Hork-Bajir: Since they were created, they don't really fit in this category, but the only problem I see with them is the reproduction thing. I seem to remember in one of the books that they only live about 6 years. I can't find the reference but it was likely either in the Hork-Bajir Chronicles or #34 - The Prophecy. They mature incredibly fast, but they would have to have a near 0% mortality rate. Too many babies dying means the species doesn't get the unique genetics it needs.
I don't think it was ever quite specific on their lifespan. Maybe you read it in a fan fiction, or maybe you caught some things and extrapolated a 6 year lifespan then. For example, maybe you noticed that Toby was born in the free colony and was "pushing seven feet [tall]" by The Resistance, which could have been only a few months apart but certainly was less than even two years so you figured "Well if she's fully adult in less than two years she couldn't have a lifespan of more than 6 or so."
Though the lifespan is never specified, it is certainly more than about 6 years, because Jara Hamee lived in the late '90s ('present day' Animorphs timeline) and Dak Hamee, his father-father (or grandfather) was a child in 1968, being only two generations apart they span 30 years time.
I extrapolated my approximation of the Hork-Bajir average lifespan as being about 25-26 years considering that, unlike Yeerks, the Hork-Bajir do not die upon their child's birth but upbring their young in the manner that humans and similar mammals do and continue living well into their child's adulthood. Jara and Ket are even shown to have another young child in The Resistance, so the time it took for Toby to grow to a height that was noted to be almost as tall as a fully grown male was not enough time for Jara and Ket to pass beyond an age capable of producing offspring (though they may be Elders by Hork-Bajir standards, they are never shown to be infirm in any way, so this is another piece of evidence that their lifespan isn't
too ridiculously short).
Still, even this has its share of innate assumptions, because comparing it to Human development is only the roughest of estimates with a completely different species. Knowing nothing about Hork-Bajir growth and development, they could just as easily reach full adulthood in only a few months then live an adulthood of the majority of their lifespan. For example, the common conversion for a dog's lifespan is seven 'dog years' to one 'people year' because an average dog's lifespan is approximately 1/7th of a human's, but dogs reach full adulthood and sexual maturity in a much smaller fraction of their lifespan than humans do, so a more accurate comparison is to say that a dog's first year is about 18 'people years' then each year after that is approximately five 'people years' in terms of its continued aging.
Of course, another important note is that the two Hork-Bajir we know the most are the worst representation of
average for their species, because the biological and physiological needs of high intelligence may also mean that Dak Hamee and Toby develop completely differently than the average Hork-Bajir due to their rare combination of DNA (Jake's noting that Toby's size is comparable to a
fully grown male of her species may well indicate that she is outside of the norm). So not too much specifically can be pinned down, but their lifespan is certainly longer than six years be well under a human's (or maybe not, maybe they just have children during a much earlier section of their lifespan than humans do).
In any case, I'm not sure I understand what you see as a problem in the Hork-Bajir from an evolutionary perspective, even if their lifespan was only six years why would that be a problem in the way that the Hork-Bajir live, provided that they are still capable of having a few (2+ children) and raising them to full adulthood; why do they have to have a lower mortality rate when they would essentially just be doing the same thing humans do but in fast-forward? Could you clarify that?
Taxxon: Many thoughts on the taxxons, as they're the most interesting to me. Why would a species develop a hunger so consuming that it would willingly eat itself given the chance. My theory is that long ago, as the taxxon were developing, there was an enormous drought (or whatever passes for a drought in a desert planet) and the taxxon began to starve. One taxxon, in desperation, turned on one of it's species and ate it. I can assume they have some sort of hive mind, a collective consciousness (not as developed as howlers, but still powerful) and the others realized that there own could be made food. I say this due to book 43: The Test. Ax and Tobias were able to call on the memory of the Taxxon nesting grounds (where they would be surrounded by food) to calm the hunger. Since it should have simply been DNA they absorbed, the memory would have been imbeded into the creature on either a cellular or instinctual level. This says something of a collective conscious (Like Jake when he morphed a howler, but not the same clarity and detail). The hunger could come from the memories of the drought (or whatever) existing in the subconscious.
I don't think that the cannibalism of this species is a big stretch at all, it's common enough on Earth. The 'all consuming hunger' is a strong evolutionary drive in itself, and the attacking of other wounded of its own species is survival of the fittest at its strongest. As far as evolving a self-control not to take a bit of your own yummy tail when it is oozing blood-goo, it doesn't negatively affect survival-to-reproduction since the Taxxon that's unable to restrain himself from eating himself is just going to be eaten by his neighbors anyways; really this isn't different from sharks at all. You're thinking from the perspective of an intelligent creature causing a social change for future generations, but an 'act of desperation' or a single 'time of drought/famine' isn't necessary, it's just a simple intra-species competition trait that can evolve in a functioning capacity just as much as many others.
I'm not sure that the collective conscious was present in the Taxxons in the same manner it was in the Howlers (though, it's never so explicitly laid out as to be undeniable, I suppose), I though the Hive better fit its portrayal in the book as being is own, separate biological structure that was symbiotic with the Taxxon species, not that it was a collective consciousness of the Taxxon minds. The images that came to Tobias and Ax's minds may not have been actual images imprinted in DNA or brought through a collective psychic link, it may just be that the biological processes being directed by a Taxxon brain going into hibernation was interpreted in their minds with this mental image. They certainly didn't recieve clear, direct mental thoughts from other local Taxxons and the ability for free Taxxons and enslaved Taxxons of The Andalite Chronicles to have separate factions and wage wars shows individual minds, not able to access a common collective conscious.
