Saccharomyces cerevisiae (baker’s yeast) shows mixed mito inheritance. You posse

Question

Saccharomyces cerevisiae (baker’s yeast) shows mixed mito inheritance. You possess a
baker’s yeast mutant missing a large amount of mito DNA (a “deletion” allele = “?”) that
nevertheless displays no deleterious phenotype. When ? is crossed to wild-type, haploid
progeny are first seen to carry both forms. After a few generations of mitotic division in culture,
however, most cells only carry the ? form. Propose an explanation for this observation including
why the wild-type mito genome is preferentially lost.

Explanation / Answer

This is an example of directional selection, a type of natural selection in which the environment selects one end of the spectrum of traits, so the mean of the population shifts in that direction. This shift will occur in a situation where the mutation in mito DNA causes a "gain of function," ie a novel property/ability which is evolutionarily adaptive over the wild type.

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