Homology describes similarities between species resulting from shared ancestors, reflecting the branching of the phylogenetic tree. Homologies abound in the biological world, such as oxidative phosphorylation, while brains are a characteristic of animals. Analogy can emerge independently in both species. Genetics has refined the concept of homology, with orthology, paralogy, and xenology.
Homology is a biological concept that describes similarities between species resulting from shared ancestors. It is a central idea in the study of evolution because it reflects the branching of the phylogenetic tree. In general, if two species share many properties and genes, they are likely to have diverged from each other—that is, speciated—relatively recently in evolutionary time.
All organisms are related to other organisms through evolution from a small number of common ancestors. Consequently, homologies abound in the biological world. Oxidative phosphorylation, the process of using oxidation to produce adenosine triphosphate, is a nearly universal biological homology. The process was used by early organisms, and subsequent developments in evolution have modified but not replaced it.
Brains, on the other hand, do not appear in all organisms. They are a characteristic that belongs only to animals. Not all animals have brains, but most do. Whether or not a species has a brain provides a clear indication of its position in the evolutionary tree. Some organisms, which have undergone less change since this time, have brains similar to older ones. It is important to remember, however, that these organisms have survived and changed over as much evolutionary time as the more complicated ones.
Homology is often discussed in contrast to analogy. Not all similarities between two organisms are the result of a common evolutionary ancestry. If two species are in an environment conducive to a particular adaptation, this adaptation can emerge independently in both species. For example, opossums developed opposable thumbs independently of primates. This result isn’t too surprising, since opposable thumbs are useful. Because evolution is unpredictable and chaotic, however, analogy is much less common than homology.
The study of genetics has refined the concept of homology with direct comparison between DNA sequences. Before genetic information could be read directly, scientists had to guess homology – and taxonomy in general – based on physiological observations. Now, the relative similarity of alleles found in different organisms may provide strong evidence regarding their evolutionary relationship. Uncertainty as to whether a shared property is homologous or analogous can be fully resolved upon DNA examination; statistically, some level of similarity between sequences could not have occurred on its own.
Genetics has also created the need for subclassifications within the concept of homology. Orthology refers to the genetic similarities that result from speciation. The paralogy describes homologies between genes resulting from duplication within the genome of a single organism. Xenology is the existence of homology resulting from lateral gene transfer: genetic material transmitted through a virus or by scientists, for example. The existence of xenologous homologies complicates the idea of a perfectly unidirectional branching of the evolutionary tree.
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