Diversification: Random Walk or Biological Determinism?

Diversity within a species is a trait that confers a major advantage for survival. Why? Because the environments that species occupy are never constant. They very greatly, not only from place to place, but from time to time. Environments from one time to another can differ like day and night. A species that has a very "tight" genome and very little phenotypic diversity will do well in an environment to which it is well adapted, and will do better than a species with a very "loose" genome and broad phenotypic diversity which will waste a lot of resources producing offspring that are not well adapted to that environment.But it is inevitable that that environment will change and the "loose" species is far more likely to have some individuals that can thrive in the new environment. We see this in the human species with its great diversity in height, skin color, hair color and hairiness, and hundreds of other traits. We see it in dogs (and wolves, foxes, dingos, and other canids) whose 1500 breeds cover the range from great danes to chihuahuas, from almost all hair to hairless. While most of the diversity in dogs results from environments and selection pressures imposed by humans, the principle still holds.We even see this principle in non-biological systems. The automobile industry of one country is narrowly focused on high performance and luxury gas guzzlers with only a grudging nod to efficient cars, while that of another diversifies to place major emphasis on fuel and resource efficient cars as well as the large carbon footprint models. Then the environment changes with high fuel prices and constrained budgets. Guess which country's automobile industry is on the verge of extinction. The Japanese and Korean auto makers are also suffering in the current environment, but with a lot of production in the now desirable models, they are well situated to survive this downturn and then fill the niches vacated by failed American makers. There is a great temptation to carry the 'extinction event' analogy beyond its logical usefulness here, but I will resist.Given that phenotypic diversity within a species is a very beneficial trait in supporting survival in the face of changing environments, an interesting question arises: What is the physical mechanism (down to the molecular level) that enforces such diversity? The most obvious is sex with its reshuffling of genomic components and 'mix and match' production of offspring. Another is that while all organisms have developed DNA proofreading mechanisms to correct most major DNA errors that occur during reproduction, these still allow many numbers and types of mutations to occur. (E. g., the genomes of every one of our ~100 trillion cells differ from the egg we originated from by several dozen to several thousand different mutations. The vast majority of these mutations cause us no problems - they are either neutral or kill the mutated cell which is then just recycled into other cells.) A third and rarely appreciated mechanism that is a major enabler of evolution is fecundity - the production of large numbers of offspring. We consider a couple (of humans) who produce 20 children to be very fecund (as well as very irresponsible), but most members of the plant world produce hundreds of seed or millions of spores each season. Fishes (and, in fact, most species) produce thousands of fertilized eggs. This fecundity permits the species to produce highly diverse offspring, the vast majority of which will not survive, but include enough that are survivable in their current environment to continue the species, but also have "Plan B" offspring deployed in case of severe environment changes.So, what happens at a major extinction event (or even a very minor localized one for that matter)? The vast majority of the members of most of the species are wiped out, but several of the species will leave a few "out-lier" members who are able to survive in the new conditions. These individuals may not be all that well adapted to these new conditions, but survival and evolution depend on been just good enough, and for a fair period of time, there is little competition for whatever resources the new environment provides. These survivors themselves now thrive and diversify with some of the more diverse forms being divergent enough to be a species distinct form the original (or it might take several of these near-extinction/reemergence events, but new species will be created). Notice that this process favors species with the most intraspecies variability and thus with the built-in mechanisms to effect such variability, i. e., diversity breeds diversity. It is therefore not surprising that major extinction events are followed by major diversity events, an "explosion" of species radiation. We also notice that there is no need to talk about mysterious (mystical?) Emergent Properties - it is all due to simple, basic easily understood processes that are amenable to a reductionist analysis.On the other hand, I may be wrong.

