SELF INSTRUCTIONAL MATERIAL
Paper III – Taxonomy and Diversity of Seed Plants
Unit – III & IV
Block – II
Madhya Pradesh Bhoj (Open) University
P - 03
BLOCK - II
TAXONOMY AND DIVERSITY OF SEED PLANTS
Taxonomy of Angiosperms
Unit – III Origin of Intra-population variation
The Species concept
Unit – IV Taxonomic Evidences & Taxonomic Tools
Editor - Dr. (Smt.) Renu Mishra,
HOD, Botany and Microbiology
Sri Sathya Sai College for Women,
Writer:- Dr. Puja Pareek
Asstt. Professor, Botany
UNIT-III TAXONOMY OF ANGIOSPERMS
3.2 Population and environment
3.5 Evolution of species:-
3.5.1 Types of variation
3.5.2 Isolating mechanisms
3.6 Differentiation of species:-
3.7 Species Concept: -
3.7.1 Ideal species
3.7.2 Nominalistic species
3.7.3 Taxonomic Concept
3.7.4 Biological Concept
3.7.5 Evolutionary Species Concept
3.8 Taxonomic groups, categories and rank.
3.9 Hierarchical Classifications.
3.10 Salient features of ICBN
3.11 Let Us Sum Up
Taxonomy, one of the oldest fields of biological science, is "the theory and practice of classification". Some of them are minute floating herbs with a very simple construction, while others are gigantic trees attaining a height of about 100 meters and with very complex structure. A conservative estimate of the number of described species of plants in the world is over 400,000. Of these, about a quarter of a million, species belong to the flowering plants. Man has been absolutely dependent on plants even in the early stages of civilization. The plants furnish most of our food, shelter, clothing, fuel, medicine, paper and a host of other useful products; and furthermore the green plants are the greatest means of utilizing energy from the sun. The primitive man thus tried to group the plants according to then' economic uses.
The primary aims of classics taxonomy were to describe and name all known species of plants.
3.2 POPULATION AND ENVIRONMENT
The population of a species generally arises as a result of reproduction, active transport of individuals, or their passive transport by such agencies as wind, water etc. All the three means of population growth are influenced by several factors of the environment as well as by the characteristics of the individuals of the species itself. Under favorable conditions, the group of individuals increases in numbers. However, environment is never static and keeps on changing from time to time i.e. is dynamic. Thus, environment acts as a natural check on population. An increase in the number of individuals of a species results in the consequences of concern to the species itself and also to other interdependent species growing in the area. Such as increase brings about harmful as well as beneficial effects (interactions) on the species. The former type of effects is mainly due to increased competition, particularly intraspecific for nutrition,space etc. The beneficial effects include the protection, influences on reproduction, and origin of division of labor.
Plants in one way or the other keep themselves adjusted (adapted) in their existing environmental conditions. An account of three ecological groups, hydrophytes, mesophytes and xerophytes, growing in their respective habitats could also make it evident that plants of each of these groups keep themselves adapted to those conditions, somehow or the other, by possessing a number of morphological, anatomical, and physiological characteristics. Thus, in such cases, we find that different species of plants belonging to widely different taxonomic genera, however, belong to the same ecological group as they possess so many features in common.
Still, there is another situation, where adaptation is a bit different one. Here the same species has an ability to grow in different types of habitat. This wide range of distribution in a single species is achieved by development of a number of distinct forms in it, differing in their growth habit etc. Thus in order to meet the challenge of different types of habitat, there develop within the same taxonomic species, distrinct populations that may be genetically similar or dissimilar we should explain briefly how a plant species reacts with its environment. Indeed a species can be pictured as a crystal of solute immersed in the solvent of its biological environment The species reacts with the environment through its genetic make up a protoplasm. If the solvent is suitable the species is dispersed homogeneously and widely or otherwise precipitates and is discarded. The demands of the solute (Species) are according to its genetic and physiological characteristics. The environment in which a species grow is not a simple physical system but a biological complex. With this complex a species is intimately connected making an 'interrelated whole', according to the ecosystem concept of Tansley (1935). But a species is somewhat different from a physical solute, as on its part it also adapts itself to various degrees in order to grow successfully in the existing environmental complex. That is, unlike the physical solute, species are discarded only rarely. The adaptation may be purely physiological or any morphological characteristics may change, or both. According to Daubenmire (1959), such features may ensure a degree of success either by allowing the plants to make especially full use of the amount of nutrients, water, heat and light etc. to it, or by bestowing a significant amount of protection against some adverse factors. All such features are known as adaptation.
