Homology and embryonic development

I imagine this is a simple one, the article is brief. Most land vertebrates/marine mammals have the same homologous pattern I the arm/wing/flipper. I’d have expected the embryonic pathways (the genetic systems that control their growth) to be the same in all those species. The article says:

“The vertebrate forelimbs show a common design, but develop in different ways, supporting the view that the different vertebrate kinds were independently created. If these two amphibian groups had really arisen from a common ancestor, the embryological pathways would not be expected to show such divergent characteristics.”

Any thoughts on this please?

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Well if it really was exactly the same, it should result in the exact same structure and thus there would be no differences. We’d all just have the same limbs. So no, if we have different limbs then genetically something must be different too of course. Exactly how they are different genetically can’t be answered a priori, except to say that we generally expect the patterns in the similarities and differences in genetics to be consilient with the morphological pattern.

Consilience of independent phylogenies.

It’s too vague to be meaningful. In what ways are their development different? How divergent is “such divergent characteristics” implied to be?

And the idea that some characters are divergent doesn’t somehow automatically support independent creation, particularly when creationists will also assert that similar characters are evidence of “common design”. They’re here trying to have their cake and eat it too. Things are similar because of common design, and dissimilar due to… uncommon design? Heads they win, tails we lose. Creationism isn’t a default explanation. The default is not knowing how something originated, and each independent hypothesis needs to explain the observations and patterns with a predictive theory. Creationism has no such theory, only ad-hoc rationalizations. Common descent predicts consilience of independent phylogenies.

You would have to cite or link the article first.

This paper goes a long way to answering how the differences in developmental polarity of salamander versus other tetrapod limbs evolved: Genetic basis for an evolutionary shift from ancestral preaxial to postaxial limb polarity in non-urodele vertebrates - ScienceDirect


To make my question clear I’ll quote part of an article from Biologos about “deep homology”:

“ … look at the animal family tree shows that there are organisms “between” insects and tetrapod’s that don’t have structures comparable to either limbs or insect legs. In other words, fins and insect legs each developed in different organisms, who originally had no appendages at all. The two branches arose independently, as near as we can tell, and so fly legs and mouse paws are not homologous. But, shockingly, the genetic systems that control their growth are strikingly similar . Fly leg development is controlled by Hox genes. More amazingly, fly leg development is controlled by many of the same Hox genes that perform similar tasks in mice. Fly legs are not homologous to mouse paws, but it looks like the genetic systems that control their development are, to a large extent, homologous.”

First of all, the fact that the same genes are responsible for legs in in insects and legs in tetrapods despite them being so different is used as evidence for common decent. Which I get. When it comes to homologous structures such as the limb of almost all mammals this is said to be evidence of common decent because the same pattern - one bone, two bones, blobs - is present across almost all land vertebrates (I kind of get this argument, but isn’t the order necessary? So that a creationist would argue it had to be that pattern because two bones together is necessary as it makes the arm stronger, one bone would make weak?). But it appears that the creationist article is saying that the genes are responsible for limbs isn’t the same in all land vertebrates. If so then they may appear homologous, but really they come from separate genes. Which means the similarities are superficial.

First off, I don’t think BioLogos is quite right. What Hox genes do is tell cells where they are, mostly their position along the long axis of the body. They don’t actually have instructions for legs; they just say “this segment is the prothorax” and such. What happens in the prothorax is up to other genes. (Pax6, similarly, doesn’t build an eye; it just says, the eye goes around here.)

Second, your link doesn’t work.


This has the quote. Limb design homology - creation.com

Thanks. This appears to be the relevant point:

Unfortunately, there is only one citation to the primary literature: Fröbisch, N.B. and Shubin, N.H., Salamander limb development: Integrating genes, morphology, and fossils, Developmental Dynamics 240 :1087–1099, 2011.

Here’s the abstract:

So this is typical creationist propaganda. Find a fact of biology, pronounce it impossible to explain under an evolutionary hypothesis, and proclaim it as evidence of creation without considering alternative explanations, including those already in the literature.

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