IndianJournals.com A Brief Review on Species

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A Brief Review on Species Concepts with Emphasis on Plants Nadia Haider Department of Molecular Biology and Biotechnology, Atomic Energy Commission of Syria (AECS), P.O. Box 6091, Damascus, Syria Email id: [email protected] Received: 4-8-2018; Accepted: 3-9-2018

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ABSTRACT Species of all living organisms have been regarded as a basic unit of biodiversity. Regardless of the importance of its rank, the species is probably the most controversial taxonomic rank in biological science, and hence, there are over 30 concepts of species in use today. The earliest concept of species was the morphological species concept. Following Linnaeus, naturalists became aware that irrespective of the levels of morphological similarities among species, they occur as reproductively isolated entities in the field. This led to the biological species concept. The development of cladistic methods increased dissatisfaction with the biological concept of the species and led to the birth of the phylogenetic species concept. After discussing these three important concepts, I referred to other less widely used species concepts and to a study that presented a summary of 26 species concepts and to another study that discussed the species problem. All species concepts in current usage have limitations, and advantages and the proposition of many different species concepts lead to the conclusion that there is still no universally accepted definition of the taxonomic rank ‘species’. Any of these concepts can be suitable under appropriate circumstances, depending on the organism under study and the question that needs to be addressed. Variation in species concept among groups of organisms was also discussed with emphasis on plants, the vast majority of which belong to flowering plants category. Keywords: Biodiversity, Biological species concept, Cladistics, Plants, Species concept, Systematic, Variation

BACKGROUND Plants are classified in several different ways. Starting from class, the highest category, plants have traditionally been classified as follows: class, subclass, superorder, order, family, subfamily, tribe, genus, species, subspecies and variety (The Seed Site, online reference). Each group has the characteristics of the level above it but has some distinguishing features. The further down the scale one goes, the more minor the differences become, until he/ she ends up with a classification which applies to only one plant (Hai, online reference).

The concept of the genus (Bartlett, 1940), which is a principal taxonomic category, is the oldest among all taxonomic categories. Since very early times, the plants in groups were named as elms, poplars, willow, pines, oaks, roses, palms and so on. According to Bhan (online reference), Tournefort (in 1700) used 698 genera of plants, whereas Linnaeus (in 1737) used about 935 genera and so on. The author believes that this made remembering all of genera very difficult and stated that the lowest category of hierarchy which is consistently used and recognised by all the botanists is the species. The author

How to cite this article: Nadia Haider, 2018. A Brief Review on Species Concepts with Emphasis on Plants. LS - An International Journal of Life Sciences, Vol. 7, No. 3, pp. 115-125.

Volume 7, Number 3, September-December, 2018, pp. 115-125

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also referred to the three characters for generic concepts used by Linnaeus in his Philosophia Botanica (a compilation of his lectures on botany which was first published in Stockholm in 1751). These are (1) natural character (complete description of plant), (2) character essentials (characters allowing easiest description) and (3) factitious character (selection of certain characters suitable for discrimination among genera in an artificial system of classification). Within the genus, species (a Latin word meaning ‘kind’) is the basic unit of classification (National Gardening Association, 1999) and evolution, and refers to a taxonomic rank which is the ‘building bricks’ in biological classification (Davis, 1978; Singh, 2012). Species of all living organisms have also been regarded as a basic unit of biodiversity, and it is the level at which most of the evolutionary studies have focused (Graybeal, 1995). In biology, the importance of species derives from their importance in systematics, which is responsible for the taxonomic framework used in all branches of biology (De Queiroz, 2005). Biological diversity or biodiversity is the term given to the variety of life on Earth and the natural patterns it forms. The biodiversity we see today is the fruit of billions of years of evolution, shaped by natural processes and, increasingly, by the influence of humans. It forms the web of life of which we are an integral part and upon which we completely depend. This diversity is often understood in terms of species diversity – the wide variety of plants, animals and microorganisms (Bulgarian Biodiversity Portal, online reference). It is worth noting that global biodiversity is being lost much faster than natural extinction. Some of the causes of this are changes in land use, unsustainable use of natural resources, invasive alien species, climate change and pollution. Species loss is also compounded by (1) the ongoing growth of human populations and unsustainable consumer lifestyles, (2) increasing production of waste and pollutants, (3) urban development and (4) international conflict (WWF, online reference). Nobody knows the total number of species that exist nor how many species are being lost each year (WWF, online

