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As far as I understand, evolution is nowadays pretty much analyzed through phylogenetic trees, that is cladograms. These are constructed using the available records and taking some key structures and deduce whether species acquired or lost it, thereby generating a node. By this approach, chronology is of utmost importance, but chronological time is not. Hence, cladograms are not 'Darwinian' evolutionary trees as the latter are based on chronological time.

However, I cannot help myself when analyzing a cladogram and interpreting the accumulation of various nodes along a branch as moving through time. Isn't evolution all about time? At the risk of pitfalling myself into a 'this is a primarily opinion based question':

Isn't it just so that along a certain branch, each successive node has taken time to develop? Would it not be plausible to determine and assign a certain 'standard evolutionary time unit (a SETU if you like) representing the average time it takes for any organism to loose or acquire a certain trait, thereby allowing each node to be interpreted as time? Of course, the SETU would be a gross estimate, but could still potentially re-animate the Darwinian look on modern-day evolution.

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Let's start with a short comment. You say [..]evolution is nowadays pretty much analyzed through phylogenetic trees[..]. Many evolutionary biologist make extensive use of phylogenetic methods but a good part of evolutionary biologist (most of them I would say) do not work directly with phylogenetic methods.


I am not sure I understand your question but I hope the following may help you either to understand the issue or to edit your question. Note that I am NOT a phylogeneticist (not sure this word exist).

What is the definition of "Node" in Phylogenetics

I think your definition of a node might be unusual. In phylogeny, a node is the most recent common ancestor shared by two sister lineages.

Time in a phylogenetic tree

Also, when you look at a phylogenetic tree, the axis along which lineages diversify represent the time (real time in years). Now, we use different methods in order to make estimation of this real time such as the rate of neutral substitutions. You may want to read about molecular clock

each successive node has taken time to develop?

I am not sure what you mean by "develop" here. Develop usually means the process by which a given individual/organism change phenotypically through its lifetime. See Developmental Biology.

average time it takes for any organism to loose or acquire a certain trait

Again, I am not sure whether you talk about development or evolution here as you talk about "any organism" and not about "any population". In any case, an average time for a population to acquire a given arbitrary trait is a concept that 1) is valid only for discrete trait, 2) that is very dependent on the kind of trait you want to consider, 3) that will varies a lot from one given trait to another and 4) is extremely dependent on when you consider being the starting point of the evolutionary process toward the creation of the given trait. Therefore, it doesn't makes much sense.

Currently used "standard evolutionary time unit"

However, you can use similar concept for quantitative traits. The Darwin (d) is a unit of evolutionary change (invented by Haldane and named after C. Darwin). It is defined as the e-fold (e≈2.7) in a mean of individual's trait in a population over million of years.

Similarly, the Haldane is a unit of evolutionary change (I don't know who invented it and it is obviously named after JBS Haldane) and correspond to the number of standard deviation (of the distribution of individual trait in the population) change in the mean of individuals trait in a population per generation. The Haldane unit is therefore dependent on the selection pressure, the additive and non-additive genetic variance and environmental variance (therefore the heritability) and the mutational pressures.

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    $\begingroup$ ...phylogeneticist (not sure this word exist). I'm going to say that it does, and just means someone who practices phylogenetics. A better word might be taxonomist. $\endgroup$ – tel Dec 29 '14 at 16:18
  • $\begingroup$ Thanks tel for you approval! Chris, let us know whether the answer(s) address your question and answer it. $\endgroup$ – Remi.b Dec 29 '14 at 17:19
  • $\begingroup$ This is definitely very helpful. And it pretty much answers my question(s) as to how I should/can interpret cladograms, their frequency of use, and the meaning of 'time' in a cladogram. Cladograms are very abstract, but very helpful and this answer clarifies a lot about them. I must say that my perception of a 'node' was likely incorrect and I could re-phrase my question to make it more appropriate, but your answer is great and I leave my question as is, if that is OK with everyone :) $\endgroup$ – AliceD Dec 30 '14 at 10:43
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Something like what you describe does exist, and is called a molecular clock. The rate of change in certain genes and non-coding sequences has been correlated to other evidence (fossil record, etc.) in such a way as to allow you to estimate in absolute terms how long a given set of mutations would take to occur naturally.

Keep in mind that only certain genes have the required property of accumulating mutations at a more or less constant rate with respect to time. Most genes accumulate mutations in a more stochastic fashion.

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