  Using Monte Carlo model of biological evolution we have discovered that
populations can switch between two different strategies of their genomes'
evolution; Darwinian purifying selection and complementing the haplotypes. The
first one is exploited in the large panmictic populations while the second one
in the small highly inbred populations. The choice depends on the crossover
frequency. There is a power law relation between the critical value of
crossover frequency and the size of panmictic population. Under the constant
inbreeding this critical value of crossover does not depend on the population
size and has a character of phase transition. Close to this value sympatric
speciation is observed.
