Hybrids between closely related species are usually sterile because they have a different number of chromosomes. The first sequencing of Neandertals found an identical chromosome count, so this wouldn't be an issue.
Edit: The best analogy to how it could work, if the populations could keep technological parity, is how polar bears and brown bears are related, or coyotes and wolves. In both cases we continue to talk about distinct species because there are morphological differences, and the main lines diverged within the last million years, but there has continued to be fully fertile crossbreeding at times across the species boundary.
Edit: The best analogy to how it could work, if the populations could keep technological parity, is how polar bears and brown bears are related, or coyotes and wolves. In both cases we continue to talk about distinct species because there are morphological differences, and the main lines diverged within the last million years, but there has continued to be fully fertile crossbreeding at times across the species boundary.
