One of the major characteristics of vertebrate immunology is thymic involution, the shrinking (involution) of the thymus with age, resulting in changes in the architecture of the thymus and a decrease in tissue mass.
This process is genetically regulated, with the nucleic material responsible being an example of a conserved sequence — one maintained through natural selection (though the pressures shaping this are unclear as will be discussed) since it arose in a common ancestor of all species now exhibiting it, via a phenomenon known to bioinformaticists as an orthologic sequence homology. [Source: Wikipedia 
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