Each organism is different from other organisms. The total number of species living on the planet is imprecisely known and published estimates vary from 12 million to over 100 million. This is because there are large numbers of species yet undiscovered and undescribed, and there is not always agreement on whether certain organisms should be regarded as separate species. The total number of known species including all animals, plants and micro-organisms is about 1.4 million, and over half of these are insects! Taxonomists have fairly complete records for the best known groups (e.g. birds with 9, 881 species world-wide). It is now also reasonably clear where the main gaps in our knowledge are, and intensive sampling of species-rich groups (e.g. insects) and species-rich areas (e.g. moist tropical forests) is now taking place to provide a more reliable picture of global and regional species richness, and a stronger basis for estimating the number of species.
Origin of species: The diversity in our planet is attributed to diversity within a species. As the world changed in climate and in geography as time passed, the characteristics of species diverged so much that new species were formed. This process, by which new species evolve, was first described by British naturalist Charles Darwin as natural selection. For an organism to change, genetic mutations must occur. At times, genetic mutations are accidental, as in the case of prokaryotes when they undergo asexual reproduction. For most eukaryotes, genetic mutations occur through sexual reproduction, where meiosis produces haploid gametes from the original parent cells. The fusion of these haploid gametes into a diploid zygote results in genetic variation in each generation. Over time, with enough arrangement of genes and traits, new species are produced. Sexual reproduction creates an immense potential of genetic variety
In order to study the enormous diversity of organisms they need to be organised into manageable groups. This grouping of organisms is known as classification and the study of biological classification is called taxonomy. Ancient Greek thinker Aristotle classified living beings on the basis of their habitat. He classified them into two groups, i.e. those living in water and those living on land. But his classification was too simple to justify inclusion of a particular organism into a particular group.
But most scientific and complete classification of organisms proposed by the Swedish naturalist Carl Linnaeus who gave each organism a two-part scientific name – a genus name and the species name .
Linneaeus developed a hierarchy of groups for taxonomy. To distinguish different levels of similarity, each classifying group, called taxon is subdivided into other groups. To remember the order, it is helpful to use a mnemonic device. The taxa in hierarchical order is as follows:
Domain – Archea, Eubacteria, Eukaryote
Kingdom – Plantae, Animalia, Fungi, Protists, Eubacteria (Monera), Archaebacteria
Phylum: In biology, a phylum is a level of classification or taxonomic rank below Kingdom and above Class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature for algae, fungi, and plants accepts the terms as equivalent.
Class: The class as a distinct rank of biological classification having its own distinctive name (and not just called a top-level genus (genus summum) was first introduced by the French botanist Joseph Pitton de Tournefort in his classification of plants that appeared in his Eléments de botanique, 1694.
Order: In biological classification, the order is. a taxonomic rank used in the classification of organisms and recognized by the nomenclature codes. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family.
Family: In taxonomy, a family is more precise than orders but less precise than genera. Organisms belonging to the same family would have evolved from the same ancestors and share relatively common characteristics
Genus: A genus is a taxonomic rank used in the biological classification of living and fossil organisms in biology. In the hierarchy of biological classification, genus comes above species and below family.
Species: A group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding. The species is the principal natural taxonomic unit, ranking below a genus and denoted by a Latin binomial, e.g. Homo sapiens.
The Three Domains
The three domains are organized based on the difference between eukaryotes and prokaryotes.
Archea (Archeabacteria): Archeabacteria consists of archeabacteria, bacteria which live in extreme environments. The kingdom Archaea belongs to this domain.
Eubacteria : Eubacteria consists of more typical bacteria found in everyday life. The kingdom Eubacteria belongs to this domain.
Eukaryote: Eukaryotes encompasses most of the world’s visible living things. The kingdoms Protists, Fungi, Plantae, and Animalia fall under this category.
Kingdoms
There are six kingdoms in the taxonomy:
Plantae: Any of various photosynthetic, eukaryotic, multicellular organisms of the kingdom Plantae characteristically containing chloroplasts, having cell walls made of cellulose, producing embryos, and lacking the power of locomotion. Plants include trees, bushes, herbs, ferns, mosses, and certain green algae.
Animalia: The major group of animals are classified under the Kingdom Animalia, also known as Metazoa. This kingdom does not contain prokaryotes. All the members of this kingdom are multicellular, eukaryotes. They are heterotrophs, they depend on other organisms directly or indirectly for food. Most of the animals ingest food and digest in the internal cavity. Most of the organisms are motile which means they can move independently and spontaneously. There are around 9 to 10 million species of animals, and about 800,000 species are identified. Fossil records of animals were found in the era of the Cambrian explosion, about 540 million years ago. Animals are divided into various sub-groups, biologists have identified about 36 phyla within the animal kingdom including birds, mammals, reptiles, fish, amphibians etc.
