If polymorphism is possible in a population, and as long as environmental stress remains stable, the cumulative effect of increasing mutational load can be essentially neutral.
That is only true for the short term. After enough loci had recessive harmful mutations, the chances would be non-negligible that loci would be homogeneous-harmful, and additional harmful mutations would have an effect.
However, if the environment changes substantially, then the formerly masked effects can reduce fitness.
Which is a problem — a population in a stable environment would develop a large, invisible burden. When — not if — the environment changed, the population would suddenly be forced to confront the massive load it had been carrying.
Note, however, that in small, isolated populations the marginal fitness due to mutational load may decrease due to other factors (such as in-breeding depression) and may ultimately lead to an extinction vortex as you noted.
This is the problem: it is likely that over time small populations will become extinct due to genetic load, so for prolonged survival, organisms need to exist in large populations. Large populations, however, are found where the environment is favorable and stable. But in such environments, organisms accumulate large genetic load, and are not equipped for environmental change, which inevitably comes.
I also want to point out something the article says:
quote:
The negative effect of permanent contamination of populations because of spontaneous mutations does not appear to be very high if judged from the relatively good health of humans or many wild and domesticated species.
The fact that populations today generally have little genetic load (despite a proclivity to attain on) can also be attributed to life not having existed for a long period of time, and thus not having had sufficient opportunity to develop a large, deleterious genetic load. If 20 million years worth of mutations should see us in bad health, and we aren’t, maybe there haven’t been 20 million years worth of mutations
Doesn't that motivate you to go make whoopie?
Your post just made my day. It just doesn't get better than mutations in the heterozygous loci of Saccharomyces cerevisiae. Heady stuff, science.
JT
Note: 20 million years is a number I pulled out of the hat just because it sounds nice. It just means the really long time a bunch of populations have had to accumulate harmful mutations.