These are also known as ecophene, habitat forms or environmentally induced variations. Different ecads of the same species differ in appearance, with variations in size of vegetative parts, number of leaves, stems, flowers, and growth habit etc. But they belong to the same genetic stock i.e. they all are genetically similar. Indeed such differences in plants are simply induced (produced) by the influences of the environment (i.e. variations are environmentally induced). Thus the variations are not fixed, but temporary, somatic and reversible. If one type of ecad is transplanted into environment of another type of ecad, its differences would change into the latter type. Thus, if different ecads are transplanted in same habitat, all would become similar in appearance. Since then a number of ecads in different species have been reported. In India, Ramakrishan (1960) has shown that in Euphorbia hirta, there are present two ecads, one growing in dry, hard soils (prostrate type), and the other along the footpaths under trampling (prostrate compact type). In Bothriochloa pertusa and Dichanthium caricosum, it is shown by Pandeya (1962) that in each of these grasses, we find distrinct large variations in morphological characters under different habitats. In each of them, variations have been noted in habit form, number of culms, number of spikes per Culm, number of spikelet’s per spike, and total seed output. Such variations have been found to be largely governed by intensity of grazing and soil moisture. In each species, there are found two types of ecads: (i) those under protection show basket form habit, where as (ii) those in overgrazed areas acquire a saucer shape. Grazing brings about reduction in size of erect stem, number of spikes per raceme, number of spikelet’s per spike, and in the length and breadth of lower glumes of spikelet’s. Besides, some physiological variations also take place, which include the development of red pigment, and initiation of early flowering.
These are also called ecological or physiological races. According to Turresson (1922), an ecotype is 'the product arising as a result of genotypical response of an acospecies to a particular habitat'. The plant in this case is genetically distinct, but as they are interfertile, they are put into one taxonomic species. The different ecotypes of a particular species may differ in their edaphic, biotic, or microclimatic requirements. Thus in ecotypes, adaptations become irreversible or genetically fixed variations. These arise only by changes in the gene structure within the chromosomes, recombination of genes through hybridization or irregularities during mitosis, or meiosis that is by chance. A species may have a mosaic of populations, each of which is distinguishable in genetically based (Ecades where these are environmentally based) physiological, and sometimes morphological features. Thus, unlike ecads, if different ecotypes are grown in identical habitat, their differences (variations) will not change, as they are genetically, and not simply environmentally, based. In India, Misra and Rao (1948) for the first time reported ecotypic differentiation in Lindenbergia polyantha. Since then, a number of workers have reported ecotypes in several species of plant, as for example in Euphorbia hirta, E. thymifolia, Cassia tora, Xanthium strumarium etc. In Euphorbia hirta (Ramakrishan, 1958, 1960, 1961), there are found two ecotypes in nature, one of which in turn has two types of ecads. These are as follows:
Ecotype 1. Erect type, growing in moist localities.
Ecotype 2. Prostrate type. It has two Ecads:
i. Ecad 1. Prostrate type, growing in dry, hard soil.
ii. Ecads 2. Prostrate compact type, derived from ecad 1, under trampling on footpaths.
The development of ecads and ecotypes in one species shows its capacity of distribution to wider areas. By developing such distinct populations the species adapts itself to the changing, new types of conditions. The response to the changing environment by the species is met with the development of variation, that may be temporary i.e. environmentally based (ecads) or genetically fixed (ecotypes). This very much depends on the degree of plasticity possessed by the responding species.
3.5 EVOLUTION OF SPECIES
It is now a universally agreed upon fact that different species are not fixed entities but systems of populations which exhibit variation and wherein no two individuals are identical. This concept of variations was first proposed by Lamarck and further developed by Darwin, culminating in his famous book Origin of Species (1859). Systematics is a unique natural science concerned with the study of individual, population and taxon relationships for purposes of classification. The study of plant systematics is based on the premise that in the tremendous variation in the plant world, there exist conceptual discrete units (Usually named as species) that can be recognized, classified, described, and named, on the further premise that logical relationships developed through evaluation exist among these units.
