A role for genetic drift: Genetic drift can cause genes that are neutral, or even slightly
disadvantageous, to be in high frequencies in small populations. The model that explains how a
population might pass through stages of low fitness on its way to high fitness is called
the shifting balance theory.
Here’s a simple example of how the shifting balance theory works. Imagine a species of
seed-eating bird (like Darwin’s finches) where beak size is controlled by a single gene (of course,
in real life, traits like this are controlled by many genes—but the idea is the same for both cases).
BB individuals have big beaks, Bb individuals have medium-sized beaks, and bb individuals have small beaks.
These birds live in a place where large and small seeds are abundant, but no medium-sized seeds are
available. Populations of all big-beaked individuals have a very high average fitness—they can
crack open big seeds. Populations of all small-beaked individuals do well (they can manipulate smaller
seeds)—but not quite as well as the big-beaked individuals. Medium-beaked individuals have the
lowest fitness—they are not particularly good with either big or little seeds (and no medium-sized
seeds are available). A graph of these gene frequencies and the population’s resulting fitness
levels is shown (below right). This sort of graph is called
an adaptive landscape.
Now imagine a small population of all small-beaked individuals (all bb genotypes). They have a high
fitness (they are at a local peak), but not as high as a population of big-beaked individuals. Through
gene flow some B alleles are introduced to
the population. If selection alone were acting, it would weed these alleles out of the population since
they would show up in Bb individuals with lower fitness. Under selection alone, the population could
never reach the higher BB fitness peak.
However, since the population is small, drift can be a powerful force. Just by chance, the frequencies
of the B alleles increase in the population over several generations (and the population moves into a valley
in the adaptive landscape). If the B alleles become frequent enough, the population will begin to have BB
individuals with high fitness. As this happens, selection begins to increase the frequency of B (the
population moves out of the valley and selection pushes it towards the global fitness peak). Eventually,
through the action of genetic drift combined with selection, the population moves from one local peak, through
a valley of low fitness, to the global fitness peak.
Of course, the above example is a simplified one based on a single locus with two fitness peaks in the adaptive landscape. In the real world, many, many loci affect the fitness of a population and an adaptive landscape may have multiple peaks and valleys. A complex landscape involving just two loci is shown below.
