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> The reason this happens is that the recessives that influence the one parent's trait are often different from the recessives for the other parent. Therefore t

by gwern 3y ago

> The reason this happens is that the recessives that influence the one parent's trait are often different from the recessives for the other parent. Therefore the child winds up expressing fewer recessives than either parent. That's wrong. Regression to the mean has nothing to do with recessives (or dominance, or epistasis), and exists with purely additive effects (and everywhere in statistics, for that matter, anywhere that a correlation is not perfect). It is simply because the phenotype has r < 1 with the genotype (ie. the trait is not 100% genetically determined), so when you select the 'high' organisms, they are lucky on both genotype and 'environmental' variables. The environmental variable goes away, however, and only the higher genotype variable persists. Since that was only a part of why the parents were 'high', the highness is not as high. > What this means is that if you select for a given complex trait in a given generation, those who survive will have that trait. But unless you also select for it in the next generation, the population will mostly revert back on its own. It is only regular selection, sustained over time, which causes the selected for trait to breed true. This is also wrong. The effect of the selection is permanent and has already 'bred true' and will persist indefinitely unless undone by some other force. It's a ratchet, not a rubber band.

wolverine876 3y ago
From the Stroud et al 2023 paper reviewed in the article: Species’ phenotypic characteristics often remain unchanged over long stretches of geological time. Stabilizing selection—in which fitness is highest for intermediate phenotypes and lowest for the extremes—has been widely invoked as responsible for this pattern. But, say Stroud et al: However, even with an explosion of microevolutionary field studies over the past four decades, evidence for persistent stabilizing selection driving long-term stasis is lacking. Stround & co's answer: Here, by directly measuring natural selection in the wild, we identified a complex community-wide fitness surface in which four Anolis lizard species each occupy a distinct fitness peak close to their mean phenotype. The presence of local fitness optima within species, and fitness valleys between species, presents a barrier to adaptive evolutionary change and acts to maintain species differences through time.
- gwern 3y ago
  Indeed. As I said, "The effect of the selection is permanent and has already 'bred true' and will persist indefinitely unless undone by some other force."
  - wolverine876 3y ago
    I gotta say, I don't quite grok your statement. Evolutionary biologists say selection effects are high in the very short-term, but very low in the long term - many creatures are basically the same for millions of years. That's the 'paradox of stability'. I don't understand how your statement explains the long-term stability part. From the Quanta article: With enough time, even the tiniest tugs should yield a measurable shift in an organism’s observable characteristics. If the beak-size changes the Grants observed [over a few years] continued over millennia, back-of-the-envelope calculations predicted some extreme phenomena, Pennell said. “You’d expect finches that were, like, 40 kilograms. This just makes no sense.” How does your theory not produce 40 kg finches? And in fairness, I sure don't have a theory other than what I read, so it's asked in the spirit of curioisty.
    - gwern 3y ago
      I don't understand your confusion. I thought the Quanta article did a perfectly reasonable job of explaining the difference between stabilizing selection for a single fixed, permanent, unaltered optimal phenotype, and having simply a bit of selection randomly different each year which nets out to no change. Lots of random changes average out to nothing, but each change was still a change. If you ratchet something a little bit one time, then ratchet it the other way randomly a bit, etc, it is still ratcheting each time, but it's not going to produce 40 consecutive clockwise spins.
      - wolverine876 3y ago
        I guess my interpretation is that the biologists say it does not average out and we would end up with 40 kg finches, and that another mechanism is needed to explain it. They used to think that mechanism was stabilizing selection, "fitness is highest for intermediate phenotypes and lowest for the extremes", but the evidence doesn't support it. As far as I can tell, the article omits the mechanism. Stroud's paper describes it thusly, though to me it doesn't seem so different from stabilizing selection, other than maybe the variations aren't smoothed out as immediately (from the abstract; in the body Stroud describes other theories as valid, but I haven't had time to sort it out): The presence of local fitness optima within species, and fitness valleys between species, presents a barrier to adaptive evolutionary change and acts to maintain species differences through time. However, instead of continuously operating stabilizing selection, we found that species were maintained on these peaks by the combination of many independent periods among which selection fluctuated in form, strength, direction, or existence and in which stabilizing selection rarely occurred. I'm getting the impression that the significance of Stroud's paper might be less the mechanism, and more that stability occurs on 'milli-scales' (my term), the short period of Stroud's data, in contrast to high variability on micro-scales (as shown in the Grants' finch research) and the well-known stability on mega-scales (millions of years). The article returns many times to the value of Stroud's long-term, high-quality data collection. Because he had followed four species for three generations, he was able to show that a long-term pattern of stasis could emerge from such short-term fluctuating selection. But I don't see how that differs from the Grants's finch data, which was over more time and was at least two generations, and I think more. Also they say: Fully resolving the paradox will require scientists to study time spans between macro- and microevolution, Porto said — on the scale of tens, hundreds or thousands of years. They need to find a sweet spot that is long enough to allow both change and stasis to emerge, he said, although at the moment biologists don’t have a long enough data set to draw from. Anyway, I am happy to keep talking, but I need to read something else today - something that I get paid to read! :)

