tag:blogger.com,1999:blog-1812431336777691886.post2402876037656727401..comments2024-02-29T03:57:00.088-05:00Comments on The Mermaid's Tale: Genes that ensure a long life? Not likelyAnne Buchananhttp://www.blogger.com/profile/09212151396672651221noreply@blogger.comBlogger11125tag:blogger.com,1999:blog-1812431336777691886.post-58361786502878281832011-10-21T07:47:45.082-04:002011-10-21T07:47:45.082-04:00Just to finish up. The effect of selection is meas...Just to finish up. The effect of selection is measured by a selection coefficient, call it 's', relative to a given genetic variant, compared to other variants in the same population. Relevant facts are how systematic or steady that 's' value is, how small the population is, how much genetic identity (kinship) there is among the members, what the mating pattern is in this regard, the average number of offspring per parent, the social dynamics of the effect (e.g., whether directly or indirectly elders help other grandchildren as well as their own), and so on.<br /><br />A parent is related by half to each of its children, but only by a quarter to each grandchild. That reduces the relative impact of any gene related to longevity leading to grandparenting.<br /><br />Reproduction is highly probabilistic, and it's just the mathematics of probability by which in a small group, small selective effects have only about the same chance of resulting in fixation (or loss, if the effect is negative) relative to what would happen by drift alone.<br /><br />If in one generation an allele helps advance the reproductive success of a grandchild because of its systematic effect on the longevity of the grandparent, it could (for example) have the opposite effect in other times, such as of resource shortage, and actually hamper the grandchild's prospects.<br /><br />The situations are complex, but since humans are not really all that longevous relative to other species in protective environments like ours, there is not that much (if any) effect to explain.<br /><br />Mathematical simulations (that I've been aware of, at least) have had to specify rather specific situations for this to work as an effective selective force. That doesn't make it impossible, but the effect of grandparenting (or analogous contributions of elders) could work in purely cultural ways, or some have argued in group-selection ways. <br /><br />And the potential effectiveness for staying healthy up to, roughly, menopausal ages to rear slow-growing human offspring are mathematically much larger and thus, if any selective explanation is needed, much more likely to account for calibrating our life histories.<br /><br />At least, these are roughly my reasons for holding the view on this that I do.Ken Weisshttps://www.blogger.com/profile/02049713123559138421noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-79921703116358284742011-10-20T22:54:39.630-04:002011-10-20T22:54:39.630-04:00Why wouldn't a selective effect still work on ...Why wouldn't a selective effect still work on the face of drift, I wonder just more slowly, maybe much more slowly. But as you say, we can agree to disagree on this point, there is so much more we obviously agree on anyway.Erick Howenstinehttps://www.blogger.com/profile/17623471996256446298noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-43328537008191202432011-10-19T12:47:38.774-04:002011-10-19T12:47:38.774-04:00The poor woman, she forwent all the fun in life, a...The poor woman, she forwent all the fun in life, and then had to live to 115! As for Ozzy Osbourne, probably somatic mutations would be of more interest!Anne Buchananhttps://www.blogger.com/profile/09212151396672651221noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-1455821458491901282011-10-19T12:13:32.639-04:002011-10-19T12:13:32.639-04:00I happened to be in that talk and there were a cou...I happened to be in that talk and there were a couple things brought up that weren't covered much in the published accounts. The dementia-free/plaque-free claim was bolstered by some specific brain function tests where she performed 30-70 years better than expected for age.<br /><br />The most problematic aspect, to me, was that she maintained a lifestyle that would be most likely to increase lifespan for anyone. She didn't drink, smoke, or have children. She kept her brain active. She had been planning to devote her body to science upon her death since the 1950s and kept up on popular science literature. While she may have some genetic variants that kept her alive, the lifespan tradeoffs that may or may not have to do with having kids or a heavy disease burden were not influencing her longevity. <br /><br />Personally, I think looking at people like Ozzy Osbourne (also sequenced) is more informative as he has done many many things that should have killed him by now.EllenQhttps://www.blogger.com/profile/04805375871164019260noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-15683190001798671522011-10-19T07:00:57.444-04:002011-10-19T07:00:57.444-04:00I was not referring to group selection, but to the...I was not referring to group selection, but to the idea that in small demes these kinds of things like grandparent babysitting are done not just for close relatives. The point about drift is that when the selective advantage is very small in a small group, chance can be as important as that advantage in terms of the actual future fate of the responsible genes. This is just the mathematical consequence of genetic sampling. <br /><br />Anyway, these are long-standing differences of viewKen Weisshttps://www.blogger.com/profile/02049713123559138421noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-13238817695003208702011-10-18T23:16:18.576-04:002011-10-18T23:16:18.576-04:00No not the group, the elders would be helping kin ...No not the group, the elders would be helping kin specifically. I rarely see group selection, though I try. But in addition to the baby sitting, gathering, care and knowledge they may provide, care of elders may result from the general growth of empathy, which would have other more direct bemeficial effects. <br /><br />But care won't make them live longer, it's the benefit of the elderly, not benefit to the elderly which would matter. So it is a mystery, still. However I don't see the countervailing force of drift. Would you explain?