Around 5,000-10,000 years ago, dairy farming changed some people’s DNA. As they began to drink milk, human adults’ genes began to accumulate mutations that would help them digest it.
Such mutations help an organism evolve. Complex organisms like humans contain thousands of genes, most of which are not essential for survival. Mutations in these genes are not lethal. As a result, evolutionary forces can act on these genes, and any beneficial mutation becomes more abundant over time.
But what if a simple organism contained only those genes essential for its survival? Any mutation in such an organism could lethally disrupt its cellular functions. How will evolutionary forces act on the genome of such an organism when it contains so few targets on which selection can act?
