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The origins of complexity

Life is complex. This is true not just in a rhetorical sense but also biologically in that living things are complicated arrangements of multicellular life. So how did this complexity arise? The most common explanation given is that it is a natural outgrowth of natural selection and that once a self-replicating molecule formed, the selection pressures would be such that given sufficient time multicellular life would be an almost inevitable consequence, though the exact form it would take would depend on many contingencies.

But via Machines Like Us, I came across this article by Carl Zimmer about a new study that suggests that complexity can arise as ‘neutral evolution’ even in the absence of having the kind of selection advantage required by natural selection. In other words, complexity can just happen and does not have to be pushed.

One question to be addressed is that of what exactly constitutes complexity and how it can be measured. The authors of the study suggest that it should not just be the number of cells that make up a organism but also the number of types of cells.

Our bodies are made of 10 trillion cells. If they were all of one type, we would be featureless heaps of protoplasm. Instead we have muscle cells, red blood cells, skin cells, and so on. Even a single organ can have many different cell types. The retina, for example, has about 60 different kinds of neurons, each with a distinct task. By this measure, we can say that we humans are, indeed, more complex than an animal such as a sponge, which has perhaps only six cell types.

The authors of the study have postulated a new law of biology.

Unlike standard evolutionary theory, McShea and Brandon see complexity increasing even in the absence of natural selection. This statement is, they maintain, a fundamental law of biology—perhaps its only one. They have dubbed it the zero-force evolutionary law.

They base their conclusions on 916 biological lines of laboratory fruit flies and find that by their measure laboratory fruit flies are more complex than wild ones, even though laboratory flies are subjected to less evolutionary pressures than those in the wild.

Other scientists question whether this conclusion is really warranted by their data and point out that fruit flies encounter selection pressures even in the laboratory.

An organism can exist without external selection—without the environment determining who wins and loses in the evolutionary race—but it will still be subject to internal selection, which takes place within organisms. In their new study, McShea and Fleming do not provide evidence for the zero-force evolutionary law, according to Erwin, “because they only consider adult variants.” The researchers did not look at the mutants that died from developmental disorders before reaching maturity, despite being cared for by scientists.

Another objection Erwin and other critics have raised is that McShea and Brandon’s version of complexity does not jibe with how most people define the term. After all, an eye does not just have many different parts. Those parts also carry out a task together, and each one has a particular job to do.

There is a lot more interesting stuff about the nature of complexity in Zimmer’s article.

Comments

  1. invivoMark says

    Of course, McShea and Brandon would have to explain to the rest of us why, if their “law” is true, do sponges still have “perhaps only six cell types.”

    After all, sponges have been subject to pretty much exactly the same amount of neutral evolution as humans.

    I don’t see this “law” being adapted by even a substantial fraction of biologists. This seems to me to be another version of the Dog’s Ass Plot.

  2. unbound says

    I don’t think most people would think of computer CPUs as anything but very complex…but they still only deal with 1s and 0s.

    @invivoMark – along the technology lines, why do we still have basic digital watches when we have smartphones that can do so very much more? Same response as to the question you posited that McShea and Brandon…digital watches still have their place in this world.

  3. Reginald Selkirk says

    The authors of the study have postulated a new law of biology…

    (roll-eyes). Biology doesn’t have many laws. That’s because it’s squishy. Evolution is about survival, and there aae many ways to survive.

    Sometimes life gets simpler. For example parasites may lose functions they no longer need because they are fulfilled by the host. Or cave fish may lose their eyesight. Etc.

    See also “left-wall saturation.”

    And simpler life forms are proably more likely to survive the occassional asteroid.

  4. invivoMark says

    But that is not at all analogous! Electronic devices are designed and created for specific purposes, and they do not undergo random neutral mutation and evolution. The lack of neutral evolution alone is enough of a reason to discard the analogy as irrelevant.

    The most dubious claim of McShea and Brandon is that differential complexity is explained by different amounts of neutral evolution. Yet this claim just isn’t supported by even a cursory glance at modern biodiversity. The organisms that are the most complex by their definition have undergone no more neutral evolution than simpler organisms. McShea and Brandon are almost guilty of committing the platypus fallacy, where they treat “simpler” animals as if they stopped evolving at the point where our lineage diverged from theirs. The whole point of this exercise, of course, is to support the notion that humans are the most highly evolved and complex of life forms (hence the similarity to DAPs).

