Inter-Species Reproduction

In science-fiction the concept of inter-species reproduction is common and widespread. Although there is little to no scientific evidence that such a thing could even be remotely possible, often though people are able to suspend their disbelief.

Sexual
Sexual reproduction occurs when two or more members of separate races engage in sexual act, this is the most common type of inter-species reproduction seen in science fiction, and like-wise the most unlikely to occur.


 * Star Trek Universe, this is incredibly prevalent in the star trek universe. Almost every sapient race that could be considered even vaguely humanoid can reproduce with the others.
 * Klingons have been demonstrated to mate with, Betazoids, Romulans (Thus they are likely compatible with Vulcans), and Humans.
 * Betazoids have been shown to reproduce with humanity.
 * Cardassians have been show to be able to reproduce with Kazan, Bajorans, and it is implied they are biologically compatible with Humans.

Invasive
There is a myriad of races that need separate organic hosts in order to further their own reproduction, this is not a consensual act and often either permanently damages the host-species, or outright kills them.


 * The Xenomorphs of the Alien Series seem to be completely dependent on foreign hosts in order to facilitate full reproduction, for without foreign hosts new queens cannot manifest.


 * The Flood seem to require foreign bodies in order to spread, while there seems to be some manifestation of the flood that can occur without hosts, hosts generally seem to be required to spread.

Grey Area?
Many of the acts of so called "Invasive" reproduction can actually occur consensually.


 * Star Treks Humans are shown to be able to carry Xyrillian offspring, the mother was not aware this was possible when she accidentally inseminated the father. While this example is invasive, the act is not inherently so, and if the couple was willing it could be carried out consensually.

Supernatural?
Some races seem to poses quasi-supernatural abilities, and often this allows a broad range of reproductive options to the race.


 * Star Trek's Q claims to be able to mate with practically any life form, note that it seems he still needs another life-form to reproduce.

Artificial
Some races are able to reproduce only through artificial means, although few examples are present there are laws against this in the Mass Effect Universe.