the new species
evolve from contiguous
populations, but nonetheless, the population does not mate
Suppose now that a population initially existed in an area to
which it was well
and that it then started to expand into a contiguous area in
environment favoured a different form.
If the transition between the two environments was sudden, a
will evolve at the border, the border would be recognized as a
worked on the population in the new area, different
accumulate in it and the two populations would diverge to become
to their respective environments.
If they diverged almost to be different species the two
populations would have separated while they were geographically
contiguous, along an environmental gradient.