Genetic drift





Genetic drift is a mechanism of evolution that acts in concert with natural
selection to change species characteristics over time. Like selection, it
acts on populations, altering which traits and which alleles predominate
among members and changing the diversity of the group. But unlike selection,
drift occurs only in small populations and results in changes that need not
be adaptive. A statistical effect, drift arises from the role of chance in
the production of offspring.

Chance affects reproduction in part, because no trait dictates exactly to
what age an individual survives or how many offspring he, she or it
produces. Even when an organism inherits "perfect genes"--that is, all the
alleles most associated with success or fitness--still an individual may be
buried in an avalanche, frozen by a frost or caught in an awkward moment by
a foe. As a result, at best a trait confers a superior average fitness to
the group of individuals that carry it.

But because individuals vary, group averages may vary too, especially one
composed of few members. When the variation has to do with number of
offspring the group produces, it affects the transmission of group traits to
the next generation, causing the prevalence of those traits to rise above or
fall below their average. Chance trends in these deviations are called
drift. Under persistent drift, an allele either disappears from the gene
pool or supplants all the other copies of a gene. This is one of the risks
that population bottlenecks pose to genetic diversity.

Law of large numbers

Small populations' greater susceptibility to drift is a manifestation of the
law of large numbers, which is especially easy to see in tossing coins. On
average, of course, coins turn up heads or tails equally. Yet just a few
tosses in a row are unlikely to produce heads and tails in equal number. The
numbers are no more likely to be exactly equal for a large number of tosses
in a row, but the inequality can be very small in percentage terms. As an
example, ten tosses turn up 70% heads about once in every six tries. The
chance of the same imbalance from a hundred tosses in a row is only about
one in 25,000. Big deviations from average reproductivity, similarly, happen
more frequently in small populations than in large ones. As a result, the
smaller the population, the faster they drift.

Sex cells and chance

Beyond its influence on sheer numbers of offspring, chance affects the
reproduction of sexual species in yet another way. Sperm and eggs each
contain only half the amount of genetic material an offspring inherits at
conception. The material in each half is selected somewhat haphazardly from
the complete set of material that each parent carries. This random selection
occurs during crossing over of the chromosomes when sex cells are produced.

In a narrow sense the random aspect is inconsequential, because every
individual produces millions of sperm or eggs. If a diploid individual
carries two versions of a gene, then each allele will be present in very
nearly exactly half of their sex cells--because the number of cells is so
large. But individuals produce few offspring, and the number produced by a
group of genetically alike individuals may not be close to a million either.
As a result, sexual reproduction can lead to a disproportionate transmission
of alleles from one generation to the next, especially when the offspring
population is small.

Adaptive, neutral, and deleterious traits

Drift and natural selection are capable of acting in parallel, even on the
same traits. At the same time as a disease, a predator or another selective
pressure is predisposing individuals with adaptive traits to a higher rate
of reproduction than their peers, runs of luck may enhance or oppose this
tendency. Even the fittest will vary in reproductivity from one generation
to the next, and whenever an advantageous allele diminishes in prevalence,
it's a neutral or even deleterious allele that takes up the slack.

The ability to affect the prevalence of even neutral traits distinguishes
drift from selection. As a result, theorists often appeal to drift to
account for evolutionary changes that might have paved a way for
adaptations, and yet offered no obvious immediate advantage. Yet while
aiding in explanation, drift also creates a challenging ambiguity. How does
one know whether a trait shared by every member of a species--yellow spots,
for example--represents an adaptation to selective conditions, or instead
represents just an accident? Not only can it be difficult to say, the answer
may be "a little bit of both."

Teasing apart the relative influence that drift and selection have had over
the course of evolution, both with respect to individual species as well as
with regard to the history of life in general, is a primary aim of
evolutionary biology.

Population genetics perspective

From the statistical perspective of population genetics, drift is a
"sampling effect". A chance over-production or under-production of offspring
compared to the average represents what statisticians call a sampling error.
According to this perspective, the frequency distribution of alleles among a
population of offspring (how many carriers there are of each allele)
reflects a sampling of the alleles of the preceding generation. When the
alleles of a gene do not differ with regard to fitness, on average the
number of carrieres in one generation is proportional to the number of
carriers in the last. But the average is never tallied, because each
generation parents the next one only once. Therefore the frequency of an
allele among the offspring often differs from its frequency in the preceding
generation.

Many sources of mortality, such as infectious diseases for which no immunity
exists, may be regarded as randomly sampling a population. In other words,
they randomly select some proportion of individuals for death. Because the
sample is random, on average alleles are picked in proportion to how common
they are. But because the sample size, the population size and the number of
carriers of an allele are finite, deviations from the average or mean often
occur. To the extent that the upward and downward deviations over successive
generations do not exactly balance out, an allele drifts.

Drifting alleles are liable to disappear all together from the gene pool.
When the number of individuals who carry an allele drifts to zero, so that
no individuals are left to reproduce it, it disappears forever. Similarly,
if all but one of the alleles for a given gene disappears, the proportion of
individuals who carry it will never stray from 100%. That is, until in at
least one individual a spontaneous mutation or other genetic change affects
that carrier's allele. It is also possible in principle for such a change to
reintroduce an allele that has disappeared from the gene pool.

When drift comes into play

Population bottlenecks, Founder's effect etc.

Other sources of chance and variability

The principle of independent assortment may also be involved in drift.
According to this principle, during gamete formation many traits combine
randomly. Thus, an individual may inherit alleles that increase fitness
along with alleles that are neutral (that neither increase nor decrease
fitness). Natural selection favors the alleles that increase fitness, but
the associated neutral alleles will also increase in frequency, as an
accidental byproduct.

Examples

Chance acts on allele frequency in a variety of ways. Perhaps the most
obvious input is lifespan. For example, imagine a collision between a car
and a bus. If the collision was caused by the fact that the driver of the
car had poor vision, that driver's death might be an example of natural
selection. But for the driver and passengers of the bus, death was random.
If these people died before reproducing, their death would alter the
frequency of their genes in the subsequent generation. In other words, even
when individuals are equally fit, they will differ in their success. Simply
by being in the wrong place at the wrong time, the death of some and the
survival of others can change the distribution of alleles in a population,
and thus be a force in its evolution. "Differential morbidity" is the most
important cause of drift in populations of asexual (or "clonal") organisms,
and it is an important cause of drift in populations of sexual organisms as well.