Consider a population that becomes isolated at some point in time. Some catastrophic event may create a physical island from a peninsula, or human activity may isolate a forested region from surrounding areas, or an artificial island might be created when a small set of plants is selected to be used in a breeding program. For simplicity assume all individuals are originally perfectly healthy from a genetic standpoint, i.e. no individual carries a mutated gene when the island is created.
Over time new segregating mutations will be introduced into the population. Some will die out, but some will spread, and eventually a few will become fixed. From this point on other mutations will become fixed at a fairly steady rate. During this period the population becomes less and less healthy, since on average mutations are slightly harmful and when a mutation is fixed it becomes part of every individual in the population.
At this point an interesting question arises: will the population continue to decline at a steady rate, until sufficient mutations have accumulated so that at some generation no individuals survive and the population is extinct? According to computer simulations, the answer is ``yes'' for certain combinations of initial population size, mutation rate, and other factors.
Interestingly, the rate of decline in overall health of the population is not constant, but instead reaches a critical point. Up to this point the decrease in mean health is linear, but beyond this point the decline is drastic and extinction occurs within a very few generations. This phenomenon is known as a mutational meltdown.
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.
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.