3.5.1 Types of variation
The recognition of taxonomic units is based on the identification of the occurrence and the degree of discontinuity in variation in the populations. The variation may be continuous when the individuals of a population are separable by infinitely small differences in any of the attributes. In a discontinuous variation, however, there is a distinct gap between two populations, each showing its own continuous variation for a particular attribute. The discontinuity between the populations primarily results from isolation in nature. Isolation plays a major role in establishing and widening the gap between the populations, allowing evolution to take its destined course with no disturbance. Variation in plants includes three fundamental types: developmental, environmental and genetic.
A distinct change in attributes is often found during different stages of development. Juvenile leaves of Eucalyptus, Salix and Populus are often different from the mature leaves, and May often causes much confusion, but may prove equally useful when both types of leaves are available from a plant. The first leaves of Phaseolus are opposite and simple, the later ones alternate and pinnately compound. As the seedling stage is most critical in a plant's life, the characters present during this period surely have survival value. Takhtajan proposed a neotenous origin for angiosperms on the assumption of juvenile simple leaves of seed ferns having persisted in the adult forms, which were the direct progenitors of angiosperms.
Environmental factors often play major role in shaping the appearance of a plant. Heterophylly is the common manifestation of environmental variation. The submerged leaves of Ranunuculus aquatilis are finely dissected, whereas the emergent leaves of the same plant are broadly lobed. The first submerged leaves of Sium suave are pinnately dissected and flaccid; the older emerged leaves are pinnately compound and stiff. The individuals of a species often exhibit phenotypic plasticity, expressing different phenotypes under different environmental conditions. Such populations are named ecophenes. In Epilobium; the sun-plants have small, thick leaves, many hairs and a short stature, whereas the shade-plants have larger thinner leaves with fewer hairs and a taller stature.
Genetic variation may result from mutation or recombination. Mutation is the occurrence of heritable change in the genotype of an organism that was not inherited from its ancestors. It is the ultimate source of variation in a species and replenishes the supply of genetic variability. A mutation may be as minute as the substitution of a single nucleotide pair in the DNA (point mutation) or as great as a major change in the chromosome structure (chromosomal mutation). Chromosomal mutation may be due to deletion, inversion, aneuploidy or polyploidy. Recombination is a reassortment of chromosomes, bringing together via meiosis and fertilization the genetic material from different parents and producing a new genotype.
3.5.2 Isolating mechanisms
Isolation is the key factor preventing intermixing of distinct species through prevention of hybridization. Based on whether isolating mechanisms operate before or after sexual fusion, two main types of mechanisms are distinguished: prezygotic mechanisms and postzygotic mechanisms. A detailed classification of isolating mechanisms is presented below.
Prezygotic mechanisms (operating before sexual fusion)
Geographical isolation: Two species are separated geographically by a gap larger than their pollen and seed dispersal. Platanus orientalis (Mediterranean region) and P. occidentalis (North America) are well established species but readily interbreed when brought into the same area (vicarious species).
Ecological isolation: Two species occupy the same general area but occupy different habitats. Silene alba grows on light soils in open places while S. dioica on heavy soils in shade. Their habitats rarely overlap, but when they do, hybrids are encountered.
Seasonal isolation: Two species occur in the same region but flower at different seasons. Sambucus racemosa and S. nigra flower nearly 7 weeks apart.
Temporal isolation: Two species flower during the same period but during different times of the day. Agrostis tenuis flowers in the afternoon, whereas A. stolonifera flowers in the morning.
Ethological isolation: Two species are interfertile but have different pollinators. Humming -birds for example, are attracted to red flowers and hawk-moths to white ones.
Mechanical isolation: Pollination between two related species is prevented by structural differences between flowers, as for example between Ophrys insectifera and O. apifera.
Gametophytic isolation: This is the commonest isolating mechanism wherein cross-pollination occurs but the pollen tube fails to germinate or if germinated, it can't reach and penetrate the embryo sac.
Gametic isolation: In such cases, reported in several crop plants, the pollen tube releases the male gametes into the embryo sac, but gametic and or endospermic fusion does not occur.
Postzygotic mechanisms (Operating after sexual fusion)
Seed incompatibility: The zygote or even immature embryo is formed but fails to develop and as such a mature seed is not formed. The phenomenon is commonly encountered in cross between Primula elatior and P. veris.
Hybrid inviability : Mature seed is formed and manages to germinate but the F1 hybrid dies before the flowering stage is reached. The phenomenon is commonly encountered in crosses between Papaver dubium and P. rhoeas.
F1 hybrid sterility: F1 hybrids are fully viable and reach flowering stage but flowers may abort or abortion may occur as late as F2 embryo formation, with the result that the F1 hybrid fails to produce viable seeds.
