Hybrid Theory
<accesscontrol>Administrators</accesscontrol> In the real world, it has the force of scientific law: only the most similar species within a genus can successfully mate, and even then the offspring will almost always be sterile. Yet in Star Trek, hybrids have been with us from the very beginning. So how is it that beings that evolved on completely different worlds, some of them with physiology fundamentally different from Earth humanity, manage to have healthy children together? And how is it their children seem to expect the same?
Programmed Convergent Evolution
We know from TNG: "The Chase" that many of the galaxy's humanoid--or cardasdanoid species, as most of my protagonists would prefer to say, are somehow the result of a billion-year-old genetic seeding program by an ancient Progenitor race. Finding themselves alone in the galaxy, these Progenitors decided to seed the primordial pools of world upon world in hopes that when their descendents looked to the stars, they would find a vast community of beings similar to themselves--though each with their own unique traits--waiting to embrace them.
What's truly remarkable about whatever seeding method the Progenitors used is that they managed to encode the pieces of a message to their descendants in the DNA of a number of worlds, and that after a billion years, this message remained intact enough to be read and played back in the 24th century. To me, this initially suggests something incredibly powerful--something that would have to be able to replenish itself over and over again as generations of evolution took place.
But perhaps the actual genetic meddling didn't take place in the primordial pools. Maybe it waited, dormant, until the planet's native life started showing signs of evolving into higher orders--even if not sentient--on its own. This lessened timespan to carry out its program of genetic engineering makes more sense. It allows less time for things to go wrong. And it may also explain how humanoid beings evolved with such a variety of biochemistries, metabolisms, and other internal and external characteristics.
Most likely, the means of transmission was a retrovirus whose genetic payload integrated itself into the DNA of each world's native species. These viruses infect the germlines--that is, the species' equivalents of egg and sperm, and introduce their mutations into the DNA of that being's offspring. Whatever the Progenitors designed, this retrovirus must have been capable of reinforcing its effects in spite of the natural appearance of knockout mutations (mutations that effectively "silence" the inserted genetic code) and recombinal deletion as evolution progresses. This retrovirus may even remain present in the environment, or encoded within the DNA in endogenous form.
The result is that over time, the genomes of the affected species begin to mould themselves towards a standard humanoid form. This process is referred to as programmed convergent evolution.
Evolutionary Lability
The degree to which the host DNA yielded to outside influence, however, varied, largely according to two factors. The first factor was the similarity of the native pre-sentient species (prior to the "activation" of the retrovirus) to those of the Progenitor race. The second was the species' evolutionary lability--that is, the degree to which the genome was in flux and open to change (or, in this case, outside influence).
Given the incredible preponderance of humanoid species resembling or identical to Earth humanity, I approach with the presumption that the ancient Terran primates were greatly similar to those of the Progenitors' ancestors, and that the human genetic code was highly labile and allowed much data to be overwritten by the retrovirus. Since the human genome subordinated itself to such a great degree to the genetic invasion, much of what was ultimately expressed was a direct copy of the original template material. Some other species, such as Bajorans, likely fall into this category as well (and I think at least in the case of Bajorans, canon evidence favors this interpretation).
On the most extreme opposite end of the spectrum, take a species like the Aurelians. In their case, the Progenitors' template material grafted onto avianoid stock and while shaping the body into a humanoid form, much of the original genetic material remained. Once enough viable specimens survived the retrovirus, the Aurelian species was able to evolve, but with a very particular balance between template and original material.
In between, we have species such as Vulcans and Cardassians. In the case of Vulcans, while their general form is very similar to Earth humanity and they are presumed to be mammalian in descent, their biochemistry is known to be significantly different. Cardassians, on the other hand, descend from therapsid stock but are not exactly mammalian (nor are they exactly reptilian), and there are known to be significant metabolic and biochemical differences. Though they certainly express a fair amount of template material, the retrovirus appears to have respected the natural evolution on their world to some extent, and has not forced them to diverge completely from the orders that have evolved there. Species like the Cardassians and Vulcans are considered descendants of the Progenitors--but neither are they divorced from the worlds from which they hail.
This may imply a sort of genetic "Prime Directive" on the part of the Progenitors--a suggestion that the diversity seen among humanoid species is intentional on their part.
