Question

geographical and reproductive isolation of organisms gradually leads to speciation

Answer 1

because it's leads to eutrophication in marine animals

Answer 2

Answer:

Speciation occurs when a single species become a two or more separate species, these species are not allowed to interbreed with each other. If they interbreed with each other then variation take place and speciation will not occur.

Reproductive isolation means the given group can not mate with each other, often because of physical barrier.

Reproductive isolation is required for speciation to occur because without it the member of species continue to mate with each other and because of it any beneficial mutation may occur which will spread throughout the group.

Speciation occur when groups are separated long enough, so that natural selection cause them to evolve differently. So, for speciation  it is required to have reproductive without it speciation can not occur.