Reproductive isolation takes place when the given group of species are not allowed to mate with each other due to physical barrier, and speciation is said to occur when a single species divide into group and become a two or more separate species, which can no longer interbreed with each other.Reproductive isolation is required for speciation to occur because if it do not occur the member of the same species continue to interbreed with each other, and because of this mating there are chances that any beneficial mutation may occur, which will cause evolution in the population.

Yes because a species is a group of population which reproductly isolated from the rest of the poulation. The origin of species from pre-existing one leads to speciation of species. If nature allow that species to survive then they can reproduce to give rise to new characters which is known as variation.

Natural selection is the phenomenon by which the nature selects those species which possess survival advantage over the other species. … Thus, we can say that both natural selection and speciation lead to the evolution of species on earth.

Hence both natural selection and speciation leads to evolution of an organism.
Yes because a species is a group of population which reproductly isolated from the rest of the poulation. The origin of species from pre-existing one leads to speciation of species. If nature allow that species to survive then they can reproduce to give rise to new characters which is known as variation.