Protists: the third kingdom, was introduced by the German biologist Ernst Haeckel in 1866 to classify micro-organisms which are neither animals nor plants. Since protists are quite irregular, this kingdom is the least understood and the genetic similarities between organisms in this kingdom are largely unknown. For example, some protists can exhibit properties of both animals and plants.
Fungi: fungi are any of about 99,000 known species of organisms of the kingdom Fungi, which includes the yeasts, rusts, smuts, mildews, molds, and mushrooms. There are also many funguslike organisms, including slime molds and oomycetes (water molds), that do not belong to kingdom Fungi but are often called fungi. Many of these funguslike organisms are included in the kingdom Chromista. Fungi are among the most widely distributed organisms on Earth and are of great environmental and medical importance. Many fungi are free-living in soil or water; others form parasitic or symbiotic relationships with plants or animals.
Eubacteria: eubacteria are prokaryotic (lacking nucleus) cells that are very common in human daily life, encounter many more times than the archaebacteria. Eubacteria can be found almost everywhere and kill thousands upon thousands of people each year, but also serve as antibiotics producers and food digesters in our stomachs. We use Eubacteria to produce drugs, wine, and cheese. Eubacteria are also known as true bacteria.
Archae (or Archaebacteria): Archaebacteria are bacteria which live in extreme environments, such as salt lakes or hot, acidic springs. These bacteria are in their own category as detailed studies have shown that they have unique properties and features (ex. unusual lipids that are not found in any other organism)which differ them from other bacteria and which allow them to live where they live. Their cell walls lack peptidoglycan.
Phylum
The living world is very diverse. There are millions of different organisms that have been discovered, and it is estimated there are millions more yet to be found. If science did not have some kind of organized way of keeping track of all these critters, it would be very difficult to conduct any research, and information about particular creatures would never pass around the scientific community.
Different animal phyla are as follows:
Protozoa: Unicellular Animals like Amoeba, Paramoecium, Monogystis and Malaria parasite. Protozoa are microscopic in size. Each individual consists of only one cell which has to carry on all the vital activities. They are abundantly found in water containing decaying organic matter. Some, such as the dysentery amoeba and the malaria parasite, live within other animals. Still others live in damp soil, or in fresh water, or in the sea.
Porifera: These are pore-bearing sedentary animals found mostly in the sea. A few species occur in the fresh water but none on the land. The sponges, like plants, are attached to a substratum. The outer surface of the sponge is perforated by numerous pores and the body wall is supported by a framework which is composed of lime, or of silica or of an organic substance called sponging.
Cnidaria: Most of the cnidaria are marine but Hydra is found in fresh water. Some, such as the corals and sea-anemones, are attached to a substratum; others are slow moving or adapted for drifting in the water. All are radially symmetrical. This means that the animal is the same all round, and has no right or left side. It is symmetrical around a median vertical axis, and can be divided into similar halves by a number of vertical planes.
Ctenophora: Ctenophores are all marine. They have bi-radially symmetrical bodies. They possess eight meridionally placed ciliated plates. They resemble the cnidarians on many counts but differ from them in not having the nematocysts. Their ectomesoderm is gelatinous and bear mesenchymal muscle cells. They possess a specialised aboral sense organ and the tentacles bear adhesive cells. All are planktonic.
Platyhelminthes : These are flat, un-segmented, worm-like creatures with soft and bilaterally symmetrical body. In a bilaterally symmetrical animal there is a right side and a left side, a fore end and a hind end, a dorsal or back surface and a ventral or front surface. There is only one plane of symmetry by which the body can be divided into two equal halves.
Nemathelmlnthes : These are cylindrical, un-segmented, worm-like animals with soft, bilaterally symmetrical body, tapering at both the ends. Digestive canal is complete, with two openings, a mouth in front and an anus behind; it is a straight tube running through the body from end to end. Most of the group are aquatic. A few inhabits damp soil. Others, such as hook-worms, thread-worms and filaria worms are parasites of man and cattle.
Annelida: These are true worms with soft, elongated, bilaterally symmetrical body, divided into a series of ring-like segments or meta- meres. The annelids are, therefore, known as the segmented worms. The annelidan body is built on the tube-within-a-tube plan.
Arthropoda: Arthropods are bilaterally symmetrical, segmented animals with soft parts of the body protected by a hard chitinous external skeleton. Each segment of the body bears paired legs or appendages which are jointed. This phylum is the largest of the animal phyla and includes nearly three-fourths of all the known species of animals.
