  Context: The evolution of solar active regions is still not fully understood.
The growth and decay of active regions have mostly been studied in case-by-case
studies.
  Aims: Instead of studying the evolution of active regions case by case, we
performed a large-scale statistical study to find indications for the
statistically most frequent scenario.
  Methods: We studied a large sample of active regions recorded by the
Helioseismic and Magnetic Imager instrument. The sample was split into two
groups: forming (367 members) and decaying (679 members) active regions. We
tracked individual dark features (i.e. those that are assumed to be intensity
counterparts of magnetised fragments from small objects to proper sunspots) and
followed their evolution. We investigated the statistically most often
locations of fragment merging and splitting as well as their properties.
  Results: Our results confirm that statistically, sunspots form by merging
events of smaller fragments. The coalescence process is driven by turbulent
diffusion in a process similar to random-walk, where supergranular flows seem
to play an important role. The number of appearing fragments does not seem to
significantly correlate with the number of sunspots formed. The formation seems
to be consistent with the magnetic field accumulation. Statistically, the
merging occurs most often between a large and a much smaller object. The decay
of the active region seems to take place preferably by a process similar to the
erosion.
