For the most part, they can cross pollinate. Check out the link below. It gives some good info and states (in referrence to Capsicum Pubescens: "One interesting point to note is that the species is 'isolated' from the other domesticated species as it cannot cross pollinate with them."
But the web page discusses all the species.
Bruce

Here is a link that might be useful: The Capsicum Genus by Chiliman.org

Here's a grid from the forum FAQ describing the likelihood of the various species to cross. But, they also self pollinate so readily it's more likely they won't cross than they will.

Here is a link that might be useful: species crossing table from faq

If they do cross save the seed you may have something new that you like.
One year I had a cross from a "Fish" pepper and a "Cali Wonder" pepper (both annuum) I got a fish pepper plant with a litle bell pepper on it. It was delicious with just a bit of heat. I regret not saving seed from it.

Eric

Hmmm, I heard species could not hybridize. If it is Annuum and Annuum they cross, But Annuum and Baccatum no cross. I am not 100% sure though.

PunkRotten not sure I understand you there, Fish and Cali Wonder are both Annuum.
Even if they were different as we can see from the chart tsheets posted, they apparently can.
Annuum and Baccatum = F1 hybrids germinate normally.

Eric

Woohooman

You remember correctly about reproductive isolation, but that turns out to be a loose rule. For example, species can be isolated by geography. That is, one lives on one island, one on another. Another example: one species flowers in April-May, and another in June-July. In each case, the pairs of species are 'reproductively isolated.' That doesn't mean that under artificial circumstances, they can't successfully cross.

It gets more complicated. A horse and a donkey can cross, but the result is a sterile mule. So that ends the mixing. There are many examples of hybrids in nature. Sometimes you get a 'hybrid zone' where two species meet. It happens in fish and frogs and irises.

Your professor was correct in that you need reproductive isolate to CREATE a new species. If a population that's a little different than the rest of the species keeps interbreeding with the 'normal' population, the mixing will prevent it from going on to differentiate more and more. If, however, the different population gets isolated, like on the far side of a big river, over time it might continue to change, and eventually become so different that it is no longs the same species as the original population.

Thanks for the replies. I thought I might be able to isolate some cultivars from others in order to have truer seed. Looks like netting it is.

Kevin