Evolution Must Happen, it is logical

All species are in transition, always. Even in a population that is completely static selection-wise, genetic drift will have shaping effects on the population. All species are transitional; they the transitionals between their parents and their offspring. That would still be a transition, don't you think? For certain we can say that the hippo is a transition between the hippos of the past and the hippos of the future. When we say that it's a transition between terrestrial and aquatic forms, we're making a prediction about where it appears to be heading in the future. You could certainly be right that it could revert to terrestrialism, or maintain as a sort of land/water half-assed hybrid. It would still be a transitional between its parents and its offspring.

That answers my query thanks (and was pretty much what I thought). It was just your original post seemed to suggest we knew for certain where it was headed whereas all we can really say it is a transitional between its parents and offspring - or more generally the future and past of the species.

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Confused ? You will be...

I appreciate the question. Like I said we make predictions about hippos based on its behavior and evolutionary history, but they aren't, of course, for certain. But thanks for the chance to clarify that.

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However, I tentatively hold that such change results in a decrease of fitness in a group.

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In what way and why?

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Via the accumulation of harmful mutations, because natural selection cannot get rid of enough of them.For example, take humans. Nature magazine published an article stating that the rate of harmful mutations in homo sapiens is possibly more than 1.6 per generation. Natural selection can filter out egregously harmful mutations, but ones that are only slightly deleterious are more insiduous. If the difference between two organisms is a small number of mutations that have only a minute effect on fitness, natural selection will take much longer to eliminate them, if it even can. In fact, the article's abstract states that: "A large number of slightly deleterious mutations may therefore have become fixed in hominid lineages."The high rate of harmful mutations also led the authors to the conclusion that: "the effects of deleterious mutations may have combined synergistically." For mutations to combine synergistically (synergistic epistasis) means that each mutation has a more harmful effect than the one before it - two equally bad mutations would be more than twice as harmful as a single bad one. If harmful mutations did combine synergystically, then a small number of harmful mutations could make enough difference for natural selection to operate on. However, another Nature article states that:

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The theoretical findings have focused the attention of experimentalists on synergistic epistasis1, 10-16. However, there is no strong experimental support for the idea that synergistic epistasis is ubiquitous13, 16. This is surprising because, in the absence of synergistic epistasis, it is difficult to explain how species with relatively small local populations (for example, mammals and trees) can survive genetic drift3, 7. Populations with high genomic mutation rates ( U) are also difficult to explain without synergy, and there is evidence that high values of U may be common3, 17-21.
the article continues on and proposes a theoretical model for the possibility of synergistic epistasis in small populations. I will be happy to debate about it if anyone wishes.[]

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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.JTP.S. I apologize for the lengthy delay -- I was busier with school than I thought I would be when I first replied to your post.

Many types of organisms are extinct, possibly for the reason you suggest.

Thank you, that is a good point. But wait - whose side are you on? JT

Same side as you, I hope - the side with the most accurate model.

[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

I don't deal with "models," or "theories," or any other constructs put forward by ATHIEST, SCUMBAG evilutionists as "Science" (to hide from KNOWLEDGE)! I deal with TRUTH! Just kidding. Yeah, that's the side I think I'm on.

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JT

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. 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. 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:

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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.

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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 Your post just made my day. It just doesn't get better than mutations in the heterozygous loci of Saccharomyces cerevisiae. Heady stuff, science.JTNote: 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.

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?

Yep. Essentially happened just the other day: I think statements like "extinction happens all the time" take on a certain abstraction, and many don't end up fully comprehending the reality of extinction. Perhaps this will serve as a small example of the reality...

delete double postThis message has been edited by pink sasquatch, 02-05-2005 15:55 AM

Hey JT, good to see you are still thinking - egads! a non-evolutionist citing the primary literature!A few posts back, in response to the question of "why can't evolution increase fitness", you answered in part: I looked over the paper you cite, and I think the conclusions are a bit overblown (perhaps someone could point out if my reading is incorrect).First, the mutation rate was determined by using comparisons between human and ape sequence, and estimating the number of mutations since the lineage split (interesting in itself - you should only really cite this paper if you accept that apes and humans have a common ancestor; if you don't accept common ancestry, you can't accept the results of the study.)It appears to me that the study examined sequence difference between chimp and humans, and they designated them neutral (no amino acid change) or deleterious (amino acid change). The authors didn't do any functional studies to determine if the accumulated mutations are deleterious, neutral, or beneficial. That is, they removed the possibility of beneficial mutations from the mix - it would seem the assumption is that chimp gene sequence is ideal, and any differences in human sequence from the chimp sequence must be deleterious.That doesn't make sense to me, which is why I wonder if I'm missing something in the study. Why has the possibility that some of these mutations have occurred to the benefit of the human species been removed from the equation?It seems that since these mutations are "invisible", the next assumption is that they must have minimal effect.Or perhaps they are beneficial. Or perhaps they contribute to the differences we see between human and chimp. Well put, except for the inclusion of "problem". Nothing in the statement is a problem for evolution.You've essentially stated: In environment A, mutations are neutral to the fitness of an individual or population; while in environment B, those same mutations are highly detrimental to fitness. Thus, selection will act only in environment B to remove the mutations from the gene pool.Do you think the statement counters evolution theory somehow? Again, I don't quite see the problem (though I agree less with this statement).Populations and species become extinct more often than not. Extinction is not a problem for evolution.Small populations may not develop the same "genetic load" seen in humans, because they inhabit a small niche or have limited resources, and thus are more subject to natural selection, and removal of deleterious alleles. Small populations will survive as long as their environment persists, or as long as they are able to adapt to changes in their environment, exactly like large populations. Yes and no. "Yes" to the likelihood that large populations "accumulate large genetic load", "no" to large populations being "not equipped for environmental change".Your use of the term "genetic load" is negative, though the same "genetic load" is the source of genetic variation, which improves the likelihood of a portion of a population surviving drastic environmental change.As an example, "genetic load"-type mutation resulted in a variant of a cell surface receptor (CCR4, maybe?) that allows its carrier to be completely resistant to HIV infection. If an aggressive HIV variant pops up, perhaps airborne and lethal, and causes a global epidemic, a portion of the population will survive because of that mutation, which the authors of the paper you cite would consider "deleterious", since it results in an amino acid change.Genetic variation is a good thing for the survival of a species. Mutations result in genetic variation.

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.