Minimum viable genetic diversity is not a particularly well-established or even objectively knowable metric. I think the relevant number is often vastly overstated, or at least stated without mentioning the strong and arbitrary assumptions that need to be made in determining it.
Wikipedia says: "The cheetah has unusually low genetic variability. This is accompanied by a very low sperm count, motility, and deformed flagella.[14] Skin grafts between unrelated cheetahs illustrate the former point, in that there is no rejection of the donor skin. It is thought that the species went through a prolonged period of inbreeding following a genetic bottleneck during the last ice age. This suggests that genetic monomorphism did not prevent the cheetah from flourishing across two continents for thousands of years.[15]"
Now - cheetahs are objectively less fit than they would have been without these population bottlenecks, but they're fit enough that they have not been outcompeted. They are probably highly vulnerable to contagion, though - maybe it's only a matter of time before something figures out how to wipe them out... but maybe not. Science isn't gonna be able to predict that one for you.
Anything we can figure out how to breed in captivity, in a controlled environment without predators and with an abundant food supply, has very low pressure on it - these bugs could all have the insect equivalent of mental retardation and missing limbs and at least some of them could probably make it to successfully reproduce.
Section 3.2 (p8)
The population at Melbourne Zoo, being descended from a single pair, appeared to experience increasing symptoms of inbreeding over two generations until four adult males were introduced, descended from a second pair held at Insektus in Sydney, and their genes began to have an influence.
After new males were introduced, no further abnormalities were observed...
Eve's eggs were harvested, incubated (though it turns out only the first 30 were fertile) and became the foundation of the zoo's new population of walking sticks.
I am not a real biologist, but I suspect it is some combination of the factors you suggest and that their genome may be simple enough that most defects make the eggs nonviable.
As such, I would not expect inbreeding to affect the viability of an insect species at all. If they can produce a viable first generation, the large majority of that generation can produce a viable second generation.
Mammals might have a harder time with inbreeding because they produce few children, and as such any problems are less likely to be resolved through another line.
There's also likely some luck involved that the initial two on that island happened to be viable colony starters.
[1] http://www.bio.miami.edu/tom/courses/bil160/bil160goods/16_r...
In the long term, inbreeding is also a problem because it limits genetic diversity, which means anything with an adverse effect on one individual will adversely affect all individuals (the same problem as "monoculture" in software).
> there is a critical mass under which there isn't enough genetic diversity to preserve the species (I've seen ~30 quoted for some rare mammals)
The Blue Book ("Viable Populations for Conservation") puts it between 50 and 1000 individuals for 95% probability of survival over 100 years, but it's highly variable between species and ecosystems on both intrinsic (r-strategy which you quoted) and historical grounds: for the second case, the Mauritius Kestrel was brought back from an effective population of 2 mated pairs. But its ecosystem has a carrying capacity of under 1000 individuals and it had already gone through several genetic bottlenecks.
The weta are r-strategist anyway, Poor Knights wetas have clutches of 200~300 eggs.
Ill effects of incest can be avoided with https://en.wikipedia.org/wiki/Genetic_purging . In case there are only very few individuals left that's probably not what one wants to do, though.
great post.
- since these insects have a r-selection curve, it's ok if 90% of the offspring are genetic defects, you can always produce another 1,000 eggs
- the insects had maintained such a small population for so long that any critical defects had been bred out
But I'd be interested to hear from a real biologist.
Edit: oops, meant r-selection, not k-selection