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reference) Estimates of global species diversity have varied from 2 million to 100 million species, with a best estimate of somewhere near 10 million of which only about 2 million species have been described and named (Conners, 2016). Problems stemming from the limits of current knowledge of species diversity are compounded by the lack of a central database or list of the world’s species (Bulgarian Biodiversity Portal, online reference). Added to the difficulty in defining the precise level of genetic similarity among individuals that constitute a species, three other reasons were proposed for explaining why scientists don’t know exactly how many species live on earth. First, some areas of earth (e.g. the deepest ocean) remain relatively unexplored. Second, a large number of organisms are extremely difficult to distinguish and tell apart from other species. Third, many species, like many fungi and bacteria, are tiny and easily overlooked (Quizlet, online reference). The estimated total number of species includes around 300,000 plant species, between 4 and 8 million insects, and about 50,000 vertebrate species (of which about 10,000 are birds and 4,000 are mammals) (WWF, online reference). First living forms (species) were observed 3.8 billion years ago, and more than 99% of species that ever lived are now extinct. Dunhill (2017) reported that most extinct species quietly disappeared during periods of ‘background extinction’, whereby in every 100,000 years or so a handful of species become extinct, mainly due to rapid ecological or environmental changes that affect adaptation of a species to the new conditions or its food resources. Aldhebiani (2017) wrote that ‘the process of species identification itself is not simplified by having a meaning of species’. He added that in many cases, we may not be able to identify species but it is clearer why species can be so difficult to identify. He believes that this difficulty has led to the cynical definition of a species as a group of individuals sufficiently distinct from other groups, to be considered by taxonomist to worth specific rank. In a previous study, Hausdorf (2011) referred to the need for precise clear definition of the term ‘species’ for larger spectators than just the academic biologists. Hence, many biological studies depend on delimitation of species. For

LS - An International Journal of Life Sciences

Concepts of species, the basic unit of biodiversity

instance, ecological and behavioural studies may examine the collaboration between species, including the interaction between species abundance with biotic and abiotic factors as described by Aldhebiani (2017).




Bhan (online reference) cited that Grant (in 1981) recognised six types of species. These are (1) taxonomic species (morphological species or phenetic species) where the taxa are groups of morphologically similar individuals, (2) biological species (genetic species: sexually reproducing population system), (3) microspecies (agamospecies: population is uniparental organism), (4) successional species (paleospecies: phyletic lineage), (5) biosystematic species (ecospecies or coenospecies: fertility group) and (6) evolutionary species (combined sexually reproducing populations iniparental groups and phyletic lineages). Mayr (1996) discriminated between the species as a taxon and as a category. In his view, the word ‘taxon’ refers to a concrete zoological or botanical object consisting of a classifiable population (or group of populations) of organisms, whereas in category, the word ‘species’ indicates the rank in the Linnaean hierarchy and contains all taxa of this rank. Mayden (1997) argued that regardless of the importance of its rank, the species is probably the most controversial taxonomic rank in biological science. This view was also expressed recently by Aldhebiani (2017), who discussed the history of the controversial issue of the definition of the term ‘species’ that has been undefined precisely for more than three centuries. Species definitions and recognising species are so contentious because (1) species are dynamic, evolving individuals, but we attempt to force them into rigid classes and (2) species are real evolutionary groups and not categories which are created as a direct function of perceived distinction (UFSCar, online reference). From the beginning of systematics, which is one of the oldest scientific disciplines, the concept of species [proposed since around 1950 (Boggs, 2001)] has been one of its central concepts (de Queiroz, 2005). The following reasons explain why the concept of species is of great importance: (1) it defines why individuals of same species are alike, (2) it explains the importance of reproduction in the perpetuation of species and (3) it has an important role in taxonomy (Bioexplorer, 2016).