<br /><br />BTW my ref on the birds, I believe is Nick Lane's Power Sex. SuicideErick Howenstinehttps://www.blogger.com/profile/17623471996256446298noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-84201059654743542182011-10-18T19:57:55.883-04:002011-10-18T19:57:55.883-04:00The survival information you refer to arguably wou...The survival information you refer to arguably would accrue to the group, not to the individual, and it has to have sufficient impact on fitness to overcome genetic drift, to be of any import. Also, in the past, if you mean the 'indigenous' long pre-agricultural past, which is the vast majority of our evolutionary history as a species distinct from others, the fraction of people who lived to really old age would have been very very small. That raises the relative impact of genetic drift vis-a-vis selection. And opposed to this is the burden that caring for the elderly imposes. There are many reasons why that could be far greater than the benefit of elders' wisdom. If anything, I would guess that baby-sitting services would be the greatest factor. But elders seem often to have baby-sat for multiple children whose mothers were out gathering. So personally, at least, I find the longevity evolution arguments to be forced.<br /><br />As to birds and some other examples,they have long raised questions about simple, or simplistic, models of the biology and evolution of aging and lifespan. Those questions aren't really answered, as far as I know, but I haven't worked directly in aging and its evolution for a number of years, and am undoubtedly out of date (if not too old).Ken Weisshttps://www.blogger.com/profile/02049713123559138421noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-65609668637277027702011-10-18T19:18:54.417-04:002011-10-18T19:18:54.417-04:00Problem almost boils down to a sample size of 1. ...Problem almost boils down to a sample size of 1. However, in the past, before written record, the elderly would have provided useful information with survival benefits. And birds are also small like mice, but can live 100 years. It has to do with metabolism required for flight, I've read.Erick Howenstinehttps://www.blogger.com/profile/17623471996256446298noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-77582909266270519272011-10-18T08:27:30.368-04:002011-10-18T08:27:30.368-04:00Thanks, Holly. Agreed, "DNA did it" can...Thanks, Holly. Agreed, "DNA did it" can be as faith-based as "God did it".Anne Buchananhttps://www.blogger.com/profile/09212151396672651221noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-17641710250974463262011-10-18T08:23:12.551-04:002011-10-18T08:23:12.551-04:00'Science', the western-culture-derived met...'Science', the western-culture-derived method of understanding Nature from a materialistic perspective, is not good at one-off events. It grew out of induction--generalizing from repeated observations of similar cause-effect pairings. <br /><br />In the 19th and 20th centuries, statistical reasoning was introduced as the formal basis of much of 'the scientific method'. Statistical reasoning is based on probabilities, another kind of repeatability or induction-based concept.<br /><br />At least we can hope to construct hypotheses based on one-off observations---if we can set up situations where we can observe the 'same' event somehow (like the effect of some particular variant we may see in the Aged Lady.<br /><br />Rearing offspring contributes to reproductive success in many ways, but that isn't the same as fitness if it is not based on genes. The effect of parenting is often taken into account indirectly by estimating fitness as the number of offspring who reach adulthood. <br /><br />The older you are the less your net or average effect of your genotype on the future fates of the variants you carry, which is why the 'grandmother hypothesis', that we live a long time because we can care for our g'children and hence enhance their fitness, is problematic.<br /><br />There were tons of research done to try to explain how selection could favor people living well past fertility (if that's what your question referred to). The upshot is related to my previous point. Selection favoring the survival of the hoary is very weak, if it works at all.<br /><br />Fortunately, there is little problem here because humans are not very different, for their body size, from other mammals in terms of their longevity. For what we were doing in the past, maybe in terms of how rapidly or not we grow to adulthood (or how immature at birth), our life-clock evolved appropriately. Likewise, there was no 'need' for mice to live more than they do (about 2 years), and they are on the same body-size/longevity line as we are.<br /><br />This has been a point very difficult for people on the NIH gravy train or anthropologists wanting some exceptionalism for humans, to digest.Ken Weisshttps://www.blogger.com/profile/02049713123559138421noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-16172113528167554942011-10-18T08:11:33.133-04:002011-10-18T08:11:33.133-04:00This is a terrific post.
Makes me wonder if ther...This is a terrific post. <br /><br />Makes me wonder if there's any difference between explaining a one-off like this with "DNA did it" as opposed to with "God did it."<br /><br />Also, rearing offspring contributes to fitness so traits can be selected for post-reproduction for sure. How far can selection reach down the road past parenthood and towards death? That's the question to my mind. (Not really relevant at all for Methusalady, just thinking about longevity.)Holly Dunsworthhttps://www.blogger.com/profile/05260104967932801186noreply@blogger.com