alloenzyme
Alloenzymes are a type of molecular marker used by phylogeneticists to gauge evolutionary histories of different species of organisms. They are enzymes that are well conserved in function throughout a phylas' or kingdoms' evolutionary history and extant members. For instance, when comparing evolutionary relationships of animals, Alcohol dehydrogenase and other analogues are commonly used examples. When comparing more diverse organisms, more well conserved and important enzymes have to be compared, such as DNA polymerase which shows relatively small differences in amino acid sequence between phyla and even kingdoms. The key to choosing a useful alloenzyme for comparing multiple species is by choosing one that is as variable as possible while still being present in all the organisms, this will show more amino acid similarities between evolutionarily related species and fewer between those that are less well related. The less well conserved the enzyme is, the more amino acid differences will be present in even closely related species.
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