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Various attempts have been made to define a species, and today there are over 30 concepts of species in use. These concepts do not only define what a species is, but by defining what a species is, they also clarify what speciation is (Aldhebiani, 2017). De Queiroz (1998) attributed the existence of diverse species concepts to the fact that different concepts are based on properties that are of greatest interest to different subgroups of biologists. Bioexplorer (2016), who believes that concepts about species are very complicated, added that biologists have already struggled with questions pertaining to the definition of what a species is, even before Charles Darwin had launched his idea about how organisms could change through time. Many concepts have been proposed to determine the variation and the limitation between species. The earliest concept of species was the morphological (or Linnaean/ phenetic/classical/typological/essentialist) species concept (Cronquist, 1978), which is based on shared traits. This concept takes no direct account of evolutionary origin of organisms but defines a species as ‘a species is recognisable by an intrinsic difference reflected by its morphology, which makes this species clearly different from any and all other species’ (Mayr, 1992). This concept dwells on the phenotypic (physical) characteristics that exist among organisms. According to this concept, organisms are classified to be under the same species only when they are physically distinct from other organisms. Typology, which is based on morphology/ phenotype, is still applied in museum research (type method) where a single specimen (type specimen) is the basis for defining the species (Biomed.brown.edu, online reference). According to typological species concept, there are a number of diversities on the surface of the earth that exist as a limited number of universals or types that do not bear any relationship to each other. The universals or types are called species (Jain, online reference). In contrast to the recognition species concept, the phenetic species concept proposes that individuals who look physically similar but tend to avoid each other during mating are still considered as the same species (Bioexplorer, 2016). In addition to the typological species concept, the other historical species concept is the nominalistic species

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concept (Carr, 2010). The nominalistic concept of species assumes that a natural property of all biological objects lies in their variability. The proponent of this concept is Occan. The latter and his followers (Buffon, Bessey, Lamarck etc.) believed that only individuals exist but do not believe in the existence of species (Jain, online reference). According to nominalists (1) the sharp boundary between species is basically artificial and, similar to higher taxa, is only a matter of convention and (2) species are man’s own creations and have no actual existence in nature. Therefore, such mental concept (i.e. species) of man has no value. This concept was popular in the eighteenth century in France and is still being used among some botanists (Jain, online reference; Frozen Evolution, online reference). Following Linnaeus, naturalists became aware that irrespective of the levels of morphological similarities among species, they occur as reproductively isolated entities in the field. This led to the biological species concept (BSC, see Mayr, 1996) (based on breeding habits) in which species are recognised on the basis of being reproductively isolated and largely irrespective of any morphological differentiation. This means that the BSC is mainly about reproduction because species need to undergo reproduction to continue to survive. Dobzhansky (1970) defined the BSC as ‘an inclusive Mendelian population; it is integrated by the bonds of sexual reproduction and parentage’. According to Mayr (1992), the BSC can be defined as ‘a species is an interbreeding community of populations that is reproductively isolated from other such communities’. Interestingly enough, there are several taxa widely accepted to be species that nevertheless are able to exchange genes at least in some parts of their genomes [e.g. Brassica rapa and Brassica napus (Brown and Brown, 1996)]. The BSC has two types: (1) the isolation species concept and (2) the recognition species concept. Mayr’s definition in 1992 represents the traditional isolation concept, whereas the recognition concept is based on the species mating recognition system (SMRS). According to this concept, organisms that can recognise other organisms as their potential partner are considered as same species. Such phenomenon is termed as a ‘shared mate recognition

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system’ (Bioexplorer, 2016). The SMRS parameters, in a sexual species, are the specific morphological and behavioural traits that make an individual of that species able to differentiate between potential partners and other reproductive communities. The isolation and recognition concepts are complementary rather than alternative (Templeton, 1989). Ehrlich (1964) believed that reproductive isolation is important in evolution but rejects basing the species concept primarily on reproductive criterion. Subsequently, Sokal and Crovello (1970) and Sokal (1973) supported this view. There have been a few criticisms of the practical utility of BSC for (1) it cannot be applied to asexually reproducing species (Wheeler, 1999) (e.g. Bdelloid rotifers) and (2) the concept does not apply on the prokaryotes due to the absence of the sexual behaviour defined in the eukaryotes. Moreover, Stuessy (1990) argued that it is difficult to accurately determine interbreeding among populations. The author also believes that there is not usually sufficient information on reproductive behaviour of living individuals to allow the BSC to be applied successfully. Other limitations of BSC noted by Bioexplorer (2016) are (1) it overlooks variations that occur within species, (2) it does not take into consideration the changes that happen over time, (3) it tends to ignore hybridisation and (4) it has problems regarding classifying non-bisexual organisms. A major limitation of BSC is that it does not, by definition, refer to evolution directly, although the BSC is useful in many ways. Therefore, evolutionary and systematic biologists have regularly engaged in long and sometimes sharp debates about species definitions and concepts. The development of cladistic methods increased dissatisfaction with the biological concept of the species and led to the birth of the evolutionary, phylogenetic and cladistic species concepts. These concepts arose, initially, out of the recognition that species do occasionally interbreed. When interbreeding occurs, genes from individuals of one species move into the gene pool of another [introgression or introgressive hybridisation (Harrison and Larson, 2014)]. However, in some cases of gene introgression, species maintain their morphological and ecological distinctness (Boggs, 2001). The evolutionary species concept (ESC)






