation ecotypes are formed. Agrostis tenuis, a temperate grass which also grown on lead concentrated soil, and becomes lead tolerant.
This character is not lost even in the absence of the metal. If the two populations grown side by side, may cross and produce hybrids that cannot tolerate the lead concentration on one side or cannot compete with the normal plants of the species on the other side. Lack of hybrid survival constitute — absolute barrier to gene exchange. There are many isolating mechanisms which operate to prevent or reduce the gene flow between populations:.
Related Species: A History of the Idea (Species and Systematics)
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