As we noted in the previous section, most multicellular organisms have many more genes than do most unicellular species. But multicellular organisms evolved from unicellular ancestors. How did the numbers of genes within the genomes of multicellular organisms increase over evolutionary time? There are two primary mechanisms that can result in such increases: genes can be transferred from other species, or genes can be duplicated within species.
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Genes can sometimes move among distantly related lineages on the tree of life.
The duplication of a gene provides opportunities for the evolution of new functions.
Some genes are present in multiple copies in the genome, and these copies often evolve together through time.