was introduced by Simpson (1961), who believed that ‘a species consists of a group of populations that shares a common evolutionary fate through time’. He emphasised that this concept would avoid the difficulties in determining actual or potential levels of interbreeding and gene flow, and it allows some degree of interspecific hybridisation. In 2002, Noor revised the BSC to encompass the evolutionary forces that cause divergence. Although several definitions of phylogenetic species were proposed, all of them suggest that classifications should reflect the phylogenetic relationships of the organisms. Boggs (2001) noted that knowledge of evolutionary history, which at present is obtained through comparisons of traits and constructions of cladograms (or genealogical trees) representing phylogenetic hypotheses, is required for application of this species concept. According to phylogenetic species concept, individuals belong to the same species if and only if they have descended from a common ancestor. By having been descended from a common ancestor, all individuals of the same species have a certain defining (distinguishable from others) and derived (common trait inherited from ancestor) traits (Bioexplorer, 2016). Wheeler (1999) stated that the phylogenetic concept is simple, broadly applicable, testable and distinguishes the smallest groups of individuals that can meet the needs of the elements of nomenclature, phylogeny and biodiversity studies. Wheeler and Platnick (2000) defined the phylogenetic species concept as ‘…. the smallest aggregation of populations (sexual) or lineages (asexual) diagnosable by a unique combination of character states’. In 2014, Quaedvlieg et al. distinguished species in the Teratosphaeriaceae based on a polyphasic approach, combining morphological, ecological and phylogenetic species concepts, named by the authors as the consolidated species concept. The distinction between the phylogenetic and cladistic species concepts has not always been clear. For instance, Wiley (1978) defined the cladistic species concept as ‘a single lineage of an ancestor-descendant population that maintains its identity from other such lineages and has its own evolutionary tendencies and historical fate’. Mishler (1985) favoured this concept, although he referred to it as the phylogenetic species concept. Ridley (1989) helped


to resolve the ambiguity when he refined the definition of the cladistic species concept as ‘the group of organisms between two speciation events or between one speciation event and one extinction event or those are descended from a speciation event (for living species)’. It has been argued, however, that species should not be regarded as classes or categories but as individuals since change via evolution is levied at the scale of the individual (van Regenmortel, 1997). In his review, Mayden (online reference) presented abbreviated discussions of 25 various species concepts. He also included synonyms, definitions, discussion and a synopsis to the suitability of the concept. In a similar study, Wilkins (2002) reported that there are numerous species concepts at the object level in the literature. He presented a summary of 26 species concepts. The author believes that metataxonomy of Mayden (1997), who listed 22 distinct species concepts along with synonyms, provides a useful starting point for a review. Wilkins (2002) has added authors where he could locate them in addition to Mayden’s references, and instead of his abbreviations, he has tried to give the concepts names, such as biospecies for biological species and so on (following George (1956), except where nothing natural suggests itself. He also referred to several additional concepts that have emerged since Mayden’s review and explained some ‘partial’ species concepts – the compilospecies concept and the nothospecies concept. From the perspective of a practicing systematist, Mayr (1996) presented a concise overview of the philosophically important aspects of the problem of the ‘species’. Recently, Zachos (2016) offered a comprehensive review of one of today’s most important and contentious issues in biology, the species problem. After setting the stage with key background information on the topic, the book provides a brief history of species concepts from antiquity to the modern synthesis followed by a discussion of the ontological status of species with a focus on the individuality thesis and potential means of reconciling it with other philosophical approaches. The author presented more than 30 different species concepts found in the literature in an annotated list, and discussed in more detail the most important ones, including the biological, genetic,

