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A student asked me this the other day and I thought that I would ask it again here. If one organism is said to be "more evolved" than another, what exactly does this mean?

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Just to be clear: are you specifically asking what “more evolved” means or what “evolved” (generally) means? – Konrad Rudolph Apr 24 '12 at 13:08

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In the strictest sense, an organism can only be said to be more evolved than another when comparing both to an outgroup, such that there is an inferred most common ancestor in reference to which to make the comparison. In this case, one organism is more evolved if there are more changes to this organism than the other, relative to the reference point.

However, when speaking, many people get lazy, and use it as shorthand. More evolved is meaningless in the following contexts:

  • when there is no outgroup understood
  • when describing increasing complexity (obligate parasites have lost complexity and have had more evolutionary changes than their non-parasitic relatives)
  • when the outgroup is poorly chosen. Mammal vs reptile comparisons should not, in general, use prokaryotes as the outgroup.
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Can you suggest sources about evolutionary comparative methods? – Gustavo Bandeira Apr 23 '12 at 4:43
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I cannot improve on Thomas Ingalls' description of when "more evolved" is appropriately used, but the inappropriate/lazy use of the phrase is so prevalent that it deserves further comment. In my experience, the most common use of the phrase "more evolved" is when describing the increased complexity of one organism versus another. This usage is not just meaningless, but wrong and harmful, and springs from a misunderstanding of what evolution implies. Evolution is emphatically not the same as increased complexity.

I try to avoid saying "more evolved" and tend to favor "more complex", "less simple" or "less primitive." "Primitive" has its own problems, since it sometimes brings with it the connotation of evolution (a "primitive eye" or "primitive nervous system" are common phrases), but it at least avoids an explicit misuse of "evolved."

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“more complex” unfortunately suffers from similar problems. – Konrad Rudolph Apr 24 '12 at 13:10
@KonradRudolph Do you mean that "more complex" has the same problems as "more evolved"? Or rather that it has the same problems as "more primitive"? Can you expand? What phrase do you prefer? I think "more complex" is very near to what people actually want to say when they use the term "more evolved." It does come with a bit of a value judgment about what complexity means, but I have a hard time thinking of a phrase that has much less baggage. If one is careful to specify the dimension, I think it can be reasonable to discuss the relative complexity of different systems. – yamad Apr 24 '12 at 17:42
Well, the first question is: what is complex? Genome? Phenotype? Specifically, behaviour? And how do you measure complexity? Once you have agreed on that (but people don’t) – what does this actually tell us? In reality, not much. People just use it as they would “more evolved”. In reality, it makes much more sense to speak about specific features, not about an ill-defined, poorly-understood and not comparable whole. – Konrad Rudolph Apr 24 '12 at 21:05
@KonradRudolph I agree with you in general, and I was not advocating using "more complex" when comparing whole organisms. But I stand by using "more complex" when discussing specific features. It's true that complexity is a flexible ill-defined concept, but at least a speaker can specify how they are using the phrase. I think it's important to have some way to express the concept of relative complexity, which is meaningful if defined for a particular domain/feature. – yamad Apr 24 '12 at 21:37
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I have mostly seen it used to mean "possessing features that appeared most recently in the historical evolutionary record". Thus a human is considered more evolved than a chimp because our bipedality, hairlessness and large brain appeared more recently than the features of chimps, and a chimp is more evolved than a fish because fish very similar to ones alive today appear in the record before anything resembling an ape, and so on.

I think it's pretty poor use of language, frankly, since it suggests linearity and ordering to evolution that is simply not there, conflates gross morphological change with change in general and incorrectly implies that stabilising selection is not evolution.

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