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| Species |
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| The sum of all the populations of same kind of organisms all over the world is called species. |
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| Generally, a species is often defined as a group of organisms similar in structure, function and behaviours. |
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| But there are numerous examples where the members of the same species have considerable differences in structure, function and behaviour. Some of them are: |
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| i) Differences in developmental stages |
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| ii) Sexual dimorphism |
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| iii) Polymorphism. |
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| 1) Differences in developmental stages |
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| These are many organisms which appear different in their developmental stages. |
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| For e.g., |
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| a) Frogs and Salamanders The adult and larval stages of frogs, toads (tadpole) and salamander (axolotyl) do not resemble in structure and function. |
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| Tadpole larva of frog is a fish like larva found in water. It shows swimming type locomotion with the help of fins, tail; respiration with gills and is herbivorous. |
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| On the other hand, adult frog shows leaping type of locomotion; cutaneous and pulmonary respiration and is carnivorous. |
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| Differences between Tadpole larva and Adult frog |
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| Similarly, caterpillar larva of moth and adult butterfly, wriggler larva of mosquitoe and adult mosquitoe, grub of beetles and a completely grown beetle do not resemble one another respectively. |
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| Caterpillar larvae - butterfly |
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| Wriggler larvae - Mosquitoe |
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| Grub of beetle |
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| 2) Sexual dimorphism |
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| It is the occurrence of two sexual forms of an organism (male and female), of the same species, differ from each other in their size, colour, structure, function and behaviour. |
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| Example: Peacock is bright coloured and has a crest on the head; a long, ocellated, colourful and attractive tail feathers, whereas Pea hen is dull coloured with small, not so attractive, tail feathers and is without crest. |
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| Peacock
Peahen |
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| Similar dimorphism is found in, |
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| a) Lion (with mane) and lioness. |
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| b) Ascaris male is smaller with its posterior curved end and Ascaris- female is more well developed and straight. |
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| c) Cock with bright plumage, long tail feathers and large wattles and hen without these features. |
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| d) Sexual dimorphism is quite evident in human beings also |
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| Sexual dimorphism is clearly seen in few plant species also. |
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| For e.g., |
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| a) Male and female plants of date palm (Phoenix dactylifera) |
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| b) Papaya (Carica papaya), |
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| Dactylifera Carica papaya |
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| 3) Polymorphism |
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| Polymorphism is the phenomenon of occurrence of more than two forms of an individual within the same kind of organism. A colony is formed of more than two types of individuals, which differ in their structure, function and behaviour. |
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| Colonialism is found in insects and coelenterates. |
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| Polymorphism increases the chances of survival and adaptation, and helps in division of labour. |
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| Polymorphism exhibits a high degree of functional interdependence. |
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| For e.g., |
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| a) Apis Indica |
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| The honeybee is a social insect and its colony consists of three castes drones, workers and queen. |
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| Drones are fertile males with stout body and large sized eyes. They copulate with queen. |
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| Queen is the fertile female with long and tapering abdomen. Its only main function is egg laying. |
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| Workers are sterile females with reduced ovaries. They have small body but with powerful wings, mouth parts and pollen collecting apparatus. These are adapted to prepare and repair the comb, to collect and store food, cooling of the hive and defence. |
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| b) Similarly, termites or white ants (Microtreme) have five castes in its colony queen, drones, workers, soldiers, and nasutes. |
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| c) Among coelenterates, the sea fur (Obelia) has three types of individuals referred as zooids - polyp, medusa and blastostyles. |
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| d) A colonial coelenterate Physalia (Portuguese-man of war) has hundreds of zooids which show polymorphism like: |
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| a) dactylozooids for defence |
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| b) gastrozooids for feeding |
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| c) gonodendra for reproduction (budding) and |
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| d) pineumatophore for floating. |
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| 4) Varieties |
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| Certain species of animals and also plants have individuals of different varieties, which show distinct features. For e.g., |
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| a) There are several varieties of dogs such as Eskimo, Terrier, Grey hound, Bull dog etc, which show differences in colour, size and shape (also behaviour). |
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| Though these varieties of a species differ in several characters, members of different varieties of the same species interbreed among themselves. |
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| Variations within a Species (Different breeds of dogs) |
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| All the varieties or races of the same kind of organisms interbreed freely among themselves. However, populations or groups belonging to different species may appear quite similar but they do not interbreed in nature. |
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| Thus the definition for species can be concluded as, a group of closely related, structurally and functionally similar organisms which interbreed with one another in nature, but not with organisms of other species. |
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