How does reducing gene flow cause speciation?

Answer 1

It allows divergence to occur

It allows populations to become genetically different.

Answer 2

It allows populations to become genetically different. APEX

Answer 3

A species is much like a crowd at a ball game, and genes can be thought of as being like particular cheers. Generally a crowd only has a few chants that "everyone" knows, and these will be repeated often. Unusual cheers (new mutations) tend to die out fast because they are unfamiliar, unless they "catch on", because they have some special advantage - making people laugh, or maybe annoying the other team or a bad ref. So the major cheers (common genes) circulate through the crowd regularly, and tend to drown out anything else - which is to say the crowd (the species) is going to sound (or look) pretty much the same indefinitely.

Reduction of gene flow can cause speciation however, because it changes the distribution of those genes - they can't be easily shared throughout the whole crowd (species). What is common on one side of the choke point can become uncommon on the other through random fluctuation, and then even disappear. Add up enough genetic differences, and what was once one group begins to separate into two (or more) distinct populations.

For example, a section of bleachers is closed for repairs separating people at one side of the stadium. This will inevitably make it hard for both sides to coordinate in a cheer, and so chants will tend to die out, rather than bridging the gap. Fairly soon, the two sides will find themselves cheering out of rhythm with each other, and then doing totally different cheers at the same time because both groups are mimicking the chants (or sharing genes with) their neighbors on the opposite sides. Eventually the crowd far away from the missing bleachers may find itself divided as people have to choose which neighboring cheer to join in, and once that split is made, they will likely choose the same side again and again - behavioral choices creating a barrier nearly as effective as the physical one.

Answer 4

Reducing gene flow between populations can lead to speciation by causing genetic differences to accumulate over time. These genetic differences can result in reproductive isolation, where individuals from different populations are no longer able to interbreed successfully. Over time, this can lead to the formation of new species as the isolated populations evolve separately.