Evolution Must Happen, it is logical

[qs]With the current evolutionary timeline and our understanding of genetics, it is surprising that many types of organisms are not extinct because of the gradual decrease of fitness caused by an accumulation of harmful mutations.[/quote]I think you and crash both may find this relevant:Szafraniec K, Borts RH, Korona R. 2001 "Environmental stress and mutational load in diploid strains of the yeast Saccharomyces cerevisiae" PNAS 98:1107-12 [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. This is partly explained by the fact that, in diploids, the new mutations are usually located in heterozygous loci and therefore are masked by wild-type alleles. The expression of mutations at the phenotypic level may also strongly depend on environmental factors if, for example, deleterious alleles are more easily compensated under favorable conditions. The present experiment uses diploid strains of yeast in which mutations arise at high rates because a mismatch-repair protein is missing. This mutagenesis resulted in a number of new alleles that were in heterozygous loci. They had no detectable effect on fitness when the environment was benign. A very different outcome was seen when thermal shock was applied, where fitness of the mutation-contaminated clones was lower and more diverse than that of the nonmutagenized clones. This shows that the genetic load conferred by spontaneous mutations can be underestimated or even overlooked in favorable conditions. Therefore, genetic variation can be higher and natural selection more intense when environmental conditions are getting poorer. These conclusions apply, at least, to that component of variation that directly originates from spontaneous mutations (as opposed to the variation resulting from the history of selection).[/qs]In other words, overall mutational load is only part of the equation. 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. However, if the environment changes substantially, then the formerly masked effects can reduce fitness.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. On the other hand, there have been populations where selection sweep and/or adaptation have overbalanced the mutational load and allowed the population to remain viable regardless of the number of mutations. Finally, within sexual populations, recombination is a key factor in overcoming mutational load. Doesn't that motivate you to go make whoopie? This message has been edited by Quetzal, 01-30-2005 22:17 AM

Sort of. As you noted elsewhere in your post, large distributed populations are substantially more likely NOT to have a heavy burden of homozygous deleterious alleles due to selection weeding out those whose load gets too high. Remember, mutations effect individual organisms while mutational load is a statistical description of populations. IOW, it may be that individuals die off when the cumulative number of mutations in their genome gets too high, but there are plenty of other organisms in the population or species to "carry on the fight". This goes back to how we define deleterious, neutral and beneficial. None of these terms has any meaning outside the context of the environment in which the organism/population lives. It is entirely possible that a population confronted by sudden changes in major (or for that matter even minor) environmental factors DOES go extinct. Happens all the time. Source-sink dynamics, for example, are well-documented instances of this. On a larger scale, if the environmental changes are substantial, regional and even global populations of multiple species may go extinct or be severely reduced. However, the beauty of evolution via RM/NS is that even in severely stressed populations, it may be that a segment of the population has genome/phenome variation that allows it to persist, regardless. If not, well, RIP. In fact, it's these periodic wipeouts that are subsumed under selection sweep - the very thing that "purges" mutational load. This type dynamic occurs in all populations at all times. If you have a really tiny population, it's possible to go extinct purely by chance regardless of what the environment does - or for that matter regardless of mutational load. OTOH, in a distributed population, as long as there's a source of new colonizers, any given population can go extinct (and does) repeatedly without effecting the overall species. It really has little to do with genetic load, and everything to do with fairly straightforward population dynamics. Or, more likely, those individual organisms whose "hidden" genetic load gets too great simply die off without leaving progeny. Remember: selection (including mutations) act on the individual, while the results of evolution act on the population/species. Ain't that the truth?

Thanks for the article, PS. This is exactly the kind of thing that happens "all the time". Many call it "background extinction" to differentiate it from more widespread extinctions (like the Big Five (or four or six, depending on how you calculate it)). At a metapopulation level, some authors have termed it the "Christmas light" phenomenon - because subpopulations of a distributed species will go extinct due to fluctuations in local conditions or simply local disasters - heavy earth moving equipment qualifies as the latter IMO. Then, if conditions are favorable, these local subpopulations may be recolonized. The effect is similar to those tiny winking lights on a Christmas tree. The species itself is generally uneffected.You can see this happening frequently in any heterogenous environment, including those we typically consider homogenous (like rainforests). None of it has anything to do with mutational load, which was the point I was trying to stress to JT. I agree with you. One of the problems with this media is that it lends itself to oversimplified explanations. The literature on extinction and its role in evolution and/or population dynamics is extensive. I have at least a dozen books and half-a-hundred articles on my own shelves. Trying to condense this into a short internet post can be problematic.