    But if I had to pick the animal that has undergone the most neutral evolution, and I had to pick between humans and sponges, I’d probably pick the one that has existed in the same ecological niche and had roughly the same physiological shape and number and function of organs for hundreds of millions of years. So by their own argument, I’d predict a sponge to be more complex than a human.

    To be sure, I’m not arguing against the idea that neutral evolution leads to complexity. That’s certainly true, but it’s also trivial given a solid understanding of modern evolutionary theory. No well-informed biologist would be surprised at that claim.

  5. Mano Singham says

    The counter-argument about sponges does not quite work.

    The authors are challenging the idea that there must to be a selection advantage for complexity to emerge. All they need to show that is one example of neutral evolution and they have used one case of the fruit flies.

    They are not saying that neutral evolution is always faster than natural selection, which would be quite an absurd assertion and so the fact that sponges are less complex than humans is not an argument against their theory.

  6. Rob Grigjanis says

    So how did this complexity arise?

    It was inevitable once the universe’s temperature dropped sufficiently to initiate hadronization. Hadrons are horribly messy beasts, and anything you build from them will be as well.

  7. invivoMark says

    But as I said, Mano, that is a trivial claim that should not be surprising for any biologist. The fact is that they predicted complexity to arise faster in situations where there is less selective pressure, and that is a prediction that doesn’t hold up.

    I’m reading the Drosophila paper now. A couple of major problems stand out to me immediately. First of all, the ways they choose to measure complexity are inconsistent with the hypothesis they’re trying to support. Their “law” claims that genetic complexity increases under neutral selection, but they only measure morphological complexity. And they do so in strange ways: an eye that is horribly misshapen is considered to be more complex than a normal, functioning eye, for instance. But an eye can become horribly misshapen by dysregulation of gene expression, which is a definite loss of genetic complexity. The eye is misshapen because the fly has lost the ability to make a useful one.

    Second, their data are severely biased. They’re taking all the mutant strains bred in captivity over a hundred years, kept alive artificially, and comparing them to a single archetypal organism. But they’re only counting the lab strains that have been kept alive, and not those that have been discarded or that failed to survive. They haven’t measured wildtype complexity, either – they’ve only assumed that the wildtype hasn’t changed in a hundred years.

    But we actually know that this isn’t true. In the 1970s, geneticists noticed that a transposable element had been introduced into wild Drosophila, which wasn’t in the lab strains bred from a common population in 1905. This element, called the P element, has little effect on wildtype fly development or survival, because wildtype females also express a P transposase inhibitor. When females from lab strains are crossed with wildtype males, the P element can cause massive defects in offspring. This is a clear instance of a complexity gain in the wildtype organisms under selective pressure.

    Of course, the existence of the P element and P transposase inhibitor doesn’t negate the authors’ point. Nor do observations of eye loss in cave fish when contrasted with the evolution of extremely complex eyes in spiders and mantis shrimp, although they might inspire one to doubt the accuracy of their law. But I think their paper is massively flawed in any case, and I don’t believe they have demonstrated what they think they have.

  8. rq says

    Very interesting. Their theory makes a certain kind of sense. So, if I have it right, according to them, external selective pressure, no matter of what kind, forces a sort of ‘direction’ onto evolution, in the sense that any characteristics that are fatal in that specific environment will be weeded out rather rapidly. If there is no external pressure at all, or very little of it, then the organism is less constrained to remain within a specific framework in order to survive, and can branch out in many directions and diversity flourishes. If external pressure (of any kind) returns, many of the newly-mutated forms will be immediately destroyed, leaving behind only those with enough of a survival advantage (whether by mutation or ancestral form). A bottleneck of sorts. And things may start over again.
    (And the fruit fly example: because lab flies are well-fed in good conditions, they don’t have to waste resources on survival, and can afford resource-costly mutations, which would otherwise leave them disadvantaged in the wild.)
    Altogether, very interesting.

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