Geographically separated sister species reproduce in isolation.
Natural and man-made barriers may arise rapidly.
For example: a population may be divided if a river breaks it's banks and takes a new
route, or a new road may separate a population.
Many generations may elapse.
Sister species are now so different - no interbreeding is possible.
Genetic variation occurring in same geographical location.
Reproductive isolation due to:
o Temporal mechanisms
o Behavioural mechanisms
Temporal (time) mechanisms may occur when two very closely related species occupy
the same habitat, differing only in the time of year that they complete their life cycles.
Reproductive isolation may develop within the population so that some members produce
gametes at distinctly different times of the year from others.
Thus, two distinctive gene pools start to evolve.
Behavioural mechanisms results when members of a population acquire distinctive
behaviour routines in their growth and development, courtship or mating process that are
not matched by all individuals of the same species.