Caught in a drift
A random process of drift can also change the genetic make-up of a species
A change in DNA might be beneficial but is sometimes harmful. Usually, though, it won’t make a blind bit of difference. This is known as a neutral change.
If the environment neither favours nor discriminates against this change, Darwinian theory suggests that it will have no evolutionary impact. In fact, neutral changes are important, thanks to a random process known as genetic drift.
Because of chance events in breeding, the frequency of alleles may fluctuate or ‘drift’ over time. How much evolutionary change is due to drift is not certain. However, the impact of drift is most pronounced in small populations, where alleles may be lost from a population purely by chance.
Such small populations can arise if a group of organisms becomes separated from a larger grouping, when it is said to pass through a ‘population bottleneck’. The alleles that these pioneering organisms carried will be the ones that spread through the new population.
This can lead to otherwise rare alleles becoming common in certain populations – known as the founder effect. Certain groups, such as the Amish religious community in the USA, have a high prevalence of disease-causing alleles because they were present in founding populations and members of the community tend not to marry outside the community.
So the presence of a common allele does not necessarily mean it has been through the filter of natural selection: it may just have been lucky.
- University of Berkeley: Genetic drift
- Pediatric medicine and the genetic disorders of the Amish and Mennonite people of Pennsylvania (2003)
Questions for discussion
- What other human genetic conditions are associated with founder effects?