I really don't think that we are disagreeing about anything, just reacting to different definitions of "emergent property". I had interpreted RADZ's use of the term (and also the direction that StevenFire seems to be going) as being the use that has emerged amongst chaos theorists (such as Stewart Kaufman), i. e., that as a system becomes more complex, certain traits and behaviors emerge that cannot be explained by the basic properties and components of the less complex system. They argue that this inability to describe such systems is not simply due to a lack of understanding or computational power, but is due to the emergence of new phenomena that must be explained with new physical laws ab initio. As I said , I believe that this is the meaning that RADZ was going after, which is why I Capitalized the term.I believe you are using the term in the common colloquial sense of convenient or simplified terms to summarize complex systems or behavior. It is much easier to say that a glass of water is 62 degrees F than to describe the instantaneous velocities of several trillion trillion molecules.I must admit that I react to the term 'emergent property' about the same way that Richard Dawkins reacts to the term 'irreducible complexity'. It's not that I doubt that the chaos theorists, who are certainly much smarter and more knowledgeable about such matters than I am, are on to something important. It's more that complex things are so far over my head, beyond my reach, and out of my limited depth, that I am driven to seek simpler answers. If the Pew Research Group were ever to contact me during one of their surveys of religious beliefs in America, I would declare myself to be an Absolute Reductionist. Our main tenet is that anyone who disagrees with us is condemned to eternal complexity. Since I seem to be in an argumentative mood, I will point out that if you were to look long enough, you would see your population of stones diversify into pebbles, gravel, sand, and dust through the processes of erosion and weathering. But such unlimited generalization of the term would just be, I think, spreading too little butter over too much toast.

You are making the mistake so commonly found in creationist diatribes: If something is partly random, then it must be completely random. They accept the randomness of mutations but ignore the deterministic aspect of natural selection. Yes, erosion has a random component, but it also has a deterministic component due to the environment in which the erosion occurs. Wind (and wind blown sand) and rain will erode rocks into fine particles, sand and dust. Freeze cracking, mechanical failure due to cyclic temperature changes, and differential erosion along faults, veins and inclusions will break big rocks into smaller rocks. Chemical erosion, due to fore example sulphuric emissions from volcanoes, will cause erosion at the molecular level.The more we explore this, the closer this erosion thing seems to get to biological evolution, but I still think the analogy is more strained than productive and is just diverting us off course from the very interesting OP you introduced (Straightree's Post#22 notwithstanding.) I'm probably most at fault for this and duly apologize.As to your questions about my understanding of the term 'emergent properties', I'd rather not show my ignorance of this issue by commenting further. I don't think it's pertinent to the discussion anyway.

First, let me apologize for taking so long to respond to your very kind words. I have never been POTM'd before and am curious as to when I will be informed of the time and place of the awards ceremony. Am I expected to wear a tux and give an acceptance speech? You raise some interesting issues in your post and I'm trying to reconcile the work required to discuss them effectively and concisely with my innate laziness, so far without success.As to your question, there was a thread a few months ago that discussed mutation rates for various types of mutations and referenced some of the original work that addressed this question I haven't figured out the key words to search on to get to that thread. As I remember, there was a range of results depending on which researcher was referenced and their experimental method, but for point mutations (base substitution, deletion, or addition) it came out to few mutations per cell division for a human size genome, i. e., about 6 billion base pairs. In that thread, iI presented an brief and simplistic analysis that indicated about one mutation per 400 million base pairs per cell division, but to repeat it here I think would be off topic. If all else fails, I'll review all my previous posts and see if I can find that thread and its references.A 100 trillion cell organism, typical of adult humans, can be produced by 47 cell divisions, which would give the several dozen to several hundred point mutations for the cells. But this is far to simple a model of how we acquire all our cells. First, most of our cells are destined to die before we do - we replace 10% to 20% of our cells every year and produce several times that 100 trillion number of cells in our life time. Second, most of our cells are not produced by a simple cascade of cell devisions - we have specialized stem cells that do most of the cell production. I don't recall all the detailed analysis, but when all this is tallied up, most of our cells have resulted from about 100 devisions on average, giving the range of average mutations stated in my post.The important thing to remember is that the vast, vast majority of mutations in the cells of individuals are inconsequential to the species. The vast majority of those mutations are even inconsequential to the individual, and because of the fecundity issue I discussed, the species can thrive even if some fraction of is members are disadvantage or non-viable. It is because of this high (but not complete) insensitivity to mutation that species experience intra-species diversity (what geneticists refer to as 'genetic drift', but I prefer the term 'genetic diffusion') and that I think this post is not off-topic for this thread. I will try to get back to this with a more detailed discussion, but the bottom line is that survival does not demand perfection, only that the species and its individual members be 'good enough'. (Please see Post #24 for some evidentiary support of this point.) The short term small changes to the environment would certainly be a factor in blurring out the meaning. of perfection and broadening the range of good enough.Again, thank you for your kind words and for the great job you and other moderators have done in making this the best conceived and managed forum on the web.Edited by AnswersInGenitals, : No reason given.