  In sexual population, recombination reshuffles genetic variation and produces
novel combinations of existing alleles, while selection amplifies the fittest
genotypes in the population. If recombination is more rapid than selection,
populations consist of a diverse mixture of many genotypes, as is observed in
many populations. In the opposite regime, which is realized for example in the
facultatively sexual populations that outcross in only a fraction of
reproductive cycles, selection can amplify individual genotypes into large
clones. Such clones emerge when the fitness advantage of some of the genotypes
is large enough that they grow to a significant fraction of the population
despite being broken down by recombination. The occurrence of this "clonal
condensation" depends, in addition to the outcrossing rate, on the heritability
of fitness. Clonal condensation leads to a strong genetic heterogeneity of the
population which is not adequately described by traditional population genetics
measures, such as Linkage Disequilibrium. Here we point out the similarity
between clonal condensation and the freezing transition in the Random Energy
Model of spin glasses. Guided by this analogy we explicitly calculate the
probability, Y, that two individuals are genetically identical as a function of
the key parameters of the model. While Y is the analog of the spin-glass order
parameter, it is also closely related to rate of coalescence in population
genetics: Two individuals that are part of the same clone have a recent common
ancestor.
