TRUE
Speciation is the creation of a new species through natural selection operating differently on 2 populations, NORMALLY due to a NEW NATURAL BARRIER (e.g. a new moutnain range or river) emerging that separates the 2 populations. The environmental conditions are different within these 2 new habitats and so natural selection operates differently for the 2 populations, favouring different characteristics to aid surivial. The new natural barrier prevents the 2 populations interbreeding. As a result, new characteristics are not genetically passed on between the 2 populations. Eventually, the 2 populations will thus become so different in their characteristics (i.e. their phenotype) that they will no longer be able to successfully interbreed/ reproduce to produce fertile offspring.
The definition of a species is: Organisms with similar characteristics that can successfully interbreed/ reproduce with each other to produce fertile offspring.
After speciation, a new species is created. Thus, by the very definition of a species, members of a population must become reproductively isolated in order for speciation to occur.