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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.
Do beings of the class Reptilia perform gas exchange in the same way amphibians do? These beings do not have permeable skin so they do not make cutaneous respiration such as a
Explain the term Phytates? We are familiar with phytates as an inhibitor of mineral absorption (calcium, iron etc.) especially in the vegetarian diets that are cereal-based. Th
Unlike DNA binding domains and dimerization domains, no general structural motifs have since been identified in the activation domains of diverse transcription factors.
Nursing Education Computer-Assisted Instruction (CAI) is a valuable teaching aid for health care students, testing them with frequent quizzes to ensure that the material rea
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How is litmus paper and testing for acid, base, and neutral, related to biology?
The various organs involved in the digestion process are: Alimentary tract which includes mouth, pharynx, esophagus stomach, small intestine, large intestine, appendix, rec
Q. Objective of nutritional management of hypertension? The objective of nutritional management of hypertension includes: • To achieve gradual weight loss in overweight and
Q. Describe the process of Rodent control? Rats and mice are destructive and cause huge loss of stored food commodities. They transmit pathogenic bacteria. Rats and mice are ge
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