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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.
R E TIN A - It is inverted. Innermost, sensitive layer. The part of ratina which is attached to choroid is pars optica. The part of retina attached to cilliary b
What are universal donors and universal recipients concerning the ABO blood system? The Universals donors of the ABO blood type system are the individuals of the type O. The Ty
Explain about the Oncogenic Viruses? Certain viruses that interfere with the functions of the regulatory genes have been identified. These viruses are called oncogenic viruses.
The parasite completes a part of its life histroy in man in two stages. They are : a) Pre-Erythrocytic cycle. b) Erythrocytic cycle. 1. When an infected female Anopheles mosquit
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In the eukaryotes, eukaryotic release factor eRF-1 recognizes all three termination codons that are UGA UAA, and UAG and, with the help of protein eRF-3, terminates translation.
Define Standardization - Nutritional Biochemistry? One of the substances involved in a titration must be used as a standard for which the amount of substance present is accura
Unconventional oilseed cakes/meal Deoiled sal (Shorea robusta) seed meal: It is a byproduct obtained after the extraction of oil from sal seeds. It is found to be high in NF
Importance of Dark Period For quite sometime the role of light period (photoperiod) was emphasised in flowering? However, based on certain experiments it was realised that it
Summary of the Phylum and protozoa and the important
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