(i) Population of different species are not capable of breeding with each other.
(ii) Intraspecific competition is more intense than interspecific competition.
(iii) Parasites tend to loose certain organs.
(i) Because of various types of biological (e.g. breeding behaviour, inability to form fertile hybrids) and physical barriers (e.g. habitat and geographical distribution).
(ii) Because the requirements of individuals of the same species are very similar, hence they compete more intensely.
(iii) Because parasites obtain nourishment and other things from host, e.g. they have reduced ingesting and egesting organs.