i'm just wondering about what kind of evolutionary impact that a sudden genetic defect leading to quadriplegia in lemmings would have.

the example i've used before is of aliens coming to earth and killing us all with a frequency gun that fries our brains via our aural circuitry; it just inputs an electrical load and pow, you've got brains all over the kitchen table. if you just go with this without asking too many questions about the actual biophysical realities around deafness, only the deaf would survive such an attack - and what is undoubtedly a disadvantage in every other conceivable scenario ends up selected. now, it's another question as to whether any lingering dominance might in the end reassert itself, or if deafness may even mutate to be dominant. but, silly examples aside, you can easily imagine scenarios where recessive traits end up selected, and you end up with a kind of de-evolving.

i just like overturning the aristotlianisms, i think. evolution isn't a tree, it's a graph with crossings. further, temporal evolution is not necessarily correlated with increases in the complexity of a species, which may be due as much to counter-intuitive adaptations as it is to a decrease in the frequency of mutation.

but, what about these quadriplegic lemmings? 

well, if they can't make it to the cliff, and there's enough of them, what do they do? they're lemmings. they might be quadriplegic, but i'd bet even quadriplegic lemmings will have a strong drive to breed - like lemmings. and, some of these lemmings will end up worse off than others, in terms of ability to locomote.

there no doubt will quickly become food shortages, and the lemmings that are the most quadriplegic will no doubt end up dying. 

now, it could very well be that they all end up dying. mutations happen all of the time; they usually fail, and this one is going to make them easy prey for birds and medium-sized canids. yeah, i think the prey issue is likely a substantive challenge for our quadriplegic lemmings, and probably the most likely outcome is that they just all get eaten.

but, let's say a few are able to roll off the other side of the cliff and land in a relatively safe space, with enough food for a few generations. 

i want to imagine that a quadriplegic lemming might exhibit convergent evolution with something like a mini-manatee - remember that lemmings are herbivores, so they're unlikely to evolve in convergence with a seal or an otter. so, if the quadriplegic lemmings could somehow roll down the hill and find their way into an aquatic environment, they might have a chance.

or they might just get eaten by other aquatic animals, like seals or otters. yeah. it's an unavoidable fate for our quadriplegic lemmings - we can imagine ironic evolutionary twists all we want, but they're just going to get eaten, clearly.