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evolutionary and different versions of the phylogenetic species concept. He also addressed specific questions that include the problem of asexual and prokaryotic species, intraspecific categories like subspecies and evolutionarily significant units and a potential solution to the species problem based on a hierarchical approach that distinguishes between ontological and operational species concepts. More recently, Aldhebiani (2017) illustrated some of species concepts starting from the typological species concepts to the phylogenetic concept. He overviewed the history of major ones and referred to some of the modern species concepts. In the same review, the author represented positive and negative aspects of these concepts in addition to their application. With criticism, Jain (online reference) discussed four important species concepts. These are (1) typological or essentialist species concept, (2) nominalistic species concept, (3) BSC and (4) ESC. Bhan (online reference) mentioned that current species concepts are morphological species concept, BSC, genetic species concept, paleontological species concept, ESC, cladistic species concept, biosystematic species concept and multispecies concept. Other less widely used species concepts include the ecological species concept described by Andersson (1990) as ‘lineages occupying minimally different adaptive zones’ and the cohesion species concept introduced by Templeton (1989), who defined the species as ‘the most inclusive group of organisms having the potential for genetic and/or demographic exchangeability’. Through cohesion species concept, the species is defined based on a set of factors, increasing genetic divergence between populations besides those increasing genetic similarity within populations, and includes factors that limit the spread of variation through genetic drift or natural selection. Biological species and recognition concepts with a more explicit incorporation of evolutionary processes are combined in this concept with a more explicit incorporation of evolutionary processes (Boggs, 2001). Bioexplorer (2016) noted that according to ‘A hierarchy of species concepts: the denouement in the saga of the species problem’ written by Dr. R.L. Mayden, other less common species concepts are agamospecies concept, evolutionary significant unit, genealogical concordance,

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genotypic cluster definition, Hennigian species concept, internodal species concept, non-dimensional species concept, polythetic species concept, reproductive competition, successional species concept and taxonomic species concept. Boggs (2001) argued that most of the discussions of species he presented assume that species are now formed as well-defined entities. However, he believes that this is clearly not the case for any of the definitions or concepts he described. But we can identify races and subspecies, which eventually may form species, using the typological approach; hybrid zones between species challenge the biological species approach and gene introgression poses problems for monophyly. The author concluded that as a result, further refinement of the species concept is likely to happen in the future. In another study, De Queiroz (2005) addressed the species problem, the metapopulation lineage concept of species, the species as a fundamental category of biological organisation and the cause of the species problem. He also suggested a solution to the species problem and discussed Ernst Mayr and the modern concept of species. He added that Ernst Mayr’s ideas had enormous importance for the development and acceptance of the modern metapopulation lineage concept of species, and they continue to provide the basis for advances regarding the theoretical concept of species and its practical application. The author concluded that Ernst Mayr is almost certainly the greatest of all biologists in terms of his contributions to the development and acceptance of modern views on species. Based on what stated above, it can be concluded that there is still no universally accepted definition of the taxonomic rank ‘species’, and all species concepts in current usage have limitations and advantages. Any of these concepts can be suitable under appropriate circumstances, depending on the organism under study and the question that needs to be addressed. VARIATION IN SPECIES CONCEPT AMONG GROUPS OF ORGANISMS Different definitions of species have been developed for each group of organisms independently. For example, although the species has long been recognised as the basic






