Viruses change rapidly. They evolve so quickly that some vaccines must be reformulated every year to remain effective. Two main factors, combined with fast growth in successful viral populations, cause viral change to be rapid. One factor is antigenic drift and one is antigenic shift. Both these mechanisms permit viruses, particularly viruses with RNA in their genetic material rather than DNA, to mutate so fast that new vaccines can hardly keep up with them.
Genetic drift, the title of this article, is a different phenomenon, though somewhat related to antigenic drift. It is responsible for a huge part of the evolution that Charles Darwin so brilliantly observed and explained.
Antigenic Drift
Antigenic drift protects viruses from vaccines, and from the human body’s immune system. In effect, antigenic drift changes the way a virus “looks” to the immune system. The influenza A virus, for example, has two important molecules on its outer coat. One, hemagglutinin, is responsible for opening up host cells so that viral cells can enter. When a host organism, through vaccination or previous infection, recognizes the shape of the hemagglutinin, it will prepare a defense against the virus, and the viral attack will be weakened or fail.
The other molecule that the immune system may recognize is neuraminidase. It is responsible mainly for releasing viruses from used up host cells, so that the virus can spread through the organism. If a flu virus is described as H1N1, for example, the H refers to hemagglutinin, and the N to neuraminidase.
The H and N are antigens. That is, they are the part of the virus that the immune system recognizes and reacts to. Antigenic drift in influenza A (and in other viruses) happens because they change readily into other shapes that are not so easily recognized by the immune system.
Because the influenza A virus is single stranded RNA, it lacks the genetic safeguards against copying errors found, for instance, in human double-stranded DNA. It is more likely to make a mistake when it reproduces itself, because it has no proofreading enzymes to make sure it has produced a true copy. In fact, essentially every new copy of influenza A will contain a copying error, a mutation.
Once these mutations change the shape of the N and H on the outside coat of the virus, they escape detection by the host’s immune system. They enter cells, and begin killing them, thanks to the disguise they have evolved through antigenic drift.
Antigenic Shift
Antigenic shift is fortunately less common than antigenic drift. In antigenic shift, two or more strains of a virus combine to create a new pathogen, one unknown to the immune system.
This could happen, for example, if someone infected with“bird flu” becomes simultaneously infected with “swine flu” from another part of her farm. Within her cells, the two viruses might recombine to produce a virus with a new structure and effect.
The new virus might be a very successful individual, fooling immune systems and spreading rapidly. It might, instead, be very unsuccessful, if it killed its host before the disease could be passed on.
Genetic drift, on the other hand, is a phenomenon of populations. It causes traits to expand or disappear in ways that have little to do with their utility to the organisms, of whatever size, involved.
Genetic Drift
Natural Selection is not the only mechanism of evolution. Genetic drift is probably as important, though it was unknown to Charles Darwin. Natural selection acts by promoting the survival of the genes of those organisms that are most suited to their environment. Genetic drift acts randomly, changing the genetic makeup of a population with no regard for whether it is improving or reducing the fitness of that group.
Genetic drift occurs because parents don’t pass all their genes on to their offspring, because some perfectly fit offspring do not survive to breed, or because some seed falls on stony ground. It’s random. In small populations, it can have a rapid effect on the makeup of subsequent generations.
An example from a human population is a family of two brown-eyed parents. If the mothers of each member of this couple had blue eyes, each parent will carry one gene for blue eyes and one for brown. Both will probably appear brown eyed, because the brown eye gene is dominant. The blue-eyed gene is still there though, hidden, recessive. If these parents have four children, according to the laws of inheritance worked out by Gregor Mendel, odds are one child will have blue eyes, two will have brown eyes but retain the capacity to bear blue-eyed offspring, and one will have brown eyes and only be able to contribute brown-eyed genes to his or her children. Half of the children of this couple will retain the capacity to have either blue-eyed or brown-eyed children. Half will not.
If the parents only have one child, chances are fifty-fifty that their effect on diversity in eye color in the next generation will be to reduce it. The parents’ potential contribution of either blue-eyed or brown-eyed children to the gene pool will vanish half the time. If the parents have only one child and he or she is blue-eyed, or brown-eyed with only the allele (form of the gene) that produces brown eyes, genetic variability decreases. This reduction in variability, and the direction in which it goes, is due to chance alone.
Most parents do not have exactly four children. If they do, not all of the children will successfully breed.
Therefore, with each generation, the distribution of alleles in any population is changed, by chance. The number of people with one allele or another will increase and decrease randomly. In successive generations, the allele for any trait can die out, and never be seen in that population again, unless it appears through mutation or in-migration. If an allele reaches 0% in the population, it is lost, and if a trait reaches 100%, it is fixed. In fact, though genetic drift is random, it acts to produce uniformity.
Another example of genetic drift is what can happen in a catastrophe. If an avalanche destroys many pine seedlings in one mountain cirque, it may happen to destroy all the trees with the most blue in the green of their needles, or those that smell least strongly of balsam. A random event can easily change the genetic makeup of a small population, while it is harder for it to change the makeup of a larger, more varied population.
Bottlenecks are great reductions in population size. After a bottleneck, the rebuilding population is almost certain to have a very different genetic makeup than it had before. For example, elephant seals were hunted almost to extinction. They are protected now (and fashions have changed). Their population is much larger, but it is quite likely that certain genes once found among these seals are gone forever. Cheetahs are another population that appears to have gone through a bottleneck several thousand years ago.
The founder effect is another mechanism of genetic drift. The few people who came to Alaska 10,000 years ago happened not to have the blood type B, or those who had it happened not to have their alleles reproduced. Today, Native Americans tend not to have that blood type, though it is reasonably common in Central Asia. Due to such changes, scientists can trace human migration through associated changes in DNA.
Genetic drift operates randomly, while natural selection operates to move a population towards a better fit with its environment. Genetic drift changes the genetic makeup of a population without regard to adaptation. Neither genetic drift nor natural selection acts individually; these two mechanisms of change interact to produce populations that constantly evolve.
Antigenic drift and antigenic shift can produce strange new organisms that inhabit new niches. Genetic drift often arises out of new niches, when isolated populations bring new characteristics to the fore.
