Fig. 1 - Antirrhinum inflorescence

Antirrhinum Hybrid Zone

Southern Europe contains 17-28 Antirrhinum species and subspecies (Fig. 1), the number depending on taxonomic criteria (Mateu and Segarra-Moragues, 2003; Rothmaler, 1956; Sutton, 1988). Even though the species display diverse morphologies and flower colours, they can all be crossed with each other and with the model species A. majus to give fertile progeny.

In most cases, Antirrhinum species occupy non-overlapping geographical regions, precluding natural hybridization. However, regions of contact provide ideal material for studying evolutionary processes at the short and medium scales.

A well studied example is a hybrid zone between yellow-flowered A.m.striatum and magenta-flowered A.m.pseudomajus in the eastern Pyrenees (Whibley et al., 2006).

A key question is how alleles conferring these distinctive phenotypes became established despite selective barriers to invasion between certain colour morphs, as observed at the hybrid zone. Have environmental shifts played a part in promoting the spread of yellow genes under some circumstances? Or did evolution take a more circuitous path that avoided barriers and incompatibilities?

To answer such questions we need to understand how factors such as drift, gene flow, epistasis, selection and environmental variation may influence the spread of alleles through diverse populations.