Genetic
diversity arises through the interplay of mutation, selection and
genetic drift. In most scientific models, mutants have a fitness value
which remains constant throughout. Based on this value, they compete
with other types in the population and either die out or become
established. However, evolutionary game theory considers constant
fitness values to be a special case. It holds that the fitness of a
mutation also depends on the frequency of the mutation.

Scientists
from the Max Planck Institute for Evolutionary Biology in Plön and the
University of British Columbia in Vancouver developed a model to address
the scenario of mutations being frequency-dependent but having random
fitness parameters. The results demonstrate that the dynamics that arise
in random mutants increase the genetic diversity within a population.
Fitness, though, may even decline.

Population
geneticists generally study mutations with constant fitness values.
However, frequency-dependent selection is a recurrent theme in
evolution: it enables the evolution of new species without geographical
separation (sympatric speciation) or a relatively rapid change in the
immune system of a population.

A
mutation may be advantageous for low frequencies, for instance, but the
fitness of the mutant decreases with rising frequency. A reverse trend
in the fitness value is also conceivable. “Our computer model combines
aspects of population genetics and evolutionary game theory in order to
obtain a new perspective on genetic evolution,” says Arne Traulsen from
the Max Planck Institute for Evolutionary Biology. Whereas mutations
have a random yet fixed fitness value in many mathematical models, this
new model also enables a change in random fitness values with the
frequency of the different types.

The
results of the simulations show that frequency-dependent selection
leads to higher genetic diversity within a population of individuals
even though diversity per se is not favoured. The interaction of
different mutations and the emergence of new mutants support the
development of dynamic diversity in the population. One mutant does not
always need to replace all other mutations or the original population.
“It is possible for different mutations to exist in parallel such that a
new mutant does not to completely replace the residents,” says Weini
Huang, lead author of the study. What is particularly interesting is the
fact that diversity in this model remains naturally limited.

Fitness,
on the other hand, does not necessarily rise with frequency-dependent
selection in contrast to constant selection. By way of example, a
mutation may arise within cells which halts the production of substances
that are passed on to other cells. This can initially be advantageous;
however, if it reaches fixation, the average fitness of the cell
population declines. Advantageous mutations can thereby be lost and
deleterious ones become established.

Source: Max Planck Insititute