How quickly can speciation occur?
Speciation can occur after a few decades or a few thousand years.
Let us first define speciation as being unable to produce fertile offsprings.
There are sveral things to take into account when talking about speciation: how a species reproduces and how long it takes to reproduce.
For the first matter, there is the important destinction between self-fertilization and fertilization by another organism. Self-fertilization means that the only way to become a new species is to have a mutation. Bacteria are an example, and so are some types of plants. This wouls lead to faster speciation as almost any organism could be considered a new spwcies. Other organisms require 2 to reproduce, which, from meisis adds genetic diversity to a population and spreads favorable traits (natural selection). This takes longer because usually the genetic pool remains constant.
Secondly, it matters how long reproduction takes. If the organism only needs to split into two cells, it would obviosly take a lot less time for a significant mutaton to occur that would result in a new species. Compare that with elephants, whose pregnancy aline is over a year, that would take a lot longer.
It must also be considered what time period is being talked about. For example, after catastrophic events the earth's bio-diversity increased. The new conditions required new organisms, and so in times like these speciation is also fatser, since the natural selection is NJ o longer stabilizing.