F2 hybrid inviability or sterility: F2 hybrid dies much before reaching the flowering stage or fails to produce seeds.
3.6 DIFFERENTIATION OF SPECIES
Speciation is a general term for a number of different processes which involve the production of new species. New species may develop through the mechanism of abrupt speciation or gradual speciaton. The phenomenon of abrupt speciation (example of sympatric speciation) is commonly met in genera such as Tragopogon and Senecio. The species are often well isolated and any chance hybridization fails to culminate into successful hybrids because of genomic differences. In some cases, however, hybridization may be accompanied by chromosome duplication resulting in the formation of allopolyploids. Such allopolyploids depict normal pairing at meiosis and thus represent well isolated, phenotypically as well as genotypically distinct species.
This is a more common phenomenon in nature. It may involve phyletic evolution when one species might evolve into something different from its ancestor over a period of time (Phyletic speciation). Alternatively, a population belonging to a single species might differentiate into two evolutionary lines through divergent evolution (additive speciation)
The concept of phyletic speciation has been the subject of considerable debate. It is the sequential production of species within a single evolutionary lineage. Species A might, over a period of time, change through species B and C into species D without ever splitting. The new species produced in this manner are variously called successional species, palaeospecies, an allophonic species. The species which have become extinct in the process are termed taxonomic extinctions. Wiley (1981), while agreeing with the concept of phyletic character transformation, rejects the concept of phyletic speciation on the grounds that:
Recognition of phyletic species is an arbitrary practice. Mayr(1942) argues that delimitation of species, which do not belong to the same time-scale, is difficult.
Arbitrary species result in arbitrary speciation mechanisms.
Phyletic speciation has never been satisfactorily demonstrated.
Additive speciation is the commonest mode of speciation, which adds to the diversity of living organisms. Mayr (1963) suggested the occurrence of reductive speciation, whereby two previously independent species fuse into a third, new species, themselves becoming extinct. Hybridization likewise produces new species but this always leads to an addition in the number of species.
It is impossible to image that two evolutionary species can actually fuse to produce a third species and they become extinct. This may happen in a particular region, but over the entire range of these species. The various modes of additive speciation are described below:
Allopatric speciation: Lineage independence and consequent speciation resole from geographical separation of lineages, i.e. the actual physical separation of two relatively large populations of a single species. Over a period of time, such separation would enable these geographical races to develop and maintain gene combinations controlling their morphological and physiological characters. The development of reproductive isolation would sooner or later result in the establishment of distinct species.
Allopatric speciation may also result from the development of new species along the boundaries of a large central population. These marginal populations (races) get separated from the main population during environmental differentiation. They then undergo adaptive radiations to develop physical and physiological differences, which sooner or later get genetically fixed (ecotypes). With further morphological and physiological differentiation, they form distinct varieties (or subspecies). Development of reproductive isolation establishes these as distinct species that will retain their identity even if a further chance should draw than together
Allo-parapatric speciation: Such speciation occurs when two populations of an ancestral species are separated, differentiate to a degree that is not sufficient for lineage independence, and then develop lineage independence during a period of preparatory (limited sympatry). It differs from allopatric speciation in the sense that speciation is complete after a period of sympatry and the process of attaining lineage independence is potentially reversible because it is possible that two partly differentiated populations could form a single evolutionary lineage showing clinal variation after they meet rather than the period of sympatry reinforcing differences between them.
Parapatric speciation: This occurs when two populations of an ancestral species differentiate despite the fact that no complete disjunction has occurred. The daughter species may share a small fraction of their respective ranges and interbreed within this narrow contact zone and yet still differentiate.
Stasipatric speciation: This is similar to parapatric speciation except that it results from spontaneous chromosomal modifications. The resultant chromosome arrangements must me fully viable in the homozygous state but of reduced viability in the heterozygous state.
Sympatric speciation: This results is the production of new species with no geographical separation of populations, even though most cases of sympatric speciation, such as those resulting from hybridization and apomixis, belong to abrupt speciation. The process of ecological sympatric speciation is a slow one of gradual speciation. The ecological differences in the habitats result in adaptive radiations in populations which gradually evolve into new species.
Check your progress-I
Note : a) Write your answer in the space given below
b) Compare your answer with those given at the end of the unit
Differentiate between Ecades and Ecotypes.
What is pre – zygotic isolating mechanisms.
Write short note on additive speciation.