Here some biological concepts that I have used for designing new creatures as well as adventures for Mutant Future. They are easy to apply to other games and settings.
Complexes are easy and yet so powerful. It is the idea that a stock species can gain the same mutations at different times and/or locations and become very different mutant species. Take a wolf and give it the complete wing development mutation. The wings could be an extra pair of limbs complete with bones, flaps of skin supported by cartilage, modified ears (Dumbo or attached to the spine near the tail) or modified legs (fore or hind). Each is its own species with different advantages and disadvantages that a GM can incorporate (such as the flaps being rigid, forcing the wolves to avoid thick brush). One idea that I really like using with complexes is regionality. The livestock of several villages could be the same mechanically but very different in appearance and behavior.
Families were the basis for Creatures of the Wastelands: Mutational Evolution. The idea is simple- take a creature, roll several sets of mutations and apply them to make several mutant creatures. Do this again and again with the resulting creatures and form a family tree. This idea is much stronger when the GM constructs his own mutation charts, adding his own mutations and playing with probabilities. Of all the ideas I have had for Mutant Future, this may be the most powerful as it can be applied to just about any game that includes monsters. A GM designing a fantasy setting that includes lots of cat monsters could write up a short mutation chart and roll up a family of a few dozen species within a few days. And the nice thing is one can always ignore or pick a mutation instead of letting the result of the roll force one's design.
The next three are real biology.
Reciprocal hybrids are a weird result of different species mating. They are the differences in hybrid offspring where the sex of the parents matter. The best known are the mule and hinny. They are crosses of horses and donkeys but look different. Mules have donkeys as fathers where hinnies have horses as fathers. When I first described this in the old form I included the idea that the sex of the offspring mattered as well. A mating of two species could result in 2-4 very different creatures. Now throw in the idea of species with more than two sexes and there is a proliferation of monsters. I will have to work on that idea.
Mesopredator release is something that more games should include, in my opinion. It is the idea that when top level predators are removed from a foodweb, the middle (meso) predators expand in numbers and cause ripples through the herbivore and plant populations. Kill the robot that was controlling the spidergoat population and suddenly there will be zillions of spidergoats. Kill a dragon and watch the orc tribes grow and raid that much more often.
Hyperpredation is a nasty way of screwing with a location. It is where a new prey species is introduced and suddenly there is an increase in predators. Say a rodent that survives just by breeding much faster than any of the native species. Not only do they push the previous residence into extinction (or very reduced numbers), they also provide a killer shrub with a lot of needed protein. Now the plant not only increases in numbers its toxin also becomes more deadly.
So, do you have any concepts you want me to explain?