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I have conversations with evolution deniers, and I've read that the best single piece of evidence for evolution is the congruence of the genetic and morphological nested hierarchies, and the fact that they are predicted on evolution, but not on intelligent design.

I think I have an example of what would falsify the nested hierarchy; please check me on this, I want to get this right.

Because evolution is a historical process, an organism can only share characteristics, or have varied characteristics, of one of its ancestors. If an organism had characteristics that came laterally (?) across the nested hierarchy, as opposed to up or down through its ancestry, that would falsify the hierarchy. An example of this would be a primate with two post-orbital fenestrae (see pic).lineage pic.

And, we don't find such organisms.

This is not predicted on design because if you design from scratch, without the necessity of varying ancestors' characteristics, you are not limited by not designing a primate with two post-orbital fenestrae.

Do I have this right?

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Your view of evolution is wrong

If an organism had characteristics that came laterally (?) across the nested hierarchy, as opposed to up or down through its ancestry, that would falsify the hierarchy

The expression "falsfying the hierarchy" does not mean much, so I won't comment on it.

Note two things

1) Convergent evolution - It is not so uncommon that different lineages evolve similar traits when facing similar selection pressures. This is called convergent evolution.

2) Horizontal transfer - It is not so uncommon that pieces of DNA find itself (via a number of different mechanism) in a different lineage. A gene can therefore jump from one lineage to another one. This causes the gene tree of a given gene to not match the species tree. Other processes can cause such mismatch though. This is called horizontal gene transfer.

So, in short, no your view of what you call "impossibility in the nested hierarchy" is wrong.

Intelligent design

Intelligent design is not a scientific hypothesis. It means that it does not make logical sense and it does not make clear predictions that can be tested. By contrast, the theory of evolution is a set of hypothesis all of which that have been formally tested have shown to be confirmed by evidence (that's why the theory of evolution is called a theory; see here for the semantic).

It is therefore impossible to say whether intelligent design is correct or wrong because it does not say anything. One cannot compare the predictions of the theory of evolution with the ones of intelligent design as intelligent design does not make clear predictions. One can only argue why the theory of evolution is a set of (confirmed) hypotheses while intelligent design is not a hypothesis (or set of hypotheses).

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While evolution generally predicts a nested hierarchy for shared traits of organisms, it's not generally that simple - there are some cases that can lead to exceptions of strictly nested hierarchy.

The first (obvious) one is convergent evolution: if there is an easy biological 'solution' to a certain problem (caused by the environment), then evolution will often find that solution independent where it started from. A good example for this are back fins in fish and marine mammals, which are very similar but not actually related. Of course this is no argument against evolution, it's just a bit more complicated to explain, why different evolutionary starting points can lead to (seemingly) the same outcome.

A second possible exception is caused by the fact that organisms do not just gain new traits during evolution, they can also loose them (if they don't serve any purpose any more).
A very good example for this are teeth: sharks have them, some fish have them, mammals have them, as do many reptiles, but birds generally don't have teeth. Still this is no argument against evolution - first because birds don't need teeth so why should they keep them? Secondly, the genetic program to make teeth is not absent or missing in birds - its just disabled: when researchers tinkered around with this, they managed to make chicken with alligator (or dinosaur) like teeth

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  • $\begingroup$ Nicolai, thanks very much. Aside from those exceptions like the ones you mentioned, would you say that my example is a good illustration of the basic idea of why the nested hierarchy is predicted by evolution? $\endgroup$ – Slothrop Aug 29 '17 at 14:42
  • $\begingroup$ @Slothrop yeah your example is definitely right. Generally you expect a new trait formed at a junction in the taxonomic tree to be present in most species that are descendent from this point. $\endgroup$ – Nicolai Aug 29 '17 at 14:49
  • $\begingroup$ As an add-on, genetic evidence is often an excellent tool in cases of convergent evolution that are not as clear-cut. In most cases, convergent phenotypic solutions come with dissimilar genetic ones. $\endgroup$ – Bryan Krause Aug 29 '17 at 15:45

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