Independent Patterns and Homology

I wasn’t ignoring you. I was in another discussion, which appears to be over now.

So I think it would be helpful to find a common frame of reference first. Why don’t you described to me in your own words what “homology” means? That’s sort of similar because it’s also mostly about pattern recognition. That way we can get into a similar mindset about what counts as “objective” and “scientific” to make sure we aren’t employing double standards.

A relation of sameness between two or more characters in two or more organisms

That’s a very poor definition of homology. The standard definition is better: similarity due to common descent. The definition you propose includes homoplasy.

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No. That’s the explanation of homology. Not definition. Homoplasy is only within an evolutionary context. Homology isn’t. The definition I used is the historical one. And it also avoids accusations of question-begging.

If that’s the definition that you want to be used then homology can’t. be used as evidence for common ancestry anymore. Which it isn’t really except for in popular writing. But I think most biologists know it’s the pattern of the characters that make it evidence of common descent.

Trying to avoid this:

Though you may be right and I may be wrong. So any thoughts that could correct my potential misunderstanding would be appreciated

I actually have a reply for this already in the can, prepared to respond to one of Jonathan Wells’s “10 questions to ask your biology teacher”:

Question 3: Homology

“Why do textbooks define homology as similarity due
to common ancestry, then claim that it is evidence for common
ancestry — a circular argument masquerading as scientific


This question stems from confusion on Wells’s part between how something is defined and how it is recognized, which are two quite different things. Homology is indeed defined as similarity due to common ancestry. But we don’t just label any similarity a homology and call it evidence for common ancestry. That would indeed be circular. What we really do is quite different. Similarity between the characteristics of two organisms is an observation. If the similarity is sufficiently detailed (“both are big” or “both are green” won’t do) we consider it a candidate for homology.

Homologies can be tested to some degree by predicting that the characters will be similar in ways we haven’t yet checked. For example, if we propose that similar-looking bones in two animals are homologous, we might predict that they would arise from similar precursors in the embryo, have similar spatial relationships to other bones in the organism, and have their development influenced by similar genes. And this is commonly the case.

But the main way of testing candidate homologies is by congruence with other proposed homologies. By congruence we mean that the two characters can plausibly belong to the same history. If the history of life looks like a tree, with species related by branching from common ancestors, then all true homologies should fit that tree; that is, each homology should arise once and only once on the tree. If a large number of functionally and genetically independent candidate homologies fit the same evolutionary tree, we can infer both that the candidates really are homologies and that the tree reflects a real evolutionary history.

And in fact that’s what we commonly find. Mammals, for example, are inferred to descend from a common ancestor because they all have hair, mammary glands, and other more obscure characteristics like seven neckbones and three earbones. All these characteristics go together: mammals have all of them and no other animals have any of them. Further, other characters support consistent groupings within mammals, and groupings within those groupings. Within most of life, groups are organized in a very special way called a hierarchy. In a hierarchy, every group is related to every other group in one of two ways: either one group entirely contained within the other (as in a below), or they share no members at all (as in b below). No two groups can partially overlap (as in c below).

[Sorry, you will have to imagine the graphic. a. is a pair of concentric circles; b. is a pair of non-overlapping circles; and c. is a pair of overlapping circles.]

What we see if we try to organize species using candidate homologies is that groups organized according to different characters fit together like a and b, but not c, so we get a pattern like this:

[If you could see this, it would be a Venn diagram showing a set of characters diagnosing mammals and various groups within mammals, e.g. a cow and a whale with a circle around them labeled “double-pulley astragalus”.]

Why should these and many other characters all go together in this consistent way? Evolutionary biology explains these characters as homologies, all evolved on a single tree of descent, like this:

[This would be a cladogram with the characters from the Venn diagram above optimized onto the branches.]

Wells gives no alternative explanation for such patterns, and indeed they are hard to explain in any other way than as reflections of an evolutionary history. Wells has it all wrong. Homology isn’t a circular argument, it’s a branching tree of evidence.


It’s an archaic definition that hasn’t been current for at least 40 years, and homoplasy was defined way back in 1870.

So there you go @BenKissling. I’ve been advised to define it as similarity due to common ancestry.

Thank you for the correction. Would you happen to have a source on its in mention in 1870

Lankester, E. R. 1870. On the use of the term homology in modern zoology, and the distinction between homogenetic and homoplastic agreements. The Annals and magazine of natural history; zoology, botany, and geology 6:34-43.

Incidentally, he used “homology” to include both homoplasy and what we now mean by the term, for which his term was “homogenetic”.


2 posts were split to a new topic: Homology according to Byers

@T.j_Runyon the interruption has been moved elsewhere. Please continue as you were. :slight_smile:

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Which is circular unless you admit that it’s not evidence for common ancestry but rather a consequence of assuming it. It also doesn’t help us in our discussion. The goal would have been to come to a way to recognize homology independently of evolutionary assumptions such that it could potentially be evidence for common descent. But hey, if it’s just a consequence of assuming common descent, then why can’t I say specifications are just a consequence of assuming intelligent design?

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No, that’s just not right. I explained this above, if you read it. If you didn’t, here it is again:

As you see, decisions about homology are not “just a consequence of assuming common descent”. Then again, specifications are not a consequence of intelligent design at all. Nested hierarchy, in particular, has no explanation from intelligent design.


i dont think its true. bascially in any case we might get a nested hierarchy. even if all creatures were created at once.

Yeah, by some unfathomably unlikely miracle. So now you need to believe in two miracles, a supernatural and a statistical one. And just to be sure, with the number of species on Earth that now live and have ever lived, to produce such a consistent nested hierarchy by a chance byproduct of instantaneous creation is, well, incalculable.

On the hypothesis that about 50 million species have existed over the entire history of life, to even get two consilient trees has a chance of 1.1359x10^-378285268

That’s a negative 378 millionth power. Turns out you do believe in Tornadoes in junkyards creating entire fleets of passenger planes.

Edit: Actually that’s just the odds of some particular tree, the odds of getting that same one twice is of course much lower.

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“we might get” is not an explanation.

Yes, you might. Or you might not.

Since you might not get a nested hierarchy from intelligent design, intelligent design provides no explanation for why there is one.

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I think you’re flirting with the sharpshooter fallacy. The odds of getting a particular tree are exactly the same as getting the same non-particular tree twice.

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You’re right. It doesn’t matter which tree it is, it’s the probability of obtaining it again, which is just to specify a particular one. I stand corrected.

20 posts were split to a new topic: SCD’s argument about Cars and Nested Hierarchy