Homologous structures are explained by shared ancestors which can be seen in the fossil record, is supported by genetic study, comparative morphology et al. Homologous structures are just one line of evidence that supports a common ancestry. Evolutionary lineages may be somewhat categorized by comparative morphology of homologous structures.
One example of homologous structures is that of the skeletal structure of the human arm and hand and the leg of a cat, the fin of a whale and the wing of a bat.
As Strahler (1987) points out:
“Corresponding organs that agree in their basic structures are said to be homologous. Of course, the same homologous relationships would also extend to all parts of the skeleton and to the soft parts, such as muscles, nerves, and internal organs. When the vast number of homologous relationships is taken into account, the evidence for descent from a common ancestor becomes overwhelming (pp. 326)”
The idea that homology implies a need is the incorrect way to look at evolution. These ‘variations on a theme’ don’t arise because some organism “needed” them and it was endowed to them. What we observe in nature is the result of natural selection. Organisms have ‘adapted’ to their environment through a weeding out of unfit geno/phenotypes. Or, formally defined by Curtis and Barnes (1994) as, “a process of interaction between organisms and their environment that results in a differential rate of reproduction of different phenotypes in the population; can result in changes in the relative frequencies of alleles and genotypes in the population – that is, evolution (pp.G-14)”.
Aside from forelimb structure, another example would be the middle ear of tetrapods which can be traced all the way back to prehistoric fish. This structure is shared by mammals, reptiles, dinosaurs, amphibians and so forth. A detailed analysis of the Devonian fish, Panderichthys, finds representations of “the earliest stages in the origin of the tetrapod middle ear architecture” (Brazeau and Ahlberg, 2006).
These similarities are not arbitrarily chosen on some whim. Homology supports what we should find as evolutionary theory states. These structures should not be confused with analogous structures which are only superficially similar such as the wing on a bird and the wing of an insect.
Homologies are not restricted to comparative anatomy either it also appears in genetics, biochemistry, neuroscience and may other areas. One well studied homologue is cytochrome c which is found to be similar in over 60 different species (Curtis and Barnes, 1994). The amygdaloid complex in the brain of amniotes share “basic developmental, subdivisions, hodological and neurochemical features” (Moreno and Gonzalez, 2006). The book lungs within species of the class Arachnida are homologous (Scholtz and Kamenz, 2006). 14% of nervous system specific genes have found orthologs across 13 different species (Noda, Ikeo and Gojobori, 2006).
Lizards, along with some other vertebrates has a “third eye” called the parietal eye which has photosensitive cells. Su, Luo, Terakita, Shichida, Liao, Kazmi et al. (2006) showed that a particular opsin, called parietopsin has orthologs across many species.
Davis, Dahn and Shubin (2007) reported on the functional Hox gene of Polyodon spathula also known as the paddle fish. They found several genetic orthologs between the Hox gene expression and regulation in this fish and tetrapods further supporting the homologous structure of forelimbs.
Strahler, A. (1987). Science and Earth History: The Evolution/Creation Controversy. Buffalo: Prometheus Books.
Curtis, H. and Barnes, N. (1994). Invitation to Biology. (5th ed.). New York: Worth Publishers.
Brazeau, M. and Alhberg, P. (2006). Tetrapod-like middle ear architecture in a Devonian fish. Nature, 439, 318-321.
Scholtz, G. and Kamenz, C. (2006). The book lungs of Scorpiones and Tetrapulmonata (Chelicerata, Arachnida): evidence for homology and a single terrestrialisation event of a common arachnid ancestor. Zoology, 109, 2-13.
Noda, A., Ikeo, K. and Gojobori, T. (2006). Comparative genome analyses of nervous system-specific genes. Gene, 365, 130-136.
Su, C., Luo, D., Terakita, A., Shichida, Y., Liao, H., Kazmi, M. et al. (2006). Parietal-eye phototransduction components and their potential evolutionary implications. Science, 311, 1617-1621.
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