unit of classification, there is still no species definition in Bacteriology that is accepted universally (Goodfellow et al., 1997). Buchanan (1955) defined a bacterial species as ‘the type culture together with such other cultures or strains of bacteria as are accepted by bacteriologists as sufficiently closely related’. Gordon (1978) proposed a very different view. He believed that species descriptions should be based on type strains, other authentic strains, fresh isolates and old stock cultures and their variants. Subsequently, Stanier et al. (1986) defined the bacterial species as ‘a species consists of an assemblage of the same general kind’. Blaxter (2003) concluded that it is difficult to apply species concepts to bacteria where sexual isolation is not relevant. There has been similar controversy regarding the definition of mammalian species, whose number rose rapidly from the eighteenth century onwards due to the widespread geographical exploration (184 species being described by Linnaeus himself). Mayr (1940) defined mammalian species as ‘a group of populations which replace each other geographically or ecologically and of which the neighbouring ones integrate or hybridise whenever they are in contact, or which are potentially capable of doing so in those cases where contact is prevented by geographical or ecological barriers’. Another widely adopted definition of the mammalian species was stated by Grant (1957), who defined it 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’. Although the BSC was used for the description of several plant species, as in the description of a new diploid species of Stephanomeria (Compositae) (Stuessy, 1990), it has far less relevance for plant species because of the common occurrence of interspecific hybridisation in the flowering plants and the varying efficacy of interspecific breeding barriers in the kingdom. In addition, the BSC concept is of questionable value for self-pollinating land plants (Cronquist, 1988). For this reason, the original species concept used for plants was essentially based on morphological distinctiveness that despite its narrowness,


it has worked well even in difficult groups such as Quercus (Fagaceae), in which hybridisation is common (Burger, 1975). Based on this concept, species were delimitated in the preparations of plant floras (the term flora is given to a book that contains a list of plant species found within a defined geographic region and that includes an associated array of diagnostic descriptions) as described in the Short Guide for Contributors to Flora Europaea that is based upon easily observable morphological features (Stuessy, 1990). The difficulty here centres on the observation that many plant species differ on the basis of a series of quantitative characters rather than clearly defined qualitative traits. In 1686, Ray therefore tried to accommodate this by adding ‘reproductive cohesion’ as an additional criterion for the recognition of species. He believed that plant species should be able to breed true within the limits of their variation (cited in Gornall (1997). According to Gornall (1997), Linnaeus adopted this view of the species concept in 1753. In subsequent years, the complexity of the criteria, used to define a species, was increased by adding geographical coherence to morphological distinctiveness and breeding behaviour. Du Rietz (1930) later went further to include the belief that species were made up of individuals of natural populations and so defined a species as ‘the smallest natural populations permanently separated from each other by a distinct discontinuity in the series of biotypes’. Mayr (1942) refined this concept to define a species as ‘a group of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups’. This definition rapidly became the most widely used definition for plant species. There have been several phylogenetic species concepts proposed for plants over the past 30 years. The most popular of these is that of Cracraft (1983), who described the species as ‘the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent’. Although species diversity in cultivated plants can be essentially similar to that of wild plants, infraspecific diversity is generally greater in the former due to human selection. Furthermore, breeders augment crop diversity through targeted introgression of genes from related species (Hawkes, 1997). Likewise,

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farmers and breeders have consciously or unconsciously altered the ploidy level of crops by polyploidy so that cultivated plants are far more commonly polyploid than wild species. For all of these reasons, the taxonomic status of crops is frequently far more complex than in wild plants. This makes the species concept more difficult to define and apply in crops. Hawkes (1997) argued that as natural and artificial selection both evoke essentially the same evolutionary pressures, systems of classification developed for wild plant groups should also be applied to cultivated taxa. Plants were on Earth for millions of years before animals. They exist on land, in oceans, and in fresh water. There are over 275,000 species of plants ranging from tiny algae that can live below the surface of the snow to a sequoia that may weigh as much as 2,500 tons (Fact Monster, online reference; Missmaggie, online reference). There are three features that distinguish plants from animals: (1) they are fixed in one place (they don’t move), (2) plants have chlorophyll, a green pigment necessary for photosynthesis and (3) their cell walls are made sturdy by a material called cellulose. Plants can be endangered too. Invasive plant species are those that do not naturally belong to an area. The vast majority of plant species belongs to flowering plants category, the largest group of the Plantae Kingdom (Ramon, 2015), which includes most trees, shrubs, vines, flowers, fruits, vegetables and legumes. Nearly all of the earth’s crop plants, orchard trees, garden plants and ornamentals are flower plants. Coffee, tea and cocoa beverages are made from flowering plants; cotton and linen are fabrics made from flowering plants. Many pharmaceuticals from aspirin to morphine come from metabolic chemicals in the flowering plants or are patterned after these chemicals. Plants in this category are also called angiosperms, Angiospermae or Magnoliophyta. The defining angiosperm feature is the enclosure of the ovules within surrounding tissue called an ovary. The ovary is part of a flower, a structure that occurs only in angiosperms. The ovary, and sometimes associated tissues, eventually forms a fruit, another unique angiosperm structure (plb.ucdavis, online reference). There are about 270,000 known angiosperm species alive 122

