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I recently read The Selfish Gene by Richard Dawkins, which I found very interesting. In one of the last chapters, he gives multiple possible explanations to the question "Why did natural selection give rise to complicated lifeforms?".

Since complex (say: multi celled) life exists, that means natural selection at some point did increase complexity, as we went from the primal soup to multi celled organisms (with many steps in between). My question is whether this trend of increasing complexity is still going on? Is natural selection a never-ending arms race, tending towards increasingly complicated genomes? Or did natural selection reach some plateau, local optimum, of biological complexity, after which the only evolution taking place is "on the same level".

I have thought a bit about a metric for complexity, but since I have no background in biology I am not at all certain. At first thought, the length of the genome sounds like a promising candidate, but the phenomenon of freeloading, not-functioning DNA (conveniently described in The Selfish Gene too) undermines this. I hope it is clear what I mean by "complexity".

Thanks for answers in advance, it has been fun speculating.

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    $\begingroup$ Hi Lourens & welcome to Bio.SE! This is an interesting topic. To make your question more answerable, try to focus in on a specific question. If you could start with your definition of "complex-life" and ask a question on what you don't understand about the definition. For example, your current definition is simply how "multicellular" an organism is. Is a blue whale more complex than a human? $\endgroup$
    – James
    Jan 13, 2020 at 10:15
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    $\begingroup$ On a side note, the Selfish gene is >40 years old. We understand far more about genes now than we did then. For example, even the definition of "non-functioning" DNA is problematic; wikipedia describes this evidence of functional "junk" DNA. $\endgroup$
    – James
    Jan 13, 2020 at 10:19
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    $\begingroup$ Answering that last question depends intimately on a definition of complexity. I'm not quite sure where people get so interested in complexity and evolution, but questions similar to this have been asked numerous times here and they have all failed (as far as I recall) to give any operational definition of complexity that can help answer the question. $\endgroup$
    – Bryan Krause ♦
    Jan 13, 2020 at 13:44
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    $\begingroup$ Possible duplicate Is variation a result of Evolution? or biology.stackexchange.com/questions/42050/… $\endgroup$
    – John
    Jan 13, 2020 at 14:58
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    $\begingroup$ Sure, a human seems to be more complex than E.coli. We would start answering this question by trying to measure & quantify the complexity of these organisms. Is the complexity the number of cells, something anatomical like the number of appendages, environmental adaptability, variety of hats, number of coding genes, or sequence complexity? Only then can we discuss if that specific feature appears to be rising, falling, or plateauing in the biosphere. Without these specific exact definitions, this question cannot be answered completely. $\endgroup$
    – James
    Jan 13, 2020 at 15:29

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There are multiple definitions of complexity, ranging from algorithmic complexity to computational complexity. In biology, I'd say that algorithmic complexity (Kolmogorov complexity) is most relevant. Unfortunately, we don't yet know enough about genetic circuits and all the other things that go on at the subcellular level even to make plausible estimates of an organism's complexity.

If the complexity of a human being were analyzed all the way from physiology down to the subcellular & molecular level, I think it would be apparent that most of the complexity is present at the subcellular & molecular level. In fact, essentially all of the complexity of a multicellular organism is encoded at the subcellular level, so it's appropriate to say that's where the complexity resides.
It is not entirely clear that a human being, whose genome contains ~3 billion base pairs, is more complex than a protozoan Amoeba Proteus whose genome is ~34-43 billion base pairs.

All of the processes that have been active in evolution since the beginning are still active, and changes in complexity - regardless of the definition of complexity - are still going on.

The closest things there is to a "plateau" is a relatively static distribution of genotypes that can last only as long as the adaptive landscape is unchanged. However, the adaptive landscape is always changing because of climate changes, plate tectonics, invasive species, disease, intra-species iteractions, and even because of chance fluctuations in the distribution of genotypes.

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    $\begingroup$ thank you for your answer. As @James also pointed out in a comment, my question fails by the definition of complexity. Not only that, the abstract idea of complexity I had in mind seems flawed after reading your answer, since I was clearly biased towards the human scale. Your fact about the amoeba dubia's genome seems to be outdated, although your point still stands. Correcting for the measurement error gives the indirect estimate of 67 billion base pairs. bionumbers.hms.harvard.edu/bionumber.aspx?id=104470&ver=6 $\endgroup$
    – Lourens
    Jan 18, 2020 at 12:13

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