[malakhi]

Organisms are said to be of differing species if their reproductive organs are functional, yet they're unable to produce viable offspring (meaning offspring which can then reproduce) due to genetic incompatibilities. At least that's what I was taught in high school - I suppose things could have changed a bit since then.

I was curious how the other poster would reply. Lions and tigers can interbreed and produce fecund offspring, but such breedings are extremely rare (suggesting, perhaps, that mate selection has a role in speciation). Most mules are sterile, but some can produce offspring. So horses and donkeys would be farther along the path of speciation than lions and tigers. My point was that there is not always a sharp line dividing what we consider as distinct "species", and that speciation ("macroevolution") is simply the result of the gradual accumulation of microevolutionary changes.

[NJ_gent]

"My point was that there is not always a sharp line dividing what we consider as distinct "species""

Indeed; nature rarely likes to cooperate with our neat little ways of organizing it. A little saying I came up with in college is that nature isn't digital, it's analog. :-)