Thursday, March 14, 2019

A question about evolution

According to the theory, as I understand it, humans evolved from critters like tarsiers. Is there an upper limit on the degree to which an organism can be scaled up given the original information base? For instance, would any amount of selective breeding produce mice the size of horses? Or would that require the infusion of new genetic information? Just in terms of the difference in scale between humans and tarsiers, is it possible for humans to evolve from tarsier-like critters without the infusion of new information?


  1. One of the factors overlooked by many is the number of generations a genetic changed needs before being fixed in a population. Also mammals breed slower so the ability for mutations to accrue relative to the fossil record time doesn't work.

  2. Not sure if it's exactly what you're looking for, but Haldane's Dilemma purports to show that there's an upper limit to the amount of mutations that can occur in a given period of time:

  3. I think there are at least two starting points from which can tackle this question, though in the end both cover much the same ground (or at worst complement one another):

    1. The perspective of the organism itself.

    a. Given current science (e.g. biomechanics) and mathematics (e.g. the cube-square law, the surface-area-to-volume ratio), I would argue there are definite upper limits. For example, there are both intrinsic as well as extrinsic factors which limit body mass. An intrinsic factor might be an organism's body plan or physiological processes, while an extrinsic factor might be the organism's environmental conditions.

    b. Other examples include Kleiber's law and Haldane's principle. JBS Haldane was the Richard Dawkins of his day except Haldane was far more intelligent than Dawkins. In fact, Haldane's principle comes from his famous essay "On Being the Right Size", which is likewise relevant to this question, I think.

    c. That said, if (say) environmental conditions change, then perhaps it's possible for the upper limits to change to some degree. Take environmental conditions in the Mesozoic Era (e.g. the Jurassic period) when dinosaurs existed. Or take water animals (e.g. whales) in comparison to land animals (e.g. elephants).

    d. However, I would tend to think even upper limits have their upper limits. That is, maybe the "ultimate" upper limit for organisms would be the Earth as a whole. Although, I suppose, we might have a thought experiment where the whole of creation including every creature could simultaneously scale up in the exact same ratio. In this respect, it's more about objects and relations within a system (e.g. Earth, the universe) being kept in the same or at least similar enough proportions to one another than ipso facto about scale itself if by scale one means sheer scale in isolation to other variables. At least that's my thinking at this point.

    2. The perspective of the organism's genetics.

    a. I think one could use an organism's genome to build a bigger version of that organism, but again with upper limits.

    b. Among other things, this would enter into questions related to an organism's body plan. Such as: how do genes control the development of an organism's structural features (e.g. the morphogenesis of limbs, the distribution of symmetry across an organism)? Short of providing supporting argumentation, for that would take too much time right now, I would want to say an organism's genome and genetics have their limitations too.

    c. By way of example, I thnk it's possible to genetically alter and create a mouse the size of a horse, but that might be lethal or fatal for the horse-sized mouse. On the one hand, smaller scale genetic changes would arguably be insufficient to create a horse-sized mouse. On the other hand, larger scale genetic changes would arguably be detrimental to the horse-sized mouse's survival.

    d. Another example is dogs. People could and have genetically altered (via breeding, but today it might arguably be via CRISPR/Cas9) an ancestral wolf to create different dogs of different sizes, from chihuahuas to great danes. However, as a result of their great size, my understanding is great danes tend to live shorter lifespans than most other dogs, suffer from issues related to its larger skeletal frame like hip dysplasia, and commonly have heart conditions such as cardiomyopathy (enlarged heart). So I think we could genetically alter dogs to become bigger than great danes, but I don't imagine these dogs could be much bigger since great danes seem to be pushing the upper limits for dogs as it is.

    1. To be fair, perhaps one could imagine dogs splintering into an entire new species which is bigger than dogs. In that case, an influx of new genetic information could account for the speciation. However, in that case, it would seem a dog wouldn't be a dog anymore, but something else.

      To be fair (again), the boundaries of species can be a bit hazy at times. Perhaps it's better to talk about biblical kinds (a la Todd Wood).