Figure 1

Schematic representation of the 'birth an death' and the 'concerted evolution' models. In phylogenetic analyses, class Ia (solid boxes) and class Ib (hatched) molecules within a species are more similar to each other than to class Ia or class Ib molecules in another species. According to the 'birth and death' model, this reflects the propensity of MHC molecules to derive from a single ancestral sequence by successive gene duplications. Another explanation, the 'concerted evolution' model, is that class Ia and class Ib molecules tend to become like each other due to frequent events of gene conversion, without necessarily deriving from a single sequence.