by gwern 3y ago

wolverine876 3y ago

From the Stroud et al 2023 paper reviewed in the article: Species’ phenotypic characteristics often remain unchanged over long stretches of geological time. Stabilizing selection—in which fitness is highest for intermediate phenotypes and lowest for the extremes—has been widely invoked as responsible for this pattern. But, say Stroud et al: However, even with an explosion of microevolutionary field studies over the past four decades, evidence for persistent stabilizing selection driving long-term stasis is lacking. Stround & co's answer: Here, by directly measuring natural selection in the wild, we identified a complex community-wide fitness surface in which four Anolis lizard species each occupy a distinct fitness peak close to their mean phenotype. The presence of local fitness optima within species, and fitness valleys between species, presents a barrier to adaptive evolutionary change and acts to maintain species differences through time.

gwern 3y ago
Indeed. As I said, "The effect of the selection is permanent and has already 'bred true' and will persist indefinitely unless undone by some other force."
- wolverine876 3y ago
  I gotta say, I don't quite grok your statement. Evolutionary biologists say selection effects are high in the very short-term, but very low in the long term - many creatures are basically the same for millions of years. That's the 'paradox of stability'. I don't understand how your statement explains the long-term stability part. From the Quanta article: With enough time, even the tiniest tugs should yield a measurable shift in an organism’s observable characteristics. If the beak-size changes the Grants observed [over a few years] continued over millennia, back-of-the-envelope calculations predicted some extreme phenomena, Pennell said. “You’d expect finches that were, like, 40 kilograms. This just makes no sense.” How does your theory not produce 40 kg finches? And in fairness, I sure don't have a theory other than what I read, so it's asked in the spirit of curioisty.
  - gwern 3y ago
    I don't understand your confusion. I thought the Quanta article did a perfectly reasonable job of explaining the difference between stabilizing selection for a single fixed, permanent, unaltered optimal phenotype, and having simply a bit of selection randomly different each year which nets out to no change. Lots of random changes average out to nothing, but each change was still a change. If you ratchet something a little bit one time, then ratchet it the other way randomly a bit, etc, it is still ratcheting each time, but it's not going to produce 40 consecutive clockwise spins.
    - wolverine876 3y ago
      I guess my interpretation is that the biologists say it does not average out and we would end up with 40 kg finches, and that another mechanism is needed to explain it. They used to think that mechanism was stabilizing selection, "fitness is highest for intermediate phenotypes and lowest for the extremes", but the evidence doesn't support it. As far as I can tell, the article omits the mechanism. Stroud's paper describes it thusly, though to me it doesn't seem so different from stabilizing selection, other than maybe the variations aren't smoothed out as immediately (from the abstract; in the body Stroud describes other theories as valid, but I haven't had time to sort it out): The presence of local fitness optima within species, and fitness valleys between species, presents a barrier to adaptive evolutionary change and acts to maintain species differences through time. However, instead of continuously operating stabilizing selection, we found that species were maintained on these peaks by the combination of many independent periods among which selection fluctuated in form, strength, direction, or existence and in which stabilizing selection rarely occurred. I'm getting the impression that the significance of Stroud's paper might be less the mechanism, and more that stability occurs on 'milli-scales' (my term), the short period of Stroud's data, in contrast to high variability on micro-scales (as shown in the Grants' finch research) and the well-known stability on mega-scales (millions of years). The article returns many times to the value of Stroud's long-term, high-quality data collection. Because he had followed four species for three generations, he was able to show that a long-term pattern of stasis could emerge from such short-term fluctuating selection. But I don't see how that differs from the Grants's finch data, which was over more time and was at least two generations, and I think more. Also they say: Fully resolving the paradox will require scientists to study time spans between macro- and microevolution, Porto said — on the scale of tens, hundreds or thousands of years. They need to find a sweet spot that is long enough to allow both change and stasis to emerge, he said, although at the moment biologists don’t have a long enough data set to draw from. Anyway, I am happy to keep talking, but I need to read something else today - something that I get paid to read! :)