Mutations can appear rare primarily because of selection.
While mutations do occur randomly, the harmful ones are often eliminated through a process known as natural or cellular selection.
The Role of Selection in Mutation Rarity
Mutations are changes in the DNA sequence. When these changes happen in crucial parts of the genome, they can disrupt vital cellular functions or developmental processes.
How Selection Filters Mutations
- Impact on Survival: If a mutation occurs in genes that are important for cell survival or organism development, it can have significant negative consequences.
- Elimination: Cells or organisms carrying these harmful mutations often do not survive or reproduce successfully. This is because of selection.
- Weeding Out Harmful Variants: The reference states, "When mutations happen in parts of the genome that code for important genes, then very often those cells don't survive." It further clarifies that "The harmful mutations are weeded out by selection, so we don't see them."
Therefore, while mutations are constantly occurring at a certain rate, the ones with negative effects are quickly removed from the population (whether it's a population of cells or a population of organisms). What we observe are the surviving cells or individuals, which are less likely to carry severe, detrimental mutations.
In essence, selection acts as a filter, making harmful mutations seem rare in the observed population because they are quickly eliminated. Mutations that are neutral or beneficial are more likely to persist and be observed.