The process of evolution is driven by natural selection, which acts on the genetic variation present in a population. This variation arises from mutations, which are constantly occurring in the gene pool. Natural selection then determines which of these mutations will be passed on to future generations.
Understanding Purifying Selection
Purifying selection, also referred to as negative selection, is the most common form of natural selection. It functions as a gatekeeper, removing new mutations that are harmful or deleterious to an organism. The primary role of this process is to preserve the existing, functional state of a gene. This form of selection is particularly active in genes that are fundamental to survival, where most changes would be detrimental.
Imagine a quality control system in a factory that is responsible for producing a complex and essential machine part. Any deviation from the established blueprint is likely to result in a faulty part that could cause the entire machine to fail. In the same way, purifying selection eliminates genetic changes that would impair the function of highly conserved genes, such as those involved in cellular respiration or DNA replication. These genes have been fine-tuned over long evolutionary periods, and most alterations to their sequence are weeded out. It is a constant and pervasive force, acting to maintain the status quo in the face of continuous mutational pressure.
Understanding Positive Selection
In contrast to the conservative nature of purifying selection, positive selection, sometimes called Darwinian selection, actively promotes the spread of new, advantageous mutations. This is the driving force behind adaptation, enabling organisms to thrive in new environments or overcome new challenges. While purifying selection is about preservation, positive selection is about change and innovation.
A classic example of positive selection can be observed in the rapid evolution of antibiotic resistance in bacteria. When a population of bacteria is exposed to an antibiotic, individuals with a pre-existing mutation that confers resistance are more likely to survive and reproduce. This beneficial mutation quickly spreads through the population, leading to a strain of bacteria that is resistant to the drug.
Another well-known example in humans is the evolution of lactose tolerance. In populations that domesticated cattle and consumed dairy products, individuals with a mutation that allowed them to digest lactose into adulthood had a nutritional advantage. This trait, driven by positive selection, became common in these populations, while remaining rare in populations that did not have a history of dairy consumption.
Key Differences in Evolutionary Impact
This difference in target alleles leads to distinct effects on genetic variation. Purifying selection acts to reduce genetic diversity at specific, important sites within the genome. By eliminating new mutations, it maintains a state of conservation, ensuring that functionally important genes remain unchanged over long evolutionary timescales.
Positive selection, on the other hand, can lead to a phenomenon known as a “selective sweep.” During a selective sweep, a beneficial allele, along with the surrounding region of DNA, rapidly increases in frequency, sometimes to the point of fixation where it is the only allele present in the population. This process drastically reduces genetic variation in that part of the genome.
How Scientists Detect Selection
Scientists can detect the signatures of these selective forces by analyzing patterns of genetic variation within and between species. A primary method involves comparing two types of mutations in the protein-coding regions of genes: synonymous and non-synonymous mutations. Synonymous mutations are changes in the DNA sequence that do not alter the resulting amino acid sequence of the protein. Non-synonymous mutations, on the other hand, do change the amino acid sequence.
The ratio of the rate of non-synonymous substitutions (dN) to the rate of synonymous substitutions (dS) is a powerful tool for inferring the type of selection acting on a gene. This dN/dS ratio, also known as the Ka/Ks ratio, provides a measure of the selective pressure on a protein. The rate of synonymous substitutions is used as a baseline for the rate of neutral evolution, as these mutations are generally not subject to selection.
A dN/dS ratio less than 1 is an indicator of purifying selection. This suggests that non-synonymous mutations are being removed from the population, which is consistent with the idea that changes to the protein sequence are generally harmful. A dN/dS ratio greater than 1 is a sign of positive selection, indicating that non-synonymous mutations are being favored and are accumulating at a faster rate than synonymous mutations. A dN/dS ratio equal to 1 suggests that the gene is evolving neutrally, with changes accumulating by random chance, a process known as genetic drift.