Are there in-universe explanations for how some species' appearances change between series?

Question

Through the course of the Star Trek franchise, some species have undergone significant changes in their appearance.

For example, here is a Klingon as seen in The Original Series.

And, here is one from The Next Generation.

Quite interestingly, if we go further back into Star Trek universe history, we find that Klingons from before the time of TOS looked much like they did in TNG. Here's one from Enterprise.

Klingons weren't the only ones who went through this, and it didn't just happen in the transition from TOS to the TNG/DS9/VOY era. Here's a Trill from an episode in TNG.

And, this is one from DS9.

I'm sure these aren't the only species to have undergone changes like this. Have there ever been in-universe explanations for these, or others?

Answer 1

Klingons

There were three major "versions" of Klingons:

1. No cranial ridges, found in the original series
2. Cranial Ridges "A", found in the first four movies
3. Cranial Ridges "B", found in the later series and movies

There is no explanation for this shift found in the original series, any of the movies, or in The Next Generation. The first hints at retconning the disparity comes from the Deep Space 9 episode "Trials and Tribble-ations"—a pseudo-crossover episode with the orignal series—wherein Bashir, Odo, and O'Brien do not recognize a group of Klingons as such, and the following conversation takes place between them and Worf:

Worf: They are Klingons, and it is a long story.
O'Brien: What happened? Some kind of genetic engineering?
Bashir: A viral mutation?
Worf: We do not discuss it with outsiders.

This was left unaddressed until the Enterprise episodes "Affliction" and "Divergence", where the in-universe reason for why the original series Klingons lacked cranial ridges is established to be the side effect of genetic augmentation gone awry.

Given this, there are a few conjectures to explain the timeline of events between "Affliction" and The Next Generation:

• The disparity between original series Klingons and the others is entirely imagined, and it was only a makeup issue. The original series Klingons should be imagined to have cranial ridges, and the Enterprise arc should be taken as a one-off about an unrelated issue. Supposedly, this was Gene Roddenberry's position (he obviously wasn't alive for the Enterprise arc, but reportedly rejected any attempts to explain the discrepancy while he was still working on Star Trek).

• The affliction affected the entire Klingon species, but was eradicated either through genetic conditioning or breeding by the time The Next Generation occurs.

• The affliction only affected part of the species, and the Enterprise in the original series happened to only encounter the subset of Klingons afflicted.

The disparity between the original series movies and the rest of Star Trek canon could then either be a period where the genetic problem was beginning to be corrected or when afflicted Klingons started to play around with cosmetic surgery.

Which theory is right has never been addressed on screen, but was, at least according to Enterprise writer Mike Sussman, intended to be left to the viewer to decide for themselves.

Trills

Like the Klingons, the change in makeup was done for aesthetic reasons:

Although Michael Westmore's alterations to make the Odan headpiece more feminine was as good as all of his work, the writers simply didn't like it. Apparently, after she had put on the Odan forehead appliance, someone looked at Terry Farrell and said to Westmore, "What did you do to her head, she used to be beautiful?" Instead of changing species, as they'd already come to like the idea of an "old man", a person with centuries of experience to guide Sisko, Westmore suggested to "just give her spots like we gave Famke", who played a Kriosian.

However, it was never addressed in canon: all Trill starting with Deep Space Nine have the Dax-style spots and the opportunity to explore the differences in-character à la "Trials and Tribble-ations" has never occurred on screen.

Answer 2

There are a few other differences between the Trill seen in the TNG episode The Host (Odan) and the Trill later seen in DS9 (Dax, Kahn, etc):

• Odan said use of a transporter would kill the symbiont. This was obviously not true of Dax, and was never mentioned by any other later Trill.
• Odan's symbiont was called a "parasite" a couple of times in the episode. I doubt Dax ever used that word and I bet she'd have objected to it.
• Odan spoke as if his personality was completely that of the symbiont, and that the personality of the host seemed to be almost completely repressed. Riker's behavior once Odan was put inside him seems to back this up.
• No one seemed to be especially concerned about Riker's health after Odan was removed from him. They seemed more concerned with his body's immune response. (This could be due to him being non-Trill.)
• The symbiont itself had fluorescent spots on it, instead of being a monotonous grey. It also pushed visibly against the host's abdomen a couple of times, and Odan used some sort of an unexplained device on this spot at the end of the cold open.

