  We discuss two different ways of chromosomes' and genomes' evolution.
Purifying selection dominates in large panmictic populations, where Mendelian
law of independent gene assortment is valid. If the populations are small,
recombination processes are not effective enough to ensure an independent
assortment of linked genes and larger clusters of genes could be inherited as
the genetic units. There are whole clusters of genes which tend to complement
in such conditions instead of single pairs of alleles like in the case of
purifying selection. Computer simulations have shown that switching in-between
complementation and purification strategies has a character of a phase
transition. This is also responsible for specific distribution of recombination
events observed along eukaryotic chromosomes - higher recombination rate is
observed in subtelomeric regions than in central parts of chromosomes - for
sympatric speciation and probably for non-monotonous relation between
reproduction potential and genetic distance between parents.
