In the Mouse Utopia experiment, an optimal environment with ample food, space, temperature, no predation etc. was created for 8 pairs of healthy breeding mice - but eventually the entire colony died, every single mouse - because after about two and a half years there were no more conceptions. The colony became sterile.
Before they died there were several phases
Phase A - 104 days - establishment of the mice in their new environment, then the first litters were born.
Phase B - up to day 315 - exponential population growth doubling every 55 days.
Phase C - from day 315-560 population growth abruptly slowed to a doubling time of 145 days.
Phase D - days 560-920; population stagnant with births just matching deaths. Emergence of many pathological behaviours.
Terminal Phase - population declining to zero. The last conception was
about day 920, after which there were no more births, all females were
menopausal, the colony aged and all of them died.
This is interpreted as a consequence of mutation accumulation leading to extinction from mutational meltdown:
As far as I can gather, mice are fully ready to reproduce at about 4 months, so the average generation time is probably about 5 months which is about 150 days.
So, starting with 104 days as zero - when reproduction began; we can convert the above timings into mouse generations
Phase B exponential growth doubling every 55 days lasted 201 days, = 1.3 mouse generations.
Phase C exponential growth doubling every 145 days lasted a further 245 days = 1.6 mouse generations.
So population growth phase in utopian conditions lasted only 3 mouse generations.
Phase D of population stagnation phase lasted a further 360 days = 2.4 mouse generations
Therefore, the last conception (and de facto inevitable extinction) was 816 days after breeding commenced = 5.4 mouse generations.
Human generations are conventionally 25 years, although these have slowed to about 30 years in Western countries in the past several decades - but let us therefore give two values - one for 25 year, and the other for 30 year human generations.
If we start at 1850 as the date when the Industrial Revolution seems to have become certainly established and child mortality rates began to drop rapidly, and start counting generations from that point, and if humans were made like mice (which they are not!)...
We would then predict that human population growth phase (B & C) would last three generations up to 1925-1940
And the stagnation phase (D) for another 2.4 generations - with 5.4 human generations taking us up to 1985-2012.
Well, clearly English people did not stop conceiving three years ago, because babies are still being born to native English - albeit not at a high rate!
I have previously guesstimated that the English situation was that the slower growth phase c began in about 1920 (not about 1880) and the plateau phase began in about 1970) not 1930-ish
So maybe England is lagged about 40 years, because 1. we are not mice, and 2. our mouse utopia emerged only incrementally and was probably not complete until about 1950.
So we do not need to worry about mutational meltdown and de facto extinction (i.e. the final English child of English parents and ancestry being conceived) for, oh, another thirty or forty years...
Nonetheless, I draw the following lessons.
1. Assuming the decline and extinction of mouse utopia was due to mutation accumulation leading to mutational meltdown - then it happened very quickly indeed: only mouse 5.4 generations to the final conception, with half of that being stagnation.
2. The decline in rate of population increase after only 1.3m mouse generations suggests that the effect of relaxed natural selection and mutation accumulation leads to genetic damage immediately, in the very first generation.
3. Although humans (maturing over 14 years and with a natural life expectancy about 70 years) are built to last longer than mice (maturing over 4 months and living about 2 years) - this may mean that humans are more vulnerable to mutation accumulation - because we have a more prolonged and multi-phasic development and depend on extremely-complex brains which use many genes make and to function, and constitution large mutational targets.
4. In mouse utopia, the mouse environment, shelter, food, hygiene etc were all managed by humans - and did not depend on the mice doing anything much for themselves except east, sleep, fight, groom and reproduce (until they altogether lost interest in sex)- but humans depend on other humans for survival.
When the human population is damaged from mutation accumulation, this will destroy the 'utopian' environment. If this destruction is severe enough and comes early enough- then mutational meltdown will be avoided.
But if there is a generational lag - and utopia is maintained sufficiently that further mutational damage to younger generations continues to accumulate - then this will hasten the meltdown and extinction; because by the time utopia comes to an end, the younger generations will be unfit to survive the harsher conditions.
Have a nice day!