Implications for Interspecies Couples
Because of this shared template material, some humanoid species are able to mate successfully. However, the evolutionary paths of the species involved are key in determining whether interspecies reproduction is possible, and if it is, the extent to which medical intervention is necessary to ensure the health of the family. Not all combinations will be able to produce viable children or even couple without health complications to one or both partners.
In the case of Earth humanity, because the species' DNA has been so heavily overwritten by template material, this same pliance tends to carry over into interspecies reproduction. Human genetic material tends to subordinate itself to alien influences, meaning that hybridization may occur with a large number of species, and in some cases a viable fetus may even result with little more intervention than the sort required for blood-type incompatibilty within the species. This is especially likely when both partners are of species that diverge little from the Progenitors' template (a human-Bajoran couple, for instance. Evidence for Bajoran genetic pliability is derived from the ability to use a Bajoran as a surrogate mother for a human child under emergency conditions.).
Hybrids between humans and less genetically-pliable species are possible, and sometimes do still occur spontaneously, but often genetic engineering or other measures are required (such as between humans and Vulcans) to ensure viable offspring. In the Sigils and Unions: Catacombs of Oralius story "Exits in the Haze," Vedek Tora Naprem successfully induces the conception of a Bajoran-Cardassian hybrid. However, she is observed injecting herself nightly with some sort of medication of unknown type. Dukat, however, had strong reason to believe that the contents of the hypo were required to bring the pregnancy to term.
(NB: In light of this revelation, the Sigils and Unions interpretation of DS9: "Indiscretion" and "Covenant" is that the canon-universe Dukat may well have surreptitiously arranged for these treatments for Naprem and Mika. While accidental conception might have happened, given the metabolic and other differences between therapsids and true mammals, it is unlikely that the pregnancy could be carried to term without some sort of assistance--at least not without severe risk to mother and child.)
However, consider the idea of a Trill-Klingon marriage. We know that both species have internal/chemical characteristics that are very specific to their races. Trill especially are presumed to have extensive neurological and biochemical differences to permit the introduction of a symbiont. Klingons are known to have a number of redundant organ systems, and some evidence suggests significantly different blood types from those observed in Earth humanity. So from that I presume both species have less malleable DNA than humanity, which leads to a much greater incompatibility as neither genome can yield successfully to the other without compromising the health of the fetus. Races with sufficient differences between them will thus remain unable to interbreed or will require much higher degrees of intervention than might be required with a different pairing. This squares perfectly with what we saw in DS9: "Tears of the Prophets."
Examples of more serious measures range from immunosuppressant treatments to the use of artificial wombs or a surrogate mother belonging to the father's race (as in some cases this resolves or lessens incompatibilty issues). Achieving conception at all, however, is by no means a guarantee. It is generally advised that couples seek genetic counseling and that interspecies pregnancies receive extra medical attention, as differences in gestation time and method, and the ability to deliver the baby by natural means may all be affected. In some cases, interspecies couples may even be advised never to have unprotected sex with each other, when the risks of complications to the partners or any potential embryo or fetus are too high.
Implications for Hybrids
Couples in which one partner is a hybrid and the other belongs to the same species as one of the hybrid's parents often still require medical intervention, but the chances of successful and healthy conception rise, and difficulties with the pregnancy tend to be diminished. (This is not always the case, however; if a signficant degree of intervention was required to bring the hybrid's birth about, serious challenges may still exist.)
However, if a hybrid seeks to have children with a member of yet another species, this may increase the level of difficulty in conceiving healthy children. For instance, a human-Betazoid hybrid may find that his or her level of genetic pliability is greatly reduced due to certain sections of the genome (in the Betazoid's case, those guiding neurological development). In many such cases, the non-hybrid's DNA must be used as the base template, and the hybrid's genetic material grafted on step by step to whatever extent this input can be tolerated. Such children tend to take more after the non-hybrid parent in appearance and physiology.
Genetic Engineering
All forms of genetic engineering are very closely regulated by Federation law, particularly by Earth law. However, an exception is made for interspecies reproduction due to its requirement for the survival of the child. Any genetic engineering undertaken must not a) cause the expression of any traits not natural to the species of either parent, b) accept outside genetic material except in cases where a so-called "patch" is absolutely required to ensure cross-species compatibility, c) expressly seek to create a "superior" species to either or both parent species, or d) attempt to create a "designer child." It is ethically required by some species that the parents and even the doctors compartmentalize their activities from each other so that some semblance of randomness remains in the conception process.
Source: Star Trek: Sigils and Unions