One possible explanation is that there are several "races" or "sub-species" of Trill and/or the symbionts.

On the other hand, there is a book that connects the differences in the Trill with the pre/post TOS differences in Klingons: "Forged in Fire", by Michael A. Martin and Andy Mangels. Featuring Curzon Dax (the book is the backstory to the DS9 episode "Blood Oath") and the Sulu-commanded Excelsior reveals that:

"...the Odan style of Trill came into existence after a Trill colony was infected by the Klingon augment virus after Klingon traders visited their world, although by the time in the novel, the Trill have accepted this "offshoot" race into their society with little objection."

(Quote from the Memory-Alpha article, emphasis is mine.) It also implies the inability to be transported to be a cover story to prevent the true nature of the Trill from becoming public.

But as Mark Trapp mentions, this has never been explained canonically on screen.

Answer 3

The only thing I found about Trills is that the woman who played Dax in s01-s06 in DS9 was allergic to the prothstetic, so they decided to make Trills look different, without prothstetics. I'm not sure if there is a Canon explanation for in-story.

Answer 4

I had a book at one time (that I no longer have and can't remember the name of) that was released shortly after TMP explaining the appearance of the new Klingons at the beginning of the movie.

According to that book the original Klingons (as seen in TOS) were taken over by another race/breed of Klingons from a nearby planet within the Klingon Empire, similar to the depiction of the Romulans & Remans in Nemesis. This was also to explain the more aggressive nature of the "new" Klingons.

This is obviously was never canon, and now explicitly is not, but I do still favor that explanation rather than the augment virus.

Answer 5

Trill: 'In the first appearance of Trill in the TNG episode "The Host", Trill were depicted physically quite differently than their later appearances. Also, they were unable to be safely transported via transporter, a weakness not shown in later appearances when Trill became a regularly used race. No explanation of the differences is ever given onscreen, but according to an article on startrek.com, humanoid Trills are actually composed of at least two races that can be used as hosts for the symbiont. It is also revealed in this episode that humans can also serve as temporary hosts to the symbionts when Commander William Riker hosted Odan's symbiont. The crew's general unfamiliarity with the race is a minor contradiction with later episodes, which stated that the Trill (such as Curzon Dax) had been working with the Federation regularly long before their first appearance in Star Trek: Deep Space Nine; "Trials and Tribble-ations" implied that a host of Dax was romantically involved with Leonard McCoy when the original Star Trek series character was still a college student. Other differences included the change in makeup style from a prosthetic forehead to a series of spots, although some makeup tests and sketches were made in the preproduction of Deep Space Nine using the earlier makeup style, but the actress who would play Jadzia Dax, Terry Farrell, proved allergic to the extensive prostheses, and so the new style was created. The Trill naming style, of using their host name as a first name and their symbiont name as a surname, differs from their first appearance, in which the Trill Odan was only known by that one name.' - http://en.wikipedia.org/wiki/Trill_%28Star_Trek%29

What I find interesting is that the trill look very similar to Kriosian (http://en.memory-alpha.org/wiki/Kriosian).

Answer 6

some Total speculation and some fact here but its kinda the best we have so far.

Klingons with flat forheads are those whos ancestors where effected by the klingon/Human augment virus that was the resault of a failed klingon experiment using human augment fertilized eggs, The changes in ridges are do to makeup limitations and later its made somewhat cannon that ridges follow family lines thus making the movies ridges make sense(though this is never made offical and is ignored sometimes such as Alexanders future self).

Romulans take a bit of guess work as its never said for sure why they have a v on forhead....its a working theory for many that they too experimented with the Human/klingon augment virus and infected the whole population sometime durring the 50years of silence and might be the reason for the silence as they tried to fix it.

Bajorans just looked bad with the unibrow makeup and so it was changed but some think in universe its do to age or ethnic group.

Trill the best explanation though again not offical is that there are two species of trill humanoids, a sybiot and a parasite species so one is like Odans and is a smaller population subpecies like a throwback, then a Dax kind that are the main population group and a more modern form of trill.