today. These angiosperms account for approximately 80% of all known living plants (Necker, 2015). Most angiosperm species are classified in a clade called core angiosperms. Phylogenetic data support dividing the core angiosperms into three subclades: magnoliids, monocots and eudicots (plb.ucdavis, online reference). According to Study (online reference), angiosperms have the following characteristics: (1) all angiosperms have flowers at some stage in their life. The flowers serve as the reproductive organs for the plant, providing them a means of exchanging genetic information; (2) they have small pollen grains that spread genetic information from flower to flower. These grains are much smaller than the gametophytes, or reproductive cells, used by nonflowering plants. This small size allows the process of fertilisation to occur quicker in the flowers of angiosperms and make them more efficient at reproducing; (3) all angiosperms have stamens. Stamens are the reproductive structures found in flowers that produce the pollen grains that carry the male genetic information; (4) angiosperms have much smaller female reproductive parts than nonflowering plants, allowing them to produce seeds more quickly, (5) angiosperms have carpels that enclose developing seeds that may turn into a fruit and (6) a great advantage for angiosperms is the production of endosperm. Endosperm is a material that forms after fertilisation and serves as a highly nutritional food source for the developing seed and seedling. In his review, Plb.ucdavis (online reference) discussed the mysterious origin of the angiosperms, the rise of angiosperms to dominance, novel features of the angiosperm life cycle and angiosperm geography and angiosperm diversity. In another review, Pullaiah (2015) discussed angiosperms origin and evolution, summarised angiosperm taxonomy and referred to their ancestors. He also presented some examples of primitive angiosperms. The flowering plants are the most diverse group of land plants, with 416 families, approximately 13,164 known genera and c. 295,383 known species. In 2011, Haider discussed the features of the chloroplast DNA (cpDNA, also called plastid DNA (pDNA)) that make it the most suited for the design of such primers. She then referred to all chloroplast-specific primers developed by that date LS - An International Journal of Life Sciences


and provided some examples of molecular studies and applications that made use of them. Haider and Wilkinson (2011) developed a set of plastid DNA-specific universal primers for flowering plants. They nominated introns of trnK and rpoC1 as potential supplementary barcode markers.

remains to be questioned by some, it is undeniable that it provides the most essential and correct definition of what a species is (Bioexplorer, 2016). I agree with Wilkins (2003) when he cited that ‘we still do not have the Ideal Species Concept. This is the best I can propose’. ACKNOWLEDGEMENTS




CONCLUSION De Queiroz (2005) considers species as one of the fundamental units of comparison in virtually all subfields of biology, from anatomy to behaviour, development, ecology, evolution, genetics, molecular biology, palaeontology, physiology and systematics. A species is often defined as a group of individuals which actually or potentially interbreed in nature. In this sense, a species is the biggest gene pool possible under natural conditions. A clear and practical definition of species is biologically and legally important. In 1996, Mayr stressed that any definition of the term ‘species’ must be based on careful study of the phenomenon of nature to which this term is applied. Many species concepts exist, each with its strengths and limitations. In classifying a group of organisms, grey areas necessarily exist during the early evolutionary diversification of a pair of species. Although the different species concepts may be equally able to recognise clear-cut species, each makes different decisions in these grey areas (Zoology, online reference). Ereshefsky (2010) cited that ‘the species problem is very much a philosophical question concerning the nature of reality: it all depends on whether we are willing to accept that a concept such as the species category “exists” as a “real” category in nature, irrespective of any human conceptualisation, or whether we consider it merely as an artefact of human thinking’. Aldhebiani (2017) believes that with this large number of concepts, it is not an easy or simple decision to adapt one. Generally, it depends on the criteria and the aim of each project. Therefore, more than one concept can be combined by using their data in numerical forms. The BSC, which is the most famous and widely accepted concept, is the most accepted and widely used concept that explains the concept of species. Although the BSC


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