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Many definitions of species have been offered, but none of them proved to be satisfactory. The definitions did not categorically provide the basis to decide whether two similar groups are distinct species or only sub-species. Again what is the criterion to decide the distinctness of a species? Some tried to specify the degree of difference that would distinguish a species from another. Apart from the difficulty in quantifying such differences, there are also other problems. Certain forms which show very little morphological differences proved to be very distinct species. In other cases, such as Homo sapiens, undoubtedly a single species, different races have pronounced differences. The discontinuity between groups, it appears, depends less on the degree of differences and more on the constancy of differences. Another approach to define species was to distinguish related organisms on their inability to breed. Many definitions of species have relied upon such interspecific sterility and sterility of offsprings, that is hybrid sterility. This proposition also has certain inherent problems. It is not easy to identify cases in which organisms do not interbreed, and those which can breed, but do not do so for certain reasons. At these times, sterility cannot be deemed to be an appiopriatc criterion for defining species. Nevertheless, reproductive isolation or inability to breed has been the commdn element in the definition of species provided by many evolutionary biologists, be it Dobzhansky, Goldschmidt or Ernst Mayr. Reproductive isolation, the discussion of which follows later in this unit, has been the basis for defining the species and speciation.' Such an isolating mechanism becomes a barrier for the flow of genes between related populations and the concept of biological species centres around this phenomenon.
What are the segments that form the body of the tapeworm called? What is their function? The body of the tapeworm is made of segments known as proglottids. The proglottids are
Question 1 List various methods used for determination of blood glucose. Explain the principle of each test. Add a note on advantages and disadvantages of each method Qu
what is a crabs excretory system?
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describ ethe genral characters of phylum colentrata
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Define the term - Cholera Cholera is caused by the gram negative, V. cholerae, which is acquired by ingesting food or water contaminated by faecal material from patients or
PO L YSACCHARIDES Polysaccharides are complex carbohydrates. Polysaccharides are also called glycans. These are insoluble in water. Polysaccharides are not swee
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