Then how would you explain ring species? Where is the demarcation?

 

There isn't a precise demarcation. This is because evolution happens. Remember, evolution happens to populations and that traits are plotted as bell-shaped curves. So, as the population gradually changes from generation to generation the curves of each generation shift either right or left, but there is a lot of overlap with the previous generation. After hundreds or thousands of generations, finally the bell-shaped curve of generation 1,000 doesn't overlap at all with the curve of generation 1. However, just where in that progression do we draw the line? Can we say "at generation 499 we had Species A but at generation 500 we have species B"? NO! We can't.

 

Ring species are undergoing speciation; they are speciation in progress. If you break the ring by having any of the intermediate propulations go extinct, then yes, we have 2 species. Mayr says that there are already breaks in many ring species. We simply haven't looked closely enough to find them. Therefore, since there are breaks, we have 2 species in those cases.

I'm necromancing this old thread to share with you this link. It's a super cool exhibit put forth by the Smithsonian exploring the question "What does it mean to be human?"

 

http://humanorigins.si.edu/

 

 

It is really well done. Enjoy.

Humans are the descendants of other humans, duh!

Turn towards the budding discipline of 'Molecular Phylogeny'. It is, at last, the first 'Falsifiable' Theory regarding such matters.