On Homology

The phenomenon of homology refers to things that are corresponding or similar in position, value, structure and purpose. For example, many mammals share a common limb design that is versatile and lends well to a number of different functions. In Origin, Darwin notes in great detail the similar expressions of pentadactyl limb design as utilized by man for grasping, moles for digging, horses for movement and bats for flying. Further considering monkey and man, coyote and wolf, or fir and pine, the fact that different types and kinds of organisms share similarities in physical structure, biochemistry and embryonic patterns of development is argued as evidence that life must have descended from a common ancestor. Darwin cited the phenomenon of homology as the strongest evidence for his general theory of evolution. In the same vein, Miller and Levine also feel that homologous resemblance amongst organisms is compelling evidence: “The structural and biochemical similarities among living organisms are best explained by Darwin’s conclusion: living organisms evolved through gradual modification of earlier forms – descent from a common ancestor.”

While most definitely a reasonable assumption, is it the only one? Homologous resemblance was one of Darwin’s big guns, a raison d’etre of his theory, and Charles defined homology as the “…relationship between parts which results from their development from corresponding embryonic parts.” This is a reasonable assumption and given the status of scientific discovery at the time, one could easily see how Darwin arrived at this definition, yet in light of new evidence we can spot the presupposition: that homologous resemblance necessarily or exclusively results from ‘corresponding embryonic parts.’ Darwin argued that similarities in physical, external parts of different organisms were arrived at through similar embryonic processes.

In terms of biological theory, homology refers to similarities of physical structure between organisms. Looking at embryos of a dog, chicken and human, to the untrained eye it would be difficult to distinguish them, and to this end Haeckel noted that in their earliest stages of life, organisms share an astounding degree of developmental similarity even amongst different species. Is homologous resemblance limited solely to physical structure? If Darwin’s interpretation of homology were complete, it would be reasonable to expect identical or even similar embryological patterns amongst organisms that displayed identical or similar structural designs. Yet in at least one example this is not the case.

Sir Gavin de Beer, embryologist and past director of the British Museum of Natural History, discussed what homology might or might not be at great length in his Homology, an Unresolved Problem. De Beer notes how homologous structures are often the result of completely different embryogenic processes, even amongst the same class of organisms. He notes that when the optic cup is removed from the embryo in the common frog Rana fusca, no lens develops. Yet, when the optic cup is removed from the embryo in the closely related edible frog Rana esculents, the lens develops perfectly fine. Homologous structures do not demand absolutely identical routes of development, and the concept of homology cannot be extended into embryology. If it could, this would be much stronger evidence for Darwin’s general theory of evolution. It seems to this author that homologous resemblance is only compelling evidence for such in the absence of irreconcilable differences, and as it stands, creatures displaying similarities in skeletal structure do not necessarily keep to their similarities at a genetic level. To me, this may suggest that what Darwin viewed as absolute(?) homology is at least partially superficial. Furthermore, homologous resemblance is also amenable to special creation.

Now I'm no biologist or geneticist, so if correction or clarification is needed, shoot from the hip. I would like to learn more about this subject.

2 Comments

  1. Freidenker says:

    I’m not a geneticist, but I undergraduate biology is sufficient for at least a few sling shots. From the hip or upwards.
    First of all, embryology is a messy process that, as far as my meager knowledge of it can account for it – isn’t solely based on genes. (!)
    Homology is something I had great doubts about when I first started learning about evolution (as a believing Jew, the whole idea seemed utterly unbelievable. Sorry for the cliche). However, certain homologies are so uncanny that the thought of their ascension without comment descent boggles the mind. I would love to have a better explanation for the existence of tetrapod limbs and ear bones in fully aquatic mammals who would die on the surface within hours. I would love to hear a better explanation why a Panda, descended from carnivores and pushed into herbivory – would need to employ a badly designed thumb that’s no more than a weird protrusion of its wrist, 4 “predatory fingers” notwithstanding.
    Frankly, homology is a bitch. I never liked it because it’s so easy to misfire. I only trust homology once it starts mounting up and is no longer deniable that certain homologies are good evidence for common descent (gill pouches in human embryos and, er, in *adult* fish spring to mind as a good example).
    Personally, I think homology is just the easiest thing to find. Last year I had a research project (no actual research, just a lot of reading and writing) with a professor who investigated the evolution of rodents. One of the first things I did was pore into a chain of studies on whale evolution. The genetic evidence for the relationship of whales with terrestrial mammals by far exceeds anything paleontology or homology that I have ever learnt about.

  2. Freidenker says:

    I’m not a geneticist, but I undergraduate biology is sufficient for at least a few sling shots. From the hip or upwards.
    First of all, embryology is a messy process that, as far as my meager knowledge of it can account for it – isn’t solely based on genes. (!)
    Homology is something I had great doubts about when I first started learning about evolution (as a believing Jew, the whole idea seemed utterly unbelievable. Sorry for the cliche). However, certain homologies are so uncanny that the thought of their ascension without comment descent boggles the mind. I would love to have a better explanation for the existence of tetrapod limbs and ear bones in fully aquatic mammals who would die on the surface within hours. I would love to hear a better explanation why a Panda, descended from carnivores and pushed into herbivory – would need to employ a badly designed thumb that’s no more than a weird protrusion of its wrist, 4 “predatory fingers” notwithstanding.
    Frankly, homology is a bitch. I never liked it because it’s so easy to misfire. I only trust homology once it starts mounting up and is no longer deniable that certain homologies are good evidence for common descent (gill pouches in human embryos and, er, in *adult* fish spring to mind as a good example).
    Personally, I think homology is just the easiest thing to find. Last year I had a research project (no actual research, just a lot of reading and writing) with a professor who investigated the evolution of rodents. One of the first things I did was pore into a chain of studies on whale evolution. The genetic evidence for the relationship of whales with terrestrial mammals by far exceeds anything paleontology or homology that I have ever learnt about.

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