PZ Myers has a post on the evolutionary origins of complexity:
αEP: Complexity is not usually the product of selection
I find it frustrating that people talk about terms that they don't clearly define, and assume everyone agrees on. Especially about complexity, which is hard to define, and I know not everyone has the same idea of. I'll just quote my comment on Pharyngula:
But, you have not quantified complexity, let alone say when there was an increase in it in the hypothetical example you give. If you don’t do this, you can’t talk about the evolution of complexity; it becomes a guessing game what we are talking about, and there is no chance that everyone will think of complexity as the same thing.
On top of that, there seems to be no distinction made anywhere between ‘complexity’ and ‘complex traits’. They need not be the same thing. Without defining complexity here(!), I’ll say that complexity can indeed easily arise by neutral processes, whereas complex traits cannot (but does have neutral and random processes involved) – it requires selection. And with that you’re going to ask me for a definition of a trait, so here is one: A single measurable component of the phenotype that has a function.
The key word here is function, without which I don’t know of any that can evolve without selection. Not selection every step of the way, as random processes are required (at least that’s how it occurs in nature), but selection at some point. The moment the trait acquires function, it becomes selected for.
On the other hand, ‘genomic complexity’ may not describe the state of a trait, but rather is the idea that the genome has many components that are intricately connected – which can arise by neutral processes.
Let's consider a simple filamentous organism. Cell division always occurs in a single plane perpendicular to the long axis of the filament. If a mutation occurs in those genetic instructions that control the orientation of cell division so that every now and again the plane of division becomes parallel with the long axis, the filament now branches and has a new growth form. The organism with this new form is subject to selection, but the trait did not arise by selection. Somehow I fail to grasp the distinction between complexity and a complex trait.
ReplyDeleteWhat is a trait? Is it any aspect of the phenotype, or does it have to have to do with function. What I am saying very specifically is that complex function cannot arise without selection (with random variation fueling it, of course). If, on the other hand, a trait is anything one can identify in a phenotype, such as the direction of filaments, then I agree that such traits can increase in complexity without selection.
ReplyDeleteAlso, but do think about how to quantify complexity. How can we compare traits? Is it really meaningful to say that in your example that complexity has increased (in a quantitative way)? Perhaps it is.
According to the definition in your 2nd paragraph, a trait necessarily has a function, but your 3rd seems to assume there can be functionless traits. Which is it?
ReplyDeleteIf the former, the claim that complex traits are necessarily selected becomes more or less vacuous because traits are by definition functional and function is necessarily selected (which one might quibble with - you don't define "function" - but your 3rd paragraph presumes it).
Andreas, you're right, it is not clear to me what a trait is. What I could say is that I have trouble believing that function can arise without selection (regardless of what we call a trait). And you are also right to ask for a definition of function. I'll have to think about it.
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