When I first saw this movie, the idea of a species with a single male seemed like evolution had made a pretty serious stuff up (and let's face it, it was
there for plot reasons). But later it occurred to me: evolution is adaptive, not predictive. A single male is not actually a weakness if that male is immortal and nothing around can hurt it! Their species had been dominant for millions of years with that approach. It only became a liability when a new species evolved with the intelligence to use tactics to target the male, and develop weapons to fight back...
- One problem with this, evolution does not work that way. You see, one of the main goals of any organism is to reproduce ensuring the species (and more specifically the genes of that paticular generation to the next) survival. Evolution and reproductive survival don't become stagnant just because an organism is "immortal" (As a comparison, there's a species of jellyfish that's considered "immortal" since it doesn't age, but it still needs to reproduce to survive). Many organisms in Real Life have adaptations that allow them to reproduce should there not be a male (or, in some cases, a female) to breed (Some fish can change sex, certain lizards reproduce via Parthenogenesis (IE: Females produce young without the need for a male to fertilize the eggs), a worker bee larvae can become a queen if fed royal jelly, and so forth). As a comparison, the dragons in this film would've gone extinct long ago since they utterly lack any of these reproductive adaptations. A single male is a HUGE weakness and evolutionary flaw in the species since they lack an alternate form of reproduction should said single male die.
- Not to mention another major problem with a single male dragon within the entire species. Basically, this would mean that not only is the Male Dragon breeding with his mates, but also his daughters, his daughters' daughters and so forth. That boils down to a lot of in-breeding. And, in-breeding causes a lot of genetic defects (Including weak immune systems, mental illness, physical deformation, and even sterility). To put it bluntly, by only a handful of generations, the dragons would've gone extinct in real-life from the numerous problems caused by having only one male. These dragons are so biologically impossible that the 1998 version of Godzilla (with all its biological problems) makes more sense.
- In their defense: it's only a single male dragon breeding with his mates, daughters, and granddaughters if we assume that A: there only is one male dragon, and B: that no more male dragons can ever be introduced into the population. It might simply be that the ratios of births are really, really, really lopsided, but it's even addressed at the end of the movie that the Dragons might come back if there actually is more than one. Also keep in mind that having remotely equal gender ratios would be *disastrous*for a species like the Reign of Fire dragons, because while the females are expendable and short lived the Bulls are not. It's possible that they might have evolved ways to slow the birth of bulls or to practice some form of eugenics (where young bulls get beaten up or killed a la young male Gorillas when they reach a certain age) because it helps conserve resources better.
- It's not just that having one male and many females is setting the species up for failure, whether by inbreeding or loss of said one male; it's that evolutionary processes actively work against such a skewed sex ratio. Any female dragon that managed to produce a son rather than just daughters would immediately qualify as a potential grandmother to half the dragons on Earth (or possibly all of them, if the young Bull can defeat and supplant his sire), meaning a HUGE genetic payoff for abandoning the lone-male reproductive strategy. That's why the standard sex ratio at birth for nearly all vertebrates is 1:1, in the first place.
- It's possible that the male simply only allows a new bull to be born at the end of his lifespan, and otherwise kills the rest to ensure his position as Alpha. From what we see, the dragons only have ONE nest. This is all one group, they just happen to be juggernauts that can wipe out all life on Earth on their own so one group is more than enough. This would make a lot more sense biologically. Of course, that's assuming these things actually are a biologically evolved species and not straight up Eldritch Abominations, which the film somewhat presents them as.