The meltdown can be explained as follows: when the critical point has been reached, the population has enough fixed mutations (as well as segregating mutations) so that barely enough individuals survive to fill the population up to the carrying capacity of the environment. In this generation there will be individuals that would otherwise have died out since they are less fit than the others in their generation. However, due to breeding pressure they are now selected for mating, and they pass their mutations to their children. Since their children are, on average, even less healthy, this new generation may not be large enough to fill the carrying capacity. Now there are even fewer choices for mates, further increasing the odds that individuals with relatively more harmful mutations will breed, and in turn the population becomes even smaller.
A plot that shows the progress of the buildup of mutations and their effect on an example population is shown in Figure 2.
Figure 2: Population health over time as mutations increase. The top line shows that the population size remains near capacity until the "meltdown point" (generation 160), even though the mean health (middle line) decreases with time. The bottom line shows that fixed mutations did not appear until generation 90, then increased slowly until extinction.
This plot is for a single population that originally started out with 32 individuals. The top curve shows the size of the population; it remains near 32 until the ``meltdown'' point is reached near generation number 160. The bottom curve is the number of fixed mutations. In this case, the first mutation to became fixed occurred at generation 95, and the second at generation 150. The middle curve shows the average health of all individuals in the population. Initially the health declines as a result of segregating mutations, and then levels off until the fixed mutations start to build up. As expected the health steadily declines, but the size of the population stays near the carrying capacity. Once the meltdown begins, however, the population size shrinks rapidly to zero.