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in this section we shall discuss a classic example of natulal selection in action in the preceding unit it was stated that natural selection always
it is generally believed that genetic drift occurs as a result of sampling error as we said earlier it occurs in small populations such as peripheral
hybrid sterility can be regarded as yet another form of interspecific sterility the offspring of the interspecific crosses are mainly sterile
in interspecific sterility the failure in mating occurs because of inability of the sperm to reach the egg in animals and the pollen to reach ovules
the second category of isolating mechanism that may permit interspecific mating but ensures reduced viability is called post-mating isolating
in certain instances differences in the morphology of genitalia between species make it impossible for normal mating to occur and sympatric species
members belonging to different species refrain from mating because of the behavioural differences between them such behavioural differences usually
ecological isolation is based on the fact that population shows preference to one habitat over the other this extensive forests become barriers to
we mentioned earlier that pre-mating mechanisms are concerned with the prevention of occurrence of interspecific crosses in such casesa the potential
in this section we are to discuss the means by which populations get separated or isolated from each other first gain the status ofsub-species and
an island may be colonised by just a few individuals or just a pair or even a single gravid female when a new population develops from these early
according to the current concept of speciation a widely distributed species should break up into partially isolated rule species the different
peripatric speciationspeciation by small populations isolated on the periphery of the distribution of the parent population can be described as
in almost all animal groups the mode of speciation is geographic speciation it is essential that a population which is a prospective new species is
sympatric speciation can be regarded as speciation where parent species gives rise to a daughter species without the individuals of a species being
in the previous section we discussed the concept of species in detail and concluded that the concept of biological species and mayrs definition of
the biologicai species concept claims that species consist of natural populations and that species are real and objective they are not man-made
the nominalists deny the existence of platos types for them only individuals exist species are man-made artificial abstractions but naturalists the
the typological species concept was suggested by plato more than 2000 years ago according to this concept the immense variety in nature can be
many definitions of species have been offered but none of them proved to be satisfactory the definitions did not categorically provide the basis to
ernst mayr for whom species and species problem have been the main concern of research points out that speciation the multiplication of species that
in the above sections we discussed in detail the fossil record of primates in general and more particularly those of apes and the humans despite the
subsequently many such fossils were known from france italy and middle east all such fossils exhibited reduced brow ridges steep forehead high
homo sapiens first appeared in the fossil record between 200000 to 300000 years ago the fossils were the swanscomb man from england and steinheim man
tools of lower paieolithic agepleistocene times there were huge herds of very large mammals bones of such large mammals were found associated with