Mollusca: Molluscs are un-segmented and without appendages. The soft parts of the body are enclosed in a Hard calcareous shell, as in snails and oysters. A fleshy muscular foot for locomotion is often present. Many of the molluscs are marine, some are fresh-water, and a few like the garden snails are terrestrial.
Echinodermata: Echinoderms are characterised by spiny skin. All are marine, inhabiting the shore and bottom of the sea. A few such as the sea-lilies are attached; but the majority are free to move about. Locomotion is very sluggish and effected by peculiar structures called tube-feet. This is the only phylum possessing a water- vascular system. The body is radially symmetrical and star-like as in starfishes, brittle-stars and basket-stars.
Chordata: The chordates possess a stiff supporting rod, called notochord. Leaving aside a few lower forms, such as balanoglossus, ascidians and amphioxus, all chordates are vertebrates. Vertebrates possess the backbone which forms the supporting skeleton for the long axis of the body.
Plant kingdoms
The plant kingdom has been divided into five subgroups. They are as follows:
Thallophyta: All the plants that lack a well-differentiated body structure belong to the subgroup Thallophyta. Thallophytes are commonly known as algae. The majority of them are aquatic. Some examples are Spirogyra, Chara, Ulothrix, etc.
Bryophyta: Bryophytes have differentiated plant body like stem, leaf structures. But they lack a vascular system for the transportation of substances across the plant body. Bryophytes are found in both land and aquatic habitats, hence are known as amphibians of the plant kingdom. Mosses and Marchantia belong to this subgroup.
Pteridophyta: Pteridophytes have well-differentiated structures such as stem, root, leaves as well as the vascular system. Ferns, horse-tails, Marsilea are some common examples of Pteridophytes.
Gymnosperms: Gymnosperms are plants that have well-differentiated plant body, vascular system and they bear seeds. The term is derived from Greek words, gymno: naked and sperma: seed. The seeds of gymnosperms are naked which means they are not enclosed within a fruit. The perennial, evergreen woody trees belong to this group. Pines, deodar, redwood, etc. are few examples.
Angiosperms: Angiosperms are also seed-bearing plants with well-differentiated plant body. The word is derived from Greek words: angio: covered and sperma: seed. Unlike gymnosperms, seeds of
angiosperms are enclosed inside the fruits. Angiosperms are commonly known as flowering plants. Examples: Mango tree, pomegranate plant, etc. Seeds germinate from embryonic leaves called cotyledons. Depending on the number of cotyledons present in seeds, angiosperms are divided into two: monocotyledons/monocots and dicotyledons/dicots.
Classification of plant families: plants are divided into families. Some important families and their characteristics are follows:
Brassicaceae:
- Leaves are often deeply lobed and pinnatifid.
- Flowers are actinomorphic and without subtending bracts (ebracteate).
- Perianth is of 4 sepals and 4 petals in a cruciform (cross-like) arrangement.
- Petals are often clawed.
Oxalidaceae:
- Oxalis or Wood-Sorrel Family Perennial or annual herbs, often with acrid juice.
- Leaves are often palmately compound; mostly trifoliate.
- Androecium of 10 stamens, often in 2 whorls, the outer whorl shorter than the inner whorl (sometimes reduced to staminodes).
Malvaceae:
- Mallow Family Plant body with stellate hairs (star-shaped).
- Flowers often are subtended by an epicalyx of distinct or connate bracts.
- Androecium has numerous stamens with its filaments grouped into a staminal column, called a monodelphous condition.
Geraniaceae (Geranium Family):
- Flowers are actinomorphic with elongated receptacle.
- All plants are annuals in the Mojave Desert.
- Perianth of 5 sepals and petals. Fruit is a capsule splitting into 5 segments with a style attached to each.
Zygophyllaceae:
- Caltrop Family Leaves mostly opposite, pinnately compound with 2 or 3 or more leaflets.
- Stipules present, sometimes spiny. Compound pistil composed of 5 carpels.
Plantaginaceae:
- Plantain Family Leaves are basal with parallel venation.
- Flowers are small, in a tight cluster at the tip of a leafless stalk (scapose).
- Corolla is papery-like, with 4 fused petals, lobes flaring.
- Fruit is a pyxis, a capsule that opens by a lid to release seeds.
Hydrophyllaceae (Waterleaf Family):
- Plant generally with erect, bristly hairs, sometimes with glandular hairs.
- The gynoecium has either 2 styles, or 1 forked or bifid style.
- Fruit is a capsule.
- Major genera in southern Nevada includes Phacelia, Nama, and Eriodictyon (yerba santa). Inflorescence a coiled cyme, resembling a scorpion.
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