Yeerk: These species make no sense start to finish. They aren't true parasites, as a parasite exists to use another body to fulfill biological needs. These creatures developed the ability to enter another creature and take complete control of it, but there is no biological need to do so, since it actually inhibits the absorption of Kandrona rays. On another note, they are extremely resilient. Three yeerks can re-create the entire species given enough time. This means they either have natural predators (salmon lay a huge amount of eggs because the likelihood of each individual one surviving is slim, but overall, the species will survive.) or they have some reason to fear extinction, but I can't think of one. The Yeerk home world is well inhabited, and repetitively safe for them (except for the andalites, of course). The only thing I can think of is they reproduced so rapidly that they overextended the planet's resources, so they had to have a way to adapt. All very sketchy.
I really don't see any of these as being a problem evolutionarily speaking. Sure, they aren't true parasites but that is just a name that is used by other species to describe what the Empire is doing, the main characters aren't biologists attempting to find a correct classification for the Yeerks, so their wording doesn't have to be technically precise. Their evolution alongside the Gedds would be best described as facultative mutualistic symbiosis (mutualistic meaning that they can mutually benefit from each other, the facultaive meaning not necessary for survival but able to be more benificial than alone as opposed to obligate when it is necessary for one or both species to survive).
And you can't say that there is no biological need to gain the ability to infest or that it is a negative trait because it makes them unable to absorb Kandrona, because even if this is true on their present day home world there isn't enough detail on the home world's evolutionary history, it's certainly likely that they did infest to escape predators (there is a lot of discussion in the
http://animorphsforum.com/forum/index.php?topic=5243.0 thread, as well as some other threads I believe) and that this was a very successful survival trait: Becoming amphibious on Earth is a trait that allows a specie to temporarily flee the medium where their prey exist (water) to escape predators, then return when the predators are not present to hunt for prey.
As far as the reproduction goes, three adults make 'many' offspring (though it's never clear exactly how many) but they die in the process, so three individual adults cannot, in a single generation, reproduce the entire species (though this wouldn't be a bad trait, it's a very positive selection trait). The ability to populate or repopulate an area in relatively few generations is a very strong survival trait, when their predators are few and their food supply is plenty it allows them to multiply quickly, when they over-produce starvation limits the increase and more predators (being able to increase due to this over-abundance of this prey) brings about a balance. The Yeerks are known to have ancestral predators, a recent extinction may not have allowed for a change in Yeerk reproduction and again, if they are living at the full capacity of their home-planet then that just means that only a few of the dozens of offspring will survive from one generation to the next, more intra-species competition is just a positive selector for the aggressive behavior Yeerks are known to have and an expansive biological imperative explains the behavior of this species as it became intergalactically capable due to Andalite interference.
Andalites: Again, these creatures are built like predators (natural weapons, ability to see multiple directions at once) but they eat plants. Maybe on the home world grass is different from Earth, but a species that large could not survive when the act of eating requires energy (running) in itself. To have a weapon like that on your tail would mean that you aim to kill or you have a good reason to defend yourself. (in the Andalite chronicles, Elfangor says that the creatures on the andalite home world are mostly non-threatening, so that's unlikely.) Their bodies can process meat and (likely) bone. (Ax mentions stepping on a snail once and absorbing it, shell and all, he then says that all the protien will keep him awake. This means that his body is capable of processing the food as well as ingesting it. A snails shell is tough, so if he can absorb that, he can likely absorb bone as well, but just a theory. Maybe some small mammals on the planet can be stunned or killed and then absorbed through the hoof (rat size or lower, most likely).
Yeah, they are pretty much agreed to be most implausible biologically, but if instead of crushing grass and absorbing nutrients at the hoof-tip like we all tend to be assuming, they instead are cutting off and 'sucking up' larger chunks of vegetation and going further breaking down throughout the length of the legs and into the torso region in stages they become more likely, the 'crushing and absorbing' description that Andalites offer may just be a colloquial simplification. Add to this the ability of the hooves to crush and separate chunks of meat from small creatures and digest them and Andalite plausibility much greater. Maybe they evolved omnivorously in this method and the shift to herbivorism was a social change allowed by their controlled expansion over their planet once they were the dominant species (they would then need to spend the majority of their time grazing while they went about other various activities, which it sounds like is exactly what they do, and they would still be needing to take in larger amounts of food and absorbing throughout the inner length of their hooves rather than just the surface area that the bottom of their hooves allows). A recent (evolutionarily speaking) change to herbivorism is a likely explaination for Ax's capability to digest non-plants but a gradually growing intolerance to non-plants throughout the species. Still, the ability for that large of an animal to create all of its own protein in the manner that small mammals do is a problem.
As far as the tails go, that offensive of a weapon is unlikely as a defensive weapon, but it is possible and that is exactly the explanation offered: Elfangor's reference to the home world as being harmless is a result of their intelligence bringing about a technological dominance, the Ellimist's encounter with Andalites well before humans had begun to evolve showed that they had quite formidable predators and that their then less flexible tails were use to allow backwards thrusts as they faced away from enemies prepared to flee when necessary.
