Before I start this essay I should warn you that I am a biologist, damn it, not a movie critic. But I also love Scifi movies and television and Doctor Who in particular. Ghost Light is noted as being the penultimate story of the TV series, tackling evolution, and for being so creepy. However reviews are often mixed, especially in discussion about the complexity of the plot and the topic of evolution. I agree with most of the reviews that the story has an effectively creepy atmosphere, great lines, and enough twists and turns to well sustain interest in the three-part story. But what I offer is a biology professors view could illuminate something about how evolution is used in this story.
Evolution is usually presented in one of several ways in scifi & fantasy. The first is to rehash the monkey trial with a dialog between hip scientists and old earth creationists. This is seen in some versions of the Lost World but is best used in the dramatization of Inherit the Wind. This is done effectively in Ghost Light with the conservative 19th Century character of Rev. Mathews.
The next way evolution is portrayed is as a weird form of metamorphosis. Pokemon, Altered States and the Outer Limits episode the Sixth Finger are notable examples. While Doctor Who already covered this in the Mutants, Ghost Light uses the metamorphosis of the aliens and discussion of everyone constantly adapting. Josiah even goes as far as causing poor old Rev. Mathews to evolve (or de-evolve) into a more primitive kind of primate. While this devise works well for cinema, it is not the way that evolution actually occurs. Evolution is genetic and occurs between generations. It is all about sex and not survival of the fittest as most people misunderstand.
The third way that evolution is portrayed is to suggest an extra-terrestrial origin or cause in the evolution of humans. 2001: A Space Odyssey, Quatermass and the Pit are notable examples. The Image of Fendahl is a clear example in Doctor Who and so perhaps is Earthshock. Although extra-terrestrials are not implicated in the evolution of our species directly in Ghost Light, the fact that they could cause the good reverend to change make it possible.
Thus far nothing is unique and nothing is complex in this story. But Ghost Light also presents a new way to use evolution as a scifi vehicle. The alien crew has the job of cataloging all of Earths species. However Light discovers that in the time that it takes to finish the task, life evolves into new species so the process has to be continually repeated. Tired of this endless pursuit, he plans to end all life on Earth so that his catalog will be forever and correct. This is a really interesting concept. But is it complex? Some reviewers seem to think not and say that the plot is implausible because life couldnt evolve that fast. A recent study showed that river spawning salmon that were released into a lake in the 1930s had adapted and evolved into a new species by the 1990s. It turns out that evolution can occur in as little as just a few generations.
In two and half centuries thousands of naturalists and scientists have named and described over a million species of plants, animals and microbes. New species are still being discovered every day and in recent years many of these require using DNA technology to distinguish forms as separate species. A totally new species of whale was recently discovered this way. The rate of discovering new species suggests that there are at least 5 million species alive today and perhaps as many as 50-100 million species. The majority of organisms are types of primitive microbes that live under the ocean floor and deep underground in places that life was not known to exist until the last 25 years. Plus it often takes scientists a year or two to identify, describe and catalog a new species. So the task of cataloging every species on the planet is quite enormous. If we use the conservative figure of 5 million species and assume that Light and his small crew could identify, describe and catalog and average of one species in a single day (an extraordinary feat), then it would take them more than 13,000 years to complete the task. This is plenty of time for new species to evolve and create the endless cycle portrayed in Ghost Light.
Overall Ghost Light is a good piece of science fiction and ranks among the better Doctor Who adventures. I rate it much higher because of the scifi new concept that it makes. Three and a half stars out of four.