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Occasionally in evolution, especially in single-celled organisms, whole genome duplication events may occur. In these events, one descendant will have two complete copies of their entire genome, which can lead to rapid speciation, as the duplicated genes can acquire new functions.

One such event occurred in the evolution of single-celled yeast. The lineage leading to the baker's yeast S. cerevisiae experienced a genome duplication event. Curiously, if we look at closely related species whose ancestors did not undergo a whole genome duplication, we find that their genome sizes are fairly similar. For example, the yeast K. lactis is descended from a pre-duplication species and has a genome size of 10.6 MB, while S. cerevisiae has a genome size of 12.2 MB (MB refers to megabases or a million base pairs).

What is the MOST likely reason that the genome sizes in the post-duplication species are not double the size of those who did not undergo a duplication?


Genomes that have not been duplicated have a greater risk of transposons becoming inserted into their genome, leading to an inflated genome size in such species.


After the duplication, many of the duplicated genes were lost as they were redundant.


With an undoubled genome, there is less selective pressure for a small genome size, allowing introns to grow much longer.


Duplication of genes can often lead to the presence of paralogs. As paralogs are disadvantageous, there is a strong